Medicine Crash Course: Medical School, A Level, and GCSE revision website: Chemistry Unit 3

With the discovery of over 50 elements by the 1860s, scientists began to try to sort the elements into a logical sequence by identifying patterns in their chemical properties. The work of John Newlands and Dmitri Mendeleev in developing early periodic tables, ultimately led to the development of the modern periodic table.

Newlands’ octaves

John Newlands

An English scientist called John Newlands put forward his Law of Octaves in 1864. He arranged all the elements [element: A substance made of one type of atom only.] known at the time into a table in order of relative atomic mass [relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom.] .

When he did this, he found a pattern among the early elements. The pattern showed that each element was similar to the element eight places ahead of it.

For example, starting at Li (lithium), Be (beryllium) is the second element, B (boron) is the third and Na (sodium) is the eighth element. He then put the similar elements into vertical columns, known as groups.

Part of Newlands' table

H	Li	Be	B	C	N	O
F	Na	Mg	Al	Si	P	S
Cl	K	Ca	Cr	Ti	Mn	Fe

Regular repeats

Newlands' table showed a repeating or periodic pattern ofproperties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] , but this pattern eventually broke down.

By ordering strictly according to atomic mass [atomic mass: The mass of an atomic particle, sub-atomic particle or molecule compared to 1/12th the mass of a carbon-12 atom.] , Newlands was forced to put some elements into groups which did not match their chemical properties.

For example, he put iron (Fe), which is a metal, in the same group as oxygen (0) and sulfur (S), which are two non-metals.

As a result, his table was not accepted by other scientists.

Mendeleev's periodic table

Dmitri Ivanovich Mendeleev

In 1869, just five years after John Newlands put forward his Law of Octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements [element: A substance made of one type of atom only.] known at the time in order of relative atomic mass [relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom.] , but he did some other things that made his table much more successful.

He realised that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.

Part of Mendeleev's periodic table

Row	Group I	Group II	Group III	Group IV	Group V	Group VI	Group VII	Group VIII
1	H
2	Li	Be	B	C	N	O	F
3	Na	Mg	Al	Si	P	S	Cl
4	K	Ca	?	Ti	V	Cr	Mn	Fe, Co, Ni, Cu

Gaps and predictions

Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered.

He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right.

For example, he predicted the properties of an undiscovered element that should fit below aluminium in his table. When this element, called gallium, was discovered in 1875, its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.

Evaluating the work of Newlands and Mendeleev

Atomic weight

Both Newlands and Mendeleev arranged the elements [element: A substance made of one type of atom only.] in order of their atomic weight (now calledrelative atomic mass [relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom.] ).

Both scientists produced tables in which elements with similarproperties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] were placed at regular intervals. However, Mendeleev did some things with his table that made it more useful than Newlands’ table – for example, he swapped the order of some elements if that fitted their properties better.

Similarities and differences

The table below summarises some similarities and differences between Newlands’ table and Mendeleev’s table.

Newlands’ Table	Mendeleev’s Table
Ordered elements by atomic weight	Ordered elements by atomic weight
Included only the elements known at the time	Left gaps for elements he predicted would be discovered later
Maintained a strict order of atomic weights	Swapped the order of some elements if that fitted their properties better
Every eighth element had similar properties (Newlands’ Law Of Octaves)	Elements in groups had similar properties
Was criticised by other scientists for grouping some elements with others when they were obviously very different to each other	Was seen as a curiosity to begin with, but then as a useful tool when the predicted elements were discovered later

Dmitri Mendeleev’s early periodic table was further refined in the early 20th century in light of the discovery of protons, neutrons and electrons. This allowed elements to be placed in appropriate groups according to atomic numbers instead of atomic masses, which produced the periodic table we use today.

The development of the modern periodic table

Dmitri Mendeleev put the elements [element: A substance made of one type of atom only.] in order of their relative atomic mass [relative atomic mass:The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom.] , and this gave him some problems.

For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table.

In order to get iodine in the same group as other elements with similar properties - such as fluorine, chlorine and bromine - he had to put it after tellurium, which broke his own rules.

However, the discovery of protons [proton: A sub-atomic particle with a positive charge and a relative mass of 1 found in the nucleus of the atom.] ,neutrons [neutron: Uncharged sub-atomic particles, with a mass of 1 relative to a proton.] and electrons [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus.] in the early 20th century allowed Mendeleev’s table to be refined into the modern periodic table. It involved an important modification – the use of atomic number to order the elements. An element’s atomic number (also called proton number) is the number of protons in its atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.] .

All atoms of the same element contain the same number of protons.

Henry Moseley

Using atomic number instead of atomic mass as the organising principle was first proposed by the British chemist Henry Moseley in 1913. It explained why Mendeleev needed to change the order of some of the elements in his table.

For example, tellurium has a higher atomic mass than iodine, but iodine has a higher atomic number than tellurium. So, even though he didn't know why, Mendeleev was (as it turned out) right to place iodine after tellurium.

Basic periodic table

The arrangement of the modern periodic table

The modern periodic table

Columns in the table - groups

The elements [element: A substance made of one type of atom only.] in avertical column are in the same group. The main groups are labelled groups 1-7, with the noble gases [noble gases: The noble gases are the elements in Group 0/Group VIII of the periodic table. They have a full outer shell of electrons and so are unreactive.] in group 0. All elements in a group have similar chemical properties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] .

The elements in a group all have the same number of electrons [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus.] in their highest occupied energy level (also referred to as the outer shell). This is why they have similar chemical properties.

An element’s group number is the same as the number of electrons in its highest occupied energy level (outer shell). For example, all the metals in Group 2 have 2 electrons in their highest occupied energy level (outer shell).

Element	Symbol	Electronic structure (written)
Beryllium	Be	2,2
Magnesium	Mg	2,8,2
Calcium	Ca	2,8,8,2

Rows in the table - periods

Elements in a horizontal row are in the same period. The periods are numbered from top to bottom.

The period number is the same as the number of occupied energy levels (shells). For example, magnesium is in period 3 – its atoms have three occupied energy levels. Calcium is in period 4 – its atoms have four occupied energy levels.

Elements within different groups within the periodic table have different physical and chemical properties. This determines the kinds of reactions these elements have. Different groups also show different trends, in terms of reactivity, as you move down a group. This can also determine how violently a reaction occurs - or whether it happens at all.

Group 1 Elements

The elements [element: A substance made of one type of atom only.] in group 1 are called the alkali metals. They belong to the left-hand column in the periodic table. They are very reactive [reactive: The tendency of a substance to undergo chemical reaction.] and must be stored in oil to avoid contact with air or water.

Periodic table Group 1

The alkali metals are soft, reactive metals. They react vigorously with water and become more reactive as you go down the group.

Common properties

The alkali metals have the following properties in common:

they are very soft and can be cut easily with a knife
they have low [density: A measure of the quantity of some physical property (usually mass) per unit length, area, or volume (usually volume).] densities (lithium, sodium and potassium will float on water)
they react quickly with water - producing hydroxides and hydrogen gas
their hydroxides dissolve in water to form alkaline solutions

In general:

group 1 metal + water → group 1 metal hydroxide + hydrogen

2M_(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

(M stands for the symbol of a Group 1 metal)

Physical and chemical trends in Group 1

Melting and boiling points

The alkali metals all have low melting points and boiling points compared to other metals. The melting points and boiling points decrease as you go down the group.

Reactivity

As you go down the group, the metals become more reactive [reactive: The tendency of a substance to undergo chemical reaction.] . Lithium (at the top) is the least reactive, while francium (which is at the bottom) is the most reactive.

You will probably see lithium, sodium and potassium at school, but rubidium and caesium are considered to be too reactive to use in the classroom. Francium isradioactive [radioactive: A substance that emits radiation is said to be radioactive.] and very rare - there are only a few grams of it in the whole of the Earth's crust at any time.

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Reactions

Group 1 metals react with non-metals to form ionic compounds. In these reactions, the metal atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.] loses its outer electron [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus.] and becomes a metal ion [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] with a charge of +1. The ionic compounds produced are white solids which form colourless solutions when they dissolve.

For example, sodium reacts vigorously with chlorine:

sodium + chlorine → sodium chloride

2Na(s) + Cl2(g) → 2NaCl(s)

In the formation of sodium chloride, the electron from the highest energy level of a sodium atom transfers to the highest energy level of a chlorine atom

Sodium burns in oxygen to form a metal oxide:

sodium + oxygen → sodium oxide

4Na(s) + O₂(g) → 2Na₂O(s)

The transition metals

The elements [element: A substance made of one type of atom only.] in the centre of the periodic table - between groups 2 and 3 - are called the transition elements. They are all metals. They include most of the commonly-used metals, such as iron, copper, silver and gold.

Periodic table transition metals

Comparing the properties of the transition elements with the Group 1 elements

	Group 1 elements	Transition elements
Melting points	Low	High (except mercury, which is liquid at room temperature)
Reactivity	High (react vigorously with water or oxygen)	Low (do not react so vigorously with water or oxygen)
Strength	Soft or liquid so cannot withstand force	Strong and hard
Density	Low	High
Compounds	White or colourless	Coloured

Check you have remembered the properties of transition metals with this activity:

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Chemical Reactions

Most transition metals form coloured compounds.

Transition metal compounds

Many transition metals act as catalysts [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.] in useful processes. For example, iron is the catalyst used catalyst in the Haber process when Making ammonia:

hydrogen + nitrogen

ammonia

3H₂(g) + N₂(g)

2NH₃(g)

Many transition elements form ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] with different charges. For example, iron forms iron(II) ions, Fe²⁺, and iron(III) ions, Fe³⁺. This means that iron oxide can exist in two forms, iron(II) oxide, FeO, and iron(III) oxide, Fe₂O₃.

Group 7 elements

The elements [element: A substance made of one type of atom only.] in Group 7 are called the halogens. They belong to the column second from the right in the periodic table.

The halogens are all toxic [toxic: Poisonous.] , but this can be a useful property. Chlorine is used to sterilise [sterilise: The process of ensuring that a sample contains no living things.] drinking water and water in swimming pools. Iodine is used in antiseptics [antiseptic: Substance that kills bacteria on skin and other surfaces.] to treat wounds.

Periodic table Group 7

Common properties

The halogens have the following properties in common:

they are non-metals
they have low melting and low boiling points
they are brittle when solid
they are poor conductors [conductor: An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. A thermal conductor allows thermal energy to be transferred through it easily.] of heat and electricity
they have coloured vapours [vapour: Vapour is a cloud of liquid particles. Steam is water vapour.]
their molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance.] are diatomic (each contain twoatoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.] ) - eg chlorine, Cl₂

Physical and Chemical trends in Group 7

Melting point and boiling point

The halogens have low melting points and low boiling points. You can see from the graph that fluorine, at the top of Group 7, has the lowest melting point and lowest boiling point in the group. The melting points and boiling points thenincrease as you go down the group.

Graph shows the melting and boiling points of halogens

Colour

The halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green and bromine is red-brown. Iodinecrystals [crystal: A regular and repeating three-dimensional arrangement of atoms found in some solids.] are shiny purple-black but easily turn into a dark purple vapour [vapour: Vapour is a cloud of liquid particles. Steam is water vapour.] when they are warmed up.

Reactivity

The halogens become less reactive [reactive: The tendency of a substance to undergo chemical reaction.] as you more down the group. Fluorine (at the top of the group) is the most reactive, while astatine (at the bottom) is the least reactive.

Reactions

Halogens react with metals to form ionic compounds [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). The ions swap electrons to achieve a full outer shell.] . In these reactions, the halogen atoms each gain one electron [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus.] to form ions with a charge [charge: In chemistry, charge usually refers to the electric charge of certain subatomic particles. Electrons have a charge of -1 while protons have a charge of +1.] of –1.

Displacement reactions in the halogens

Halogens [halogen: The halogens, or halogen elements, are the elements in Group VII of the periodic table. They have seven electrons in the outer shell.] react with metals to form ionic [ionic: An ionic bond forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair.] compounds [compound: A substance formed by the chemical union (involving bond formation) of two or more elements.] , which dissolve in water. The reacting [reactivity: The rate at which a substance undergoes a chemical reaction.] of the halogens also decreases as you move down the group.

These two principles can be used to explain displacement reactions [displacement reaction: Displacement reactions happen when a more-reactive element replaces a less-reactive element in a compound.] . In these reactions, a more reactive halogen can displace a less reactive halogen from an aqueous [aqueous: Dissolved in water.] solution of its salt [salt: A compound formed by neutralisation of an acid by a base, eg a metal oxide, as the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride, common salt, is one such compound.] .

For example, chlorine is more reactive than bromine, so it can displace bromine from bromide compounds:

chlorine + sodium bromide → sodium chloride + bromine

Cl₂(g) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)

A displacement reaction

You can see that the Cl and Br have ‘swapped places’, forming sodium chloride and bromine (which turns the mixture brown).

In the exam, make sure your answers avoid terms like ‘stronger’ and ‘weaker’. Instead, write ‘more reactive’ and ‘less reactive’.

Reactivity series

If you test different combinations of the halogens and their salts you can work out a reactivity series for the halogens.

The most reactive halogen displaces all the other halogens from solutions of their salts, while the least reactive halogen is always displaced. It works just the same whether you use a sodium salt or a potassium salt.

Test your understanding using this animation in which chlorine, bromine and iodine are added to various halogen salts. Note carefully the products which are present in the test tube after each reaction.

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Read on if you're taking the higher paper.

Trends in reactivity – Higher tier

The reactivity [reactivity: The rate at which a substance undergoes a chemical reaction.] of an element [element: A substance made of one type of atom only.] depends on how easily its atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.] lose or gain electrons [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus.] . Remember that only the electrons in the highest occupied energy level (outer shell) of an atom are used in bonding.

Metals

Metal atoms lose electrons when they react with non-metals.

For example, elements in Group 1 lose the electron from their highest occupied energy level (outer shell) to form ions with a +1 charge.

As you go down the group, the number of occupied energy levels (filled shells) increases. The higher the energy level of the outer electrons, the greater the distance from the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.] , and the more easily electrons are lost. This is why elements in Group 1 become more reactive as you go down the group.

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Non-metals

Non-metal atoms gain electrons when they react with metals.

For example, elements in Group 7 gain one electron into their highest occupied energy level (outer shell) to form ions with a –1 charge.

As you go down the group, the number of occupied energy levels (filled shells) increases. The higher the energy level of the outer electrons, the greater the distance from the nucleus, and the less easily electrons are gained. This is why elements in Group 7 become less reactive as you go down the group.

The electron from the second energy level of a lithium atom transfers to the second energy level of a fluorine atom. This creates a positively charged lithium ion and a negatively charged fluoride ion

The electron from the fourth energy level of a potassium atom transfers to the third energy level of a chlorine atom. This creates a positively charged potassium ion and a negatively charged chloride ion

Water in different parts of the UK varies in the amount of dissolved mineral ions it contains. This determines whether it is hard or soft water. There are advantages and disadvantages to each, but the damage that can be caused to water pipes and appliances by hard water means that water may need to be softened.

Hard and soft water

Rainwater is naturally weakly acidic [acidic: Having the properties of an acid, such as a pH below 7.] because it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks or soaks into the ground and then passes through layers of rock. Compounds [compound: A substance formed by the chemical union (involving bond formation) of two or more elements.] from the rocks dissolve into the water.

Hard water [hard water: Water, containing magnesium ions and calcium ions, that does not easily form lather with soap.] contains dissolved compounds, usually calcium or magnesium compounds. For example, limestone contains calcium carbonate, CaCO₃. Carbonic acid in rainwater reacts with this to produce soluble calcium hydrogencarbonate:

carbonic acid + calcium carbonate → calcium hydrogencarbonate

H₂CO₃(aq) + CaCO₃(s) → Ca(HCO₃)₂(aq)

The presence of calcium ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] and magnesium ions in the water makes it hard. Soft water [soft water: Water that readily forms lather with soap.] readily forms lather with soap, but it is more difficult to form lather with hard water. The dissolved calcium ions and magnesium ions in hard water react with the soap to form scum [scum: An insoluble precipitate that forms with soap and hard water.] , so more soap is needed. Soapless detergents do not form scum with hard water.

The types of rocks found in different regions determines how hard or soft the water will be.

The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.

There are varying degrees of hardness in water - from slightly hard to very hard.

Water softness in England and Wales

Measuring hardness

One way to measure the hardness in water is to perform a titration [titration:A quantitative procedure in which two solutions react in a known ratio, so if the concentration of one solution is known and the volumes of both are measured, the concentration of the other solution can be determined.] with soap solution.

A known volume of water is put into a conical flask. Soap solution is added to it from a burette or pipette. The mixture is swirled to help it form lather. The volume of soap solution that needs to be added to form permanent lather is recorded. The harder the water, the greater the volume of soap solution needed.

The table shows the results of titration experiment to measure the hardness of water.

Water sample	Volume of soap solution needed to form permanent lather/cm³
Distilled water	0.1
A	6.4
B	3.8

In this example, the distilled water acts as a control. Water A is harder than water B because more soap solution was needed to form permanent lather.

Types of hardness

Temporary hard water can be softened by boiling it. Permanent hard waterstays hard, even when it is boiled.

You should be able to tell temporary hard water from permanent hard water. The table shows the results of a titration experiment to distinguish between the two types.

Water sample	Volume of soap solution needed to form permanent lather/cm³
Distilled water	0.1
A	6.4
A after boiling	1.6
B	3.8
B after boiling	3.8

Water B contained only permanent hardness because boiling made no difference to the volume of soap solution needed to form permanent lather. Water A contained both types of hardness. Less soap solution was needed after boiling, but more was needed to form permanent lather than was needed for the distilled water control.

Explaining temporary hardness – Higher tier

Temporary hard water contains dissolved hydrogen carbonate ions, HCO_3–. When heated, these ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] decompose (break down) to form carbonate ions, CO₃^2–. The carbonate ions in the boiled water react with dissolved calcium and magnesium ions to form insoluble [insoluble: Unable to dissolve (usually in water).] precipitates [precipitate: A suspension of particles in a liquid formed when a previously dissolved substance becomes insoluble, ie in a precipitation reaction.] (calcium carbonate and magnesium carbonate).

Permanent hard water contains dissolved sulfate ions, SO₄^2–. These do not decompose when heated. They remain dissolved and do not react with calcium and magnesium ions - so the water stays hard even when boiled.

The benefits and drawbacks of hard water

You need to be able to evaluate the environmental, social and economic aspects of water hardness.

Hard water has some benefits compared to soft water. For example, the dissolved calcium compounds in hard water:

can improve the taste of the water
are good for the development and maintenance of bones and teeth
can help to reduce heart disease

But hard water also has some drawbacks compared to soft water. For example:

More soap is needed to produce lather, which increases costs. This happens with temporary or permanent hardness.
The scum produced is unsightly - spoiling the appearance of baths and shower screens, for example.
Temporary hardness can reduce the efficiency of kettles and heating systems. This is because limescale (a solid containing calcium carbonate) is produced when the water is heated. It coats the heating element in kettles, and the inside of boilers and hot water pipes. This means more energy is needed to heat the water, again increasing costs. Pipes may become blocked by limescale - causing the heating system to break down.

Softening hard water

The damaging effect that hard water can have means that it may be beneficial to soften the water. Methods for softening hard water involve the removal of calcium ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] and magnesium ions from the water.

There are two methods for softening hard water:

adding sodium carbonate to the water
using ion exchange columns

Adding sodium carbonate

Sodium carbonate, Na₂CO₃, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.

The carbonate ions from sodium carbonate react with the calcium and magnesium ions in the water to produce insoluble [insoluble: Unable to dissolve (usually in water).] precipitates [precipitate: A suspension of particles in a liquid formed when a previously dissolved substance becomes insoluble, ie in a precipitation reaction.] . For example:

calcium ions + sodium carbonate → calcium carbonate + sodium ions

Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na+(aq)

The water is softened because it no longer contains dissolved calcium ions and magnesium ions. It will form lather more easily with soap.

However, the calcium carbonate and magnesium carbonate precipitates to formlimescale [limescale: Deposit of calcium carbonate formed when water with temporary hardness is boiled.] . As well as being unsightly on your taps, it can also clog up pipes in heating systems (causing them to break down). This makes treatment with sodium carbonate suitable for softening water only in certain circumstances - such as softening water for hand washing clothes.

Ion exchange columns

Commercial water softeners often use ion exchange resins [resin: 'Raw' plastic, especially when in semi-liquid form (Resistant materials). Substances (eg acrylics and urethanes) which, applied to a fabric's surface, evaporate to leave a film upon it (Textiles technology).] . These substances are usually made into beads, which are packed into cylinders called ion exchange columns. These can be built into machines, such as dishwashers, or plumbed into water systems to continuously soften the water.

The resin beads have sodium ions attached to them. As the hard water passes through the column, the calcium and magnesium ions swap places with the sodium ions.

The calcium and magnesium ions are left attached to the beads, while the water leaving the column contains more sodium ions. The hard water is softened because it no longer contains calcium or magnesium ions. Some ion exchange resins use hydrogen ions instead of sodium ions.

An ion exchange column: calcium ions in hard water get replaced with sodium ions to produce soft water

Once the resin beads in dishwashers become saturated with calcium and magnesium ions, they must be regenerated [regenerate: To restore something to its original form. For example, a catalyst is regenerated at the end of a reaction.] by adding sodium chloride (common salt). The sodium ions it contains replace the calcium and magnesium ions on the beads. Sodium chloride is cheap and widely available, making this a convenient and cost-effective system.

When we turn our taps on, we naturally assume the water is safe to drink. This is because it is treated before it is supplied to our homes. In some parts of the country, fluoride is added to the water supply but this is controversial. Water can also be filtered at home – to help improve its taste and quality. In parts of the world where water is more scarce, sea water is distilled to provide drinking water.

Supplying safe water

Water is essential for life - it acts as the solvent [solvent: A solvent is the liquid in which the solute dissolves to form a solution.] in our cells for chemical reactions to take place.

Water covers about two-thirds of our planet, but the vast majority of it cannot be drunk directly. This is because humans need drinking water with low levels of dissolved salts [salt: A compound formed by neutralisation of an acid by a base, eg a metal oxide, as the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride, common salt, is one such compound.] and microbes [microbe: Another name for a microorganism. Microscopic (too small to see) organisms such as bacteria and viruses.] . To produce water of a sufficient quality, we must:

choose an appropriate source of water
filter the water
chlorinate the water

Sources of water

Sources of water for drinking should be reliable, and they should also be fresh and free of toxic [toxic: Poisonous.] chemicals (such as heavy metals). In the UK, water resources include lakes, rivers, aquifers and reservoirs.

An aquifer [aquifer: Naturally occurring underground water stores.] is an underground layer of permeable rock, gravel or sand that is soaked with water, while a reservoir is usually an artificial lake, made by building a dam to accumulate and save river water in the valley behind.

In countries where water is scarce, boreholes are drilled to reach water underground.

Filtering the water

Solids in the water, such as leaves and soil, must be removed. The water is sprayed onto specially-prepared layers of sand and gravel called filter beds.

Different-sized insoluble solids are removed as the water trickles through the filter beds. These are cleaned every so often by pumping clean water backwards through the filter.

The water is then passed into a sedimentation tank. Aluminium sulfate is added to clump tiny particles together to make larger particles, which settle out more easily. The water is then passed through a fine filter, such as carbon granules, to remove very small particles.

Water is purified by filtration, sedimentation and the addition of chlorine

Chlorinating the water

Chlorine is added to drinking water to sterilise [sterilise: The process of ensuring that a sample contains no living things.] it. The chlorine killsmicrobes [microbe: Another name for a microorganism. Microscopic (too small to see) organisms such as bacteria and viruses.] - including microbes that cause potentially-fatal diseases such as typhoid, cholera and dysentery.

Adding fluoride to the water supply

Results from scientific research indicate that fluoridated water can improve dental health by reducing tooth decay. Many areas of the country naturally have low fluoride levels present in the water supply. However, some local health authorities have made water companies add additional fluoride to the water.

The main areas of water fluoridation in England are around Birmingham, Northampton, Lincoln and Newcastle-upon-Tyne

Some people argue that extra fluoride should not be added to water, even if it does improve dental health. They claim that fluoridation:

has been linked to tooth mottling (staining), bone disease and pain
forces people to consume fluoride when they drink tap water - taking away their personal choice (making it unethical)

Filtering water at home

Water treatment in the UK means that the water from your tap is safe to drink. However, the water is not pure because it contains dissolved mineral ions [ion:The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] and chlorine.

Some people prefer to filter their water rather than use it straight from the tap. Filtering removes impurities and this can improve the taste and quality of the water. Filtering also helps to soften the water.

Commercially-available systems use cartridges containing water filters. These may be fitted in jugs or kettles, or plumbed in to the home water supply pipework.

Water filter

The filter cartridges can contain:

silver to kill bacteria
carbon (‘activated charcoal’) to absorb impurities, eg chlorine
ion exchange resins to soften the water, and remove heavy metal ions (such as lead ions)

Silver nanoparticles [nanoparticle: A particle with dimensions less than 100 nanometres.] have an antibacterial effect. Their presence in the filter prevents the growth of bacteria within the filter if water is left inside it for long periods. Silver nanoparticles also help break down harmful pesticides [pesticide:Chemicals used to kill insects, weeds and micro-organisms that might damage crops.] which might be in the water.

Note that you do not need to recall details of specific water filters, or details of the structure and chemical nature of ion exchange resins. However, you should understand why they are used. You may be given information in the exam so that you can use your scientific knowledge and understanding to make comparisons between different water filters.

Obtaining water from other sources

Seawater is a very abundant source of water, but its high salt content make seawater unsuitable as drinking water. However, pure water can be produced from seawater by distillation [distillation: The process of separating two liquids with different boiling points.] .

During distillation, the seawater is boiled. The water vapour is then cooled andcondensed [condense: A change in state where gas becomes liquid by cooling.] to form pure water - leaving the salt behind.

The disadvantages of producing drinking water this way include:

it is expensive because large amounts of energy are needed to heat the seawater
it increases the use of fossil fuels [fossil fuel: Fuel, such as coal, oil and natural gas, made from the remains of ancient plants and animals.] - which are non-renewable resources
carbon dioxide emissions from burning fossil fuels contribute to global warming [global warming: The gradual increase in the average temperature of the Earth.]

Distillation is common in some Middle Eastern countries that have little rainfall, but are wealthy due to their oil reserves.

Testing water purity

The purity of water can be tested by:

measuring its boiling point
evaporating [evaporate: The process in which a liquid turns into a gas.] it (to dryness) on an evaporating dish

Pure water boils at 100°C, but its boiling point increases as the concentration of dissolved salts increases.

Pure water will leave no solids behind when it is evaporated, whereas impure water will leave solids behind on the evaporating dish.

Energy changes take place during chemical reactions. Exothermic reactions give out thermal energy and endothermic reactions take in thermal energy. These changes can be measured experimentally or calculated before being analysed. Knowing the amount of energy involved in a reaction can be used to ensure that resources are used efficiently.

Measuring energy transfers

Heat energy can be given out or taken in from the surroundings during chemical reactions. The amount of energy transferred can be measured. This is calledcalorimetry.

Energy changes from combustion

The diagram shows a simple calorimetry experiment to measure the heat energy released from burning a fuel. You should be able to recognise and label apparatus like this.

Calorimetry

To do the experiment:

measure cold water into a calorimeter (a metal or glass container)
record the starting temperature of the water
heat the water using the flame from the burning fuel
record the final temperature of the water

The spirit burner containing the fuel is usually weighed before and after the experiment - in this way, the mass [mass: The amount of matter an object contains. Mass is measured in 'kg'.] of the fuel burned can be found. Knowing the mass of fuel burnt and the temperature change in the water, it is then possible to calculate the energy released by the fuel. This method also works for finding the amount of energy released by foods.

The biggest source of error is usually heat loss to the surroundings. This can be reduced by insulating [insulate: To help maintain the temperature by reducing heat loss.] the sides of the calorimeter and adding a lid.

Energy changes from reactions in solution

Energy changes also happen when chemicals in solution react [reacting: When particles of two substances collide with enough energy to produce a new chemical.] . For example, heat energy is given out to the surroundings whenacids [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] and alkalis [alkali: A base which is soluble in water.] react together.

The apparatus below is used to find the energy changes forneutralisation [neutralisation: Neutralisation is the reaction between an acid and a base to form a salt plus water.] reactions or for reactions of solids with water.

Calcium powder is mixed with sulphuric acid in an insulated container which incorporates a thermometer and a glass stirring rod

To do this experiment:

add a known volume of the first reactant (in solution) to theinsulated [insulate: To help maintain the temperature by reducing heat loss.] container
record the starting temperature of the liquid
add the second reactant (either in solution or as a solid powder)
replace the lid and stir the reaction mixture
record the maximum temperature that the reaction mixture reaches

Knowing the mass [mass: The amount of matter an object contains. Mass is measured in 'kg'.] of reactant and/or volumes [volume: A measurement of the amount of three-dimensional space something takes up.] of solution and the temperature change, it is possible to calculate the energy change during the reaction.

Calculating energy changes

The amount of energy transferred during a chemical reaction (either from the burning of a fuel or a chemical reaction in solution) can be calculated using the equation:

Q = mc ΔT

Where:

Q = the heat energy transferred (joule, J)

m = the mass of the liquid being heated (grams, g)

c = the specific heat capacity of the liquid (joule per gram degree Celsius, J/g°C)

ΔT = the change in temperature of the liquid (degree Celsius, °C)

The specific heat capacity of water is 4.2 J/g°C. This value is also used when the liquid being heated is not water. For example if an acid [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] , alkali [alkali: A base which is soluble in water.] or other solution is being used.

Energy is normally measured in joules, J. However sometimes the amount of energy can be given in other units, including kilojoules, kJ (1 kJ = 1000 J), kJ per mole [mole: The unit of amount of substance in chemistry. One mole of any substance always has the same number of formula particles in it.] and kJ per gram.

The energy content of food is often measured in calories and calories per gram. In the exam, you will be given a conversion factor if you are asked to convert from calories to joules.

Worked example 1 – neutralisation

50 cm³ of an acid was added to 50 cm3 of an alkali. The mixture was stirred and the temperature increased from 18°C to 28°C. What was the amount of energy released in J?

Step 1: Calculate the temperature change, ΔT

28 – 18 = 10°C

Step 2: Use Q = mc ∆T

Remember that c = 4.2 J/g°C for liquids (unless you are told otherwise):

Q = mc ∆T

Q = (50 + 50) × 4.2 × 10 = 4200 J

Worked example 2 – energy from a fuel

In an experiment, ethanol [ethanol: The alcohol found in alcoholic drinks.] was burnt from a spirit burner and the energy released was used to heat 50 g of water.

The starting temperature of the water was 19°C but by the end of the reaction, the temperature had risen to 41°C. The mass of fuel in the spirit burner was initially 40.0 g, but this had decreased to 38.5 g by the end of the reaction.

Calculate the energy change in kJ/g of fuel.

Step 1: Calculate the temperature change, ΔT

41 – 19 = 22 °C

Step 2: Use Q = mc ∆T

Remember that c = 4.2 J/g°C for liquids (unless you are told otherwise):

Q = mc ΔT

Q = 50 × 4.2 × 22 = 4620 J

Q = 4.62 kJ

Step 3: Calculate the mass of fuel burnt

40.0 – 38.5 = 1.5 g

Step 4: Divide energy released by mass of fuel burnt

Energy change = 4.62 ÷ 1.5 = 3.08 kJ/g

Bonds and chemical reactions

During a chemical reaction:

bonds in the reactants [reactant: One of the starting substances in a chemical reaction.] are broken
new bonds are made in the products [product: A substance formed in a chemical reaction.]

Energy is needed to break bonds, and energy is released when bonds are made.

Exothermic reactions

Exothermic reactions give out heat energy to the surroundings. Exothermic reactions have a negative energy change. This is shown in the energy level diagram below.

Energy released in an exothermic reaction

Some examples of exothermic reactions are:

combustion [combustion: The process of burning by fire.]
neutralisation [neutralisation: Neutralisation is the reaction between an acid and a base to form a salt plus water.] reactions between acids [acid:A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] and alkalis [alkali: A base which is soluble in water.]
the reaction between water and calcium oxide

Endothermic reactions

Endothermic reactions absorb heat energy from the surroundings, making the temperature of the surroundings cooler. Endothermic reactions have a positiveenergy change. This is shown in the energy level diagram below.

Energy absorbed in an endothermic reaction

Some examples of endothermic reactions are:

electrolysis [electrolysis: The decomposition (separation or break-down) of a compound using an electric current.]
the reaction between ethanoic acid and sodium carbonate
the thermal decomposition [thermal decomposition: Type of reaction in which a compound breaks down to form two or more substances when it is heated.] of calcium carbonate in a blast furnace

Bond breaking and making – Higher tier

In an exothermic reaction, more energy is released when new bonds are made than is needed to break existing bonds.

In an endothermic reaction, more energy is needed to break existing bonds than is released when new bonds are made.

Activation energy and catalysts

Simple energy level diagrams only show the energy levels at the beginning and end of a reaction. Energy levels change gradually during a reaction, and this can be shown using a curve between the reactant [reactant: One of the starting substances in a chemical reaction.] and product [product: A substance formed in a chemical reaction.] energy levels.

Energy changes in chemical reactions

This is an exothermic [exothermic: Reaction in which energy is given out to the surroundings.] reaction because the energy level of the reactants is higher than the energy level of the products.

However, the energy curve goes up from the reactants’ energy level to begin with, then drops to the products’ energy level. This is because many reactions need an input of energy to start the reaction off. This is energy is called theactivation energy. It is represented on an energy level diagram as the difference between the reactants’ energy level and the top of the curve.

For example, burning methane in a Bunsen burner:

methane + oxygen → carbon dioxide + water

CH₄ + 2O₂ → CO₂ + 2H₂2O

The activation energy must be supplied in the form of a flame or a spark to get the methane to ignite [ignite: To catch, or cause to catch, fire.] . Once the reaction begins, it gives out energy to the surroundings so it is exothermic.

Catalysts

A catalyst is a substance that speeds up the rate of a chemical reaction without being used up in the reaction.

Catalysts can do this because they provide a different pathway for the reaction to follow. This pathway has lower activation energy than the one followed by the uncatalysed reaction. As a result, a greater proportion of reacting particles have enough energy to react.

The energy level diagram below shows the effect of a catalyst in lowering activation energy.

Energy changes in chemical reactions

Lowering the activation energy has many advantages. It means that reactions happen more quickly and are more economical in terms of the energy required for industrial-scale reactions.

Hydrogen as a fuel

Hydrogen is often seen as an environmentally-friendly alternative to fossil fuels [fossil fuel: Fuel, such as coal, oil and natural gas, made from the remains of ancient plants and animals.] . Some car manufacturers have developed cars than run on hydrogen rather than petrol or diesel.

There are two ways in which hydrogen is used to power cars:

Burning hydrogen directly in the engine

Water is the only product formed when hydrogen burns:

hydrogen + oxygen → water

2H₂ + O₂ → 2H₂O

There are no carbon dioxide emissions that could contribute to global warming [global warming: The gradual increase in the average temperature of the Earth.] .
Hydrogen fuel cells

In a hydrogen fuel cell, hydrogen reacts with oxygen without burning. The energy released is used to generate electricity, which is used to drive an electric motor.

Problems with hydrogen

At the moment, most hydrogen is made by reacting steam with coal or natural gas - both non-renewable resources.

Hydrogen can also be made by passing electricity through water. Unfortunately, most electricity is generated using coal and other fossil fuels, so pollution from burning these fuels happens at the power station. Pollution therefore still occurs.

However, some countries are producing hydrogen using electricity from renewable sources, such as geothermal [geothermal: Of or relating to the internal heat of the Earth.] energy in Iceland.

Advantages and disadvantages of hydrogen

There are some benefits to using hydrogen as a fuel:

unlike petrol and diesel, hydrogen does not generate carbon dioxide when burnt
hydrogen fuel cells are very efficient

However, there are also some downsides too:

few filling stations sell hydrogen
hydrogen must be compressed and liquefied, and then stored in tough, insulated fuel tanks
atmospheric pollution may be generated during the production of hydrogen
hydrogen fuel cells do not work at very low temperatures, and they may also require a platinum catalyst (platinum is expensive and prone to contamination by impurities)

Read on if you're taking the higher paper.

Calculating bond energies – Higher tier

You can calculate the energy change in a reaction using bond energies. A bond energy is the amount of energy needed to break a mole [mole: The unit of amount of substance in chemistry. One mole of any substance always has the same number of formula particles in it.] of a particular bond. You will be given any bond energies you need in the exam.

Method

Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
Add together the bond energies for all the bonds in the products [product: A substance formed in a chemical reaction.] – this is the ‘energy out’.
Calculate the energy change: energy in – energy out.

Worked example 1– an exothermic reaction

Hydrogen and chlorine react to form hydrogen chloride gas:

H−H + Cl−Cl → 2 × (H−Cl)

The table below shows the bond energies relevant to this reaction.

Bond	Bond Energy (kJ/mole)
H−H	436
Cl−Cl	243
H−Cl	432

Energy in = 436 + 243 = 679 kJ/mole
Energy out = 2 × 432 = 864 kJ/mole
Energy change = in – out = 679 – 864 = –185 kJ/mole

The energy change is negative, showing that energy is released to the surroundings in an exothermic [exothermic: Reaction in which energy is given out to the surroundings.] reaction.

Worked example 2 – an endothermic reaction

Hydrogen bromide decomposes [decompose: If a substance decomposes, it breaks down into simpler compounds or elements.] to form hydrogen and bromine:

2 × (H−Br) → H−H + Br−Br

The table below shows the bond energies relevant to this reaction.

Bond	Bond Energy (kJ/mole)
H−Br	366
H−H	436
Br−Br	193

Energy in = 2 × 366 = 732 kJ/mole
Energy out = 436 + 193 = 629 kJ/mole
Energy change = in – out = 732 – 629 = +103 kJ/mole

The energy change is positive, showing that energy is taken in from the surroundings in an endothermic [endothermic: Reaction in which energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls.] reaction.

In many scientific fields, such as forensics, it is useful for scientists to be able to detect particular elements or compounds, or to identify unknown substances. To do this, they use a range of chemical tests. They also carry out titrations to determine how much acid or alkali is in a solution.

Using flame tests to identify metal ions

Flame tests are used to detect the presence of a particular metal ion [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] in acompound [compound: A substance formed by the chemical union (involving bond formation) of two or more elements.] . Metal ions change the colour of a flame when they are heated in it. Different metal ions give different colours to the flame - so flame tests can be used to identify the presence of a particular metal in a sample.

This is how you would carry out a typical flame test:

dip a clean flame test loop in the sample solution
hold the flame test loop at the edge of a Bunsen burner flame
observe the changed colour of the flame, and decide which metal it indicates
clean the loop in acid [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] and rinse with water, then repeat steps 1 to 3 with a new sample

Flame test

Flame colours and the metal ion they represent

Metal ion	Flame colour	Picture showing flame colour
Lithium	Crimson
Sodium	Yellow
Potassium	Lilac
Calcium	Red
Barium	Green

Using precipitation to identify metal ions

Some reactions form a precipitate - this is an insoluble [insoluble: Unable to dissolve (usually in water).] solid formed in the reaction. Precipitates often appear as small particles suspended in a solution.

A precipitate may be formed when a few drops of sodium hydroxide are added to a solution of a metal compound [compound: A substance formed by the chemical union (involving bond formation) of two or more elements.] . For example, a blue precipitate of copper(II) hydroxide forms when sodium hydroxide solution is added to copper(II) sulfate solution:

copper(II) sulfate + sodium hydroxide → copper hydroxide + sodium sulfate

CuSO_4(aq) + 2NaOH_(aq) → Cu(OH)_2(s) + Na₂SO_4(aq)

Precipitation

The colour and the properties of the precipitate can be used to identify the metal ion present.

Calcium, magnesium and aluminium

Calcium, magnesium and aluminium all form white precipitates whenreacted [reacting: When particles of two substances collide with enough energy to produce a new chemical.] with sodium hydroxide.

However, it is possible to identify whether the white precipitate is due to the presence of aluminium ions, as adding excess sodium hydroxide causes a precipitate of aluminium hydroxide to dissolve. This does not happen for the precipitates formed by calcium and magnesium ions.

Metal ion	Colour of precipitate	What happens when excess sodium hydroxide is added?
Aluminium	White	Aluminium hydroxide precipitate dissolves
Calcium	White	No change
Magnesium	White	No change

Transition metal ions

The distinctive colour of the precipitate formed when particular transition metal ions react with sodium hydroxide, allows them to be identified.

You need to know the colours for the metal ions below.

Transition metal ion	Colour of precipitate
Copper (II)	Blue
Iron (II)	Green
Iron (III)	Brown

Using precipitation to identify non-metal ions

Testing for carbonate ions

Metal carbonates contain carbonate ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.] , CO₃^2-. The presence of carbonate ions can be confirmed using a two-step experiment:

Step 1: Carbonates react with dilute acids to produce carbon dioxide and water

For example:

magnesium carbonate + sulfuric acid → magnesium sulfate + carbon dioxide + water

MgCO_3(s) + H₂SO_4(aq) → MgSO_4(s) + CO_2(g) + H₂O(l)

Step 2: Collect the gas given off and bubble it through limewater

Limewater is calcium hydroxide solution. It turns cloudy white if carbon dioxide is bubbled through it:

calcium hydroxide + carbon dioxide → calcium carbonate + water

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H₂O(l)

The presence of the white precipitate [precipitate: A suspension of particles in a liquid formed when a previously dissolved substance becomes insoluble, ie in a precipitation reaction.] confirms that carbonate ions were originally present in step 1.

Carbon dioxide test

Testing for halide ions

The halogens are the elements in Group 7 of the periodic table. They include chlorine, bromine and iodine. Their ions are called halide ions. You can find out more about these by studying Trends within the periodic table

You can test to see if a solution contains chloride ions, bromide ions or iodide ions using silver nitrate solution. To do this:

a few drops of dilute nitric acid are added to the solution
a few drops of silver nitrate solution are then added
the colour of any precipitate formed is recorded

the table below summarises the colours of each precipitate

Halide ion	Precipitate formed	Colour of precipitate
Chloride, Cl–	Silver chloride, AgCl	White
Bromide, Br–	Silver bromide, AgBr	Cream
Iodide, I–	Silver iodide, AgI	Yellow

Testing for sulfate ions

You can test to see if a solution contains sulfate ions SO₄^2- using barium chloride solution. To do this:

a few drops of dilute hydrochloric acid are added to the solution
a few drops of barium chloride solution are then added

The presence of a white precipitate of barium sulfate shows the presence of sulfate ions in the solution.

Test showing the presence of sulfate ions

For example:

barium chloride + sodium solution → barium sulfate + sodium chloride

BaCl_2(aq) + Na₂SO_4(aq) → BaSO_4(s) + 2NaCl_(aq)

Titrations

Titrations are used to determine the volumes [volume: A measurement of the amount of three-dimensional space something takes up.] of acid [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] and alkali [alkali: A base which is soluble in water.] needed to react together to produce a neutral [neutral: A neutral solution has a pH of 7 because it has an equal concentration of hydrogen ions and hydroxide ions.] solution. Titrations are carried out using a piece of apparatus called a burette, along with a suitable indicator [indicator: Substance that changes colour depending on the properties of the substance it is added to.] .

Acid from a burette is added to a flask containing a known volume of alkali and a few drops of indicator

Carrying out a titration

A titration is carried out using a number of steps:

A pipette is used to accurately measure a volume of an alkali, often 25 cm³. A pipette filler is used to draw solution into the pipette safely. The alkali is emptied into a conical flask.
A few drops of a suitable indicator are then added to the conical flask. This will show a change of colour when the acid and alkali haveneutralised [neutralise: To be made neutral by removing any acidic or alkaline nature.] one another and the titration is complete.
The acid is placed in a burette and the starting volume of acid is read against the scale marked on the burette.
The acid from the burette is added to the conical flask, and the flask is swirled to mix its contents. When the acid in the burette has almost run in, it is added one drop at a time. Eventually, a colour change shows that the correct amount has been added to react completely with the alkali in the conical flask.
The volume of acid added from the burette is noted. The titration results can then be used to calculate the concentration of the acid or alkali (if the concentration of the other is known).

Universal indicator [universal indicator: A chemical solution that produces many different colour changes corresponding to different pH levels.] is unsuitable for titrations because it has a range of colours. Phenolphthalein is often used instead. It changes from pink in alkali to colourless in acid.

Read on if you're taking the higher paper.

Calculating chemical quantities in titrations – Higher tier

Calculating concentration using the number of moles

If you know the concentration of one of the reactants [reactant: One of the starting substances in a chemical reaction.] present in a titration [titration: A quantitative procedure in which two solutions react in a known ratio, so if the concentration of one solution is known and the volumes of both are measured, the concentration of the other solution can be determined.] , you can work out the concentration of the other reactant.

Worked example 1

25 cm³ of dilute hydrochloric acid is neutralised by 20 cm³ of 0.5 mol/dm³sodium hydroxide. What is the concentration of the hydrochloric acid?

Step 1: Convert volumes to dm³

25 cm³ of HCl = 25 ÷ 1000 = 0.025 dm³

20 cm³ of NaOH = 20 ÷ 1000 = 0.020 dm³

Step 2: Determine the number of moles of sodium hydroxide

moles of NaOH = concentration × volume

moles of NaOH = 0.5 × 0.020 = 0.010 mol

Step 3: Work out the number of moles of acid using the balanced equation

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

In this reaction, one mole of HCl reacts with one mole of NaOH. This is a 1:1 ratio.

Therefore, in our titration, 0.010 mol of NaOH must neutralise 0.010 mol of HCl.

Step 4: Calculate the concentration of the acid

concentration of HCl = number of moles ÷ volume

concentration of HCl = 0.010 ÷ 0.025 = 0.4 mol/dm³

Answer

The concentration of the HCl is 0.4 mol/dm³.

Worked example 2

You need to be able to calculate the chemical quantities in titrations involving masses in grams per dm³.

A sample of vinegar contains 0.1 mol/dm³ ethanoic acid. What is its concentration in g/dm³? (The relative formula mass, Mr, of ethanoic acid is 60)

concentration in g/dm³ = concentration in g/dm³ × Mr

concentration = 0.1 × 60 = 6 g/dm³

Answer

6 g/dm³

Ammonia is a raw material used in the manufacture of fertilisers, explosives and cleaning fluids. It is produced using a reaction between nitrogen and hydrogen called the Haber process. Production costs of making ammonia are based on factors including the rate of reaction, and the cost of energy, labour, raw materials and equipment.

Ammonia and the Haber process

Ammonia, NH₃, is a compound [compound: A substance formed by the chemical union (involving bond formation) of two or more elements.] of nitrogen and hydrogen. It is a colourless gas with a choking smell, and a weakalkali [alkali: A base which is soluble in water.] that is very soluble in water.

Ammonia is used to make fertilisers (as a source of nitrogen for plants), explosives, dyes, household cleaners and nylon. It is also the most important raw material in the manufacture of nitric acid.

Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process.

An industrial ammonia plant

Raw materials

The raw materials for this process are hydrogen and nitrogen:

Hydrogen is obtained by reacting natural gas (mostly methane) with steam, or by cracking [cracking: Cracking is the breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vapourizing them and passing them over a hot catalyst.] oilfractions [fractions: The fractions of crude oil are outputs of the process of fractional distillation and include petrol, diesel oil and kerosine.] .
Nitrogen is obtained from the air. Air is 78 per cent nitrogen; nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen - leaving nitrogen behind.

The reaction conditions

The reaction between nitrogen and hydrogen is reversible:

nitrogen + hydrogen

ammonia

N_2(g) + 3H_2(g)

2NH_3(g)

The symbol

indicates that the reaction between nitrogen and hydrogen can proceed in both directions.

In the Haber process, nitrogen and hydrogen react together under these conditions:

a high temperature - about 450°C
a high pressure - about 200 atmospheres (200 times normal pressure)

An iron catalyst [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.] is used to increase the rate of reaction.

Stages of the Haber process

Part of the equipment used in the Haber process

Stage 1	Having obtained the hydrogen and nitrogen gases (from natural gas and the air respectively), they are pumped into thecompressor [compressor: A machine that increases the pressure of a gas.] through pipes.
Stage 2	The gases are pressurised to about 200 atmospheres of pressure inside the compressor.
Stage 3	The pressurised gases are pumped into a tank containing beds of iron catalyst at about 450°C. In these conditions, some of the hydrogen and nitrogen will react to form ammonia.
Stage 4	The unreacted nitrogen and hydrogen, together with the ammonia, pass into a cooling tank. The cooling tank liquefies the ammonia, which can be removed into pressurised storage vessels.
Stage 5	The unreacted hydrogen and nitrogen gases are recycled by being fed back through pipes to pass through the hot iron catalyst beds again.

Read on if you're taking the higher paper.

Reversible reactions – Higher tier

A closed system [closed system: A system in which inputs loop around continuously, for example, the water cycle. No reactants or products enter or leave the system.] is a system in which no reactants [reactant: One of the starting substances in a chemical reaction.] are added and no products are removed. When a reversible reaction happens in a closed system,equilibrium [equilibrium: In chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.] is reached in which the rate of the forward reaction is the same as the backward reaction.

For example, the production of ammonia is a reversible reaction:

the forward reaction is: N_2(g) + 3H_2(g) → 2NH_3(g)
the backward reaction is: 2NH_3(g) → N_2(g) + 3H_2(g)

So while nitrogen and hydrogen continually combine to form ammonia, ammonia is continually breaking up to form nitrogen and hydrogen:

N2_(g) + 3H_2(g)

2NH_3(g)

The percentage yield [yield: The yield in a reversible reaction is usually expressed as the percentage of product in the reaction mixture.] of ammonia at equilibrium depends on the balance between the forward and backward reactions.

Changing pressure

If the pressure is increased during reactions involving gases, then the reaction that produces the least number of molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance.] of gas is favoured.

For example:

2SO_2(g) + O_2(g)

2SO_3(g)

If the pressure is increased, the forward reaction is favoured and the equilibrium yield of SO3 is increased. This is because there are 2 + 1 = 3 molecules of gas on the left of the chemical equation, and only 2 molecules of gas on the right.

Changing temperature

In a reversible reaction, the reaction in one direction will beexothermic [exothermic: Reaction in which energy is given out to the surroundings.] and the reaction in the opposite direction will beendothermic [endothermic: Reaction in which energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls.] .

If the temperature is increased, the yield from the endothermic reaction increases (and the yield from the exothermic reaction decreases).
If the temperature is decreased, the yield from the endothermic reaction decreases (and the yield from the exothermic reaction increases).

In the example above, the backward reaction is endothermic and the forward reaction is exothermic. If the temperature is decreased, the forward reaction is favoured and the equilibrium yield of SO₃ is increased.

Changing conditions in the Haber process – Higher tier

Effect of pressure on percentage yield of ammonia

There are fewer molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance.] of gas on the right-hand side of the chemical equation than there are on the left hand side:

N_2(g) + 3H_2(g)

2NH_3(g)

If the pressure is increased, the reaction that produces the least number of molecules of gas is favoured. This means that the equilibrium [equilibrium: In chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.] yield [yield: The yield in a reversible reaction is usually expressed as the percentage of product in the reaction mixture.] of ammonia is increased if the pressure is increased.

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However, achieving high pressures requires a lot of energy. It also requires stronger pipes and tanks to withstand that pressure. This is costly for companies.

Therefore, a compromise must be made between optimising [optimum: The most favourable.] the amount of product formed, and the cost remaining economically viable. The pressure used is around 200atmospheres [atmosphere: The envelope of gasses that surround the Earth. The important gasses in the atmosphere are nitrogen, oxygen and carbon dioxide.] .

Effect of temperature on percentage yield of ammonia

In the Haber process, the forward reaction is exothermic [exothermic:Reaction in which energy is given out to the surroundings.] and the backward reaction is endothermic [endothermic: Reaction in which energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls.] . If the temperature is decreased, the yield from the exothermic direction is increased.

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However, by decreasing the temperature the molecules move more slowly and collide less frequently. This lowers the rate of reaction.

Therefore, a compromise has to be made between achieving a reasonable rate of reaction and still achieving a reasonable percentage yield of ammonia. The optimum temperature for this compromise is around 450°C.

Production costs – Higher Tier

The graph below shows the effects of changing the pressure and temperatureon the equilibrium [equilibrium: In chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.] yield [yield: The yield in a reversible reaction is usually expressed as the percentage of product in the reaction mixture.] of ammonia.

Graph shows the yield of ammonia at different temperatures

At 200 atmospheres and 450°C, the yield of ammonia is only about 25 per cent. However, at 400 atmospheres and 350°C the yield is about 65 per cent. This means that chemical companies using 450°C temperatures and 200 atmospheres of pressure are not maximising their percentage ammonia yield.

There must be other factors they consider when deciding on the conditions to use.

Economic and environmental considerations

Chemical companies are in business, so they must consider all factors associated with cost to ensure they generate profits. They must also consider the effect of production on the environment.

The cost of making any new substance is made up of different components, including the cost of:

energy (gas and electricity) needed
the starting materials
equipment(plant)
labour (the wages of the people needed)

The higher the temperature used, the higher the energy cost and the higher the impact on the environment. This impact has economic costs associated with it as companies are charged for the pollution they generate.

Unused reactants [reactant: One of the starting substances in a chemical reaction.] are recycled to minimise the cost of raw materials.

An industrial plant is expensive to build and maintain. When high pressures are used - as in the manufacture of ammonia - the cost is particularly high. Reaction vessels have to be very strong and there must be rigorous safety precautions.

Labour costs can be reduced by automating processes.

The longer a process takes, the more expensive it is likely to be. A large yield produced over a long period of time is more costly and less desirable than a reduced yield produced quickly. Catalysts [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.] are therefore often used to speed up reactions.

Many people think of alcoholic drinks when the term alcohol is used. However, alcoholic drinks mainly contain only one type of alcohol – ethanol. Alcohols are actually a family of organic compounds, all of which contain a –OH functional group. Some alcohols are very important as fuels and solvents. Alcohols have similar chemical properties to each other and undergo similar chemical reactions.

Alcohols

The alcohols are a homologous series [homologous series: A family of compounds that share the same functional group and therefore take part in similar chemical reactions. They have a trend in physical properties such as boiling point.] of organic compounds [organic compound: A massive range of chemicals that are based on carbon and were once part of living things. Organic compounds are now generally obtained from, or made from, crude oil.] . They all contain the functional group [functional group: A group of atoms bonded in a specific arrangement that makes a compound behave in a particular way. For example, all alcohols have the functional group -OH and take part in similar chemical reactions.] –OH. This group is responsible for theproperties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] of alcohols.

The names of alcohols end with ‘-ol’ – eg ethanol.

The first three alcohols in the homologous series are methanol, ethanol and propanol. These alcohols are highly flammable [flammable: Able to catch fire.] , making them useful as fuels. They are also used as solvents [solvent: A solvent is the liquid in which the solute dissolves to form a solution.] in marker pens, medicines, and cosmetics (such as deodorants and perfumes).

Ethanol is the alcohol found in alcoholic drinks such as wine and beer. Ethanol is usually mixed with petrol for use as a fuel (see the Biology revision bite on Biofuels).

In the exam, you will need to be able to recognise the following alcohols from their names and formulae.

Alcohol	Number of carbon atoms	Structural formula
Methanol	1	CH₃OH
Ethanol	2	CH₃CH₂OH
Propanol	3	CH₃CH₂CH₂OH

Note that you are not expected to remember the names and formulae of other alcohols.

Properties of methanol, ethanol and propanol

The alcohols methanol, ethanol and propanol all have the followingproperties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] :

They are colourless liquids that dissolve in water to form a neutral solution(pH [pH: Scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline.] 7).
They react with sodium to produce hydrogen and a salt [salt: A compound formed by neutralisation of an acid by a base, eg a metal oxide, as the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride, common salt, is one such compound.] . For example:

ethanol + sodium → hydrogen + sodium ethoxide

This reaction is similar but less vigorous to the reaction of water with sodium. This is due to the similarity in structure between water and the –OH group in alcohols.
They burn in the air, releasing energy and producing carbon dioxide andwater.

You should be able to write balanced equations for the combustion reactions of alcohols. For example:

methanol + oxygen → carbon dioxide + water

2CH₃OH(l) + 3O_2(g) → 2CO_2(g) + 4H₂O(l)

ethanol + oxygen → carbon dioxide + water

2C₂H₅OH(l) + 6O_2(g) → 4CO_2(g) + 6H₂O(l)

propanol + oxygen → carbon dioxide + water

2C₃H₇OH(l) + 9O_2(g) → 6CO_2(g) + 8H₂O(l)

Carboxylic acids are a group of important organic chemicals. Vinegar contains ethanoic acid, which is a carboxylic acid. All carboxylic acids have a –COOH functional group, and have similar reactions as a result. They are weak acids because this functional group is only partly ionised in solution.

Carboxylic acids

The carboxylic acids are a homologous series [homologous series: A family of compounds that share the same functional group and therefore take part in similar chemical reactions. They have a trend in physical properties such as boiling point.] of organic compounds [organic compound: A massive range of chemicals that are based on carbon and were once part of living things. Organic compounds are now generally obtained from, or made from, crude oil.] . They all contain the same functional group [functional group: A group of atoms bonded in a specific arrangement that makes a compound behave in a particular way. For example, all alcohols have the functional group -OH and take part in similar chemical reactions.] –COOH.

The names of carboxylic acids end in ‘-oic acid’ – eg ethanoic acid.

In the exam, you will need to be able to recognise the following carboxylic acids from their names and formulae.

Carboxylic acid	Number of C atoms	Structural formula
Methanoic acid	1	HCOOH
Ethanoic acid	2	CH₃COOH
Propanoic acid	3	CH₃CH₂COOH

You are not expected to remember the names and formulae of other carboxylic acids.

Ethanoic acid from ethanol

Vinegar is an aqueous [aqueous: Dissolved in water.] solution containingethanoic acid. Ethanoic acid is formed from the mild oxidation [oxidation:Oxidation is either a reaction in which oxygen combines with a substance (oxygen is gained) or electrons are lost.] of the ethanol (which is analcohol [alcohol: Family of substances (including ethanol) that contain carbon, hydrogen and oxygen atoms.] ). This can be achieved through:

The addition of chemical oxidising agents - such as acidified potassium dichromate.
The action of microbes [microbe: Another name for a microorganism. Microscopic (too small to see) organisms such as bacteria and viruses.] in aerobic conditions (in the presence of oxygen). This happens on a small scale when a bottle of wine is left open and exposed to air. On a commercial scale, it is achieved in a fermenter [fermenter: Vessels used to cultivate microorganisms on a large scale.] using acetic acid bacteria [bacteria:Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium.] .

Properties of carboxylic acids

Carboxylic acids have the following properties:

They dissolve in water to produce acidic solutions (pH [pH: Scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline.] less than 7).
They react with carbonates to produce carbon dioxide and a salt and water. For example:

calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide
They all react with alcohols, in the presence of an acid catalyst, to form esters. For example:

ethanol + ethanoic acid → ethyl ethanoate + water

Read on if you're taking the higher paper.

Ionisation of weak acids – Higher tier

Strong acid, such as hydrochloric acid, ionise [ionise: To ionise is to convert an uncharged atom or molecule into a charged particle by adding or removing electrons.] fully in water:

HCl(aq) → H+(aq) + Cl–(aq)

Their aqueous [aqueous: Dissolved in water.] solutions have a high concentration of hydrogen ions, H+. This gives them a low pH.

Carboxylic acids are weak acids. They do not completely ionise when they are dissolved in water. Instead only some of their molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance.] ionise to form H+ ions:

CH₃COOH(aq)

CH₃COO–(aq) + H+(aq)

This means that an aqueous solution of a weak acid will have a higher pH compared to the same concentration of an aqueous solution of a strong acid.

Note that weak acids still have a pH below pH 7.

Esters are organic compounds formed by the reaction of an alcohol with a carboxylic acid. They have the functional group –COO–.

Esters

Esters are a group of organic compounds [organic compound: A massive range of chemicals that are based on carbon and were once part of living things. Organic compounds are now generally obtained from, or made from, crude oil.] which all contain the functional group [functional group: A group of atoms bonded in a specific arrangement that makes a compound behave in a particular way. For example, all alcohols have the functional group -OH and take part in similar chemical reactions.] –COO–. They have these properties [property: A chemical property is any characteristic that gives a substance the ability to undergo a change that results in a new substance.] in common:

they are volatile - they are liquids that become vapours [vapour: Vapour is a cloud of liquid particles. Steam is water vapour.] easily
they have distinctive ‘fruity’ smells

These properties make esters very useful as food flavourings, and asperfumes in cosmetics. Some esters are obtained from natural sources, such as fruits. Others are manufactured.

Making ethyl ethanoate

Ethyl ethanoate is the ester made from ethanol and ethanoic acid. Sulfuric acid is added to act as a catalyst [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.] in the reaction.

ethanol + ethanoic acid

ethyl ethanoate + water

CH₃CH₂OH(aq) + CH₃COOH(aq)

CH₃CH₂OOCCH₃(aq) + H₂O(l)

The distinctive smell of ethyl ethanoate (which is like modelling glue) can be detected as the reaction proceeds. Excess ethanoic acid in the reaction mixture is neutralised [neutral: A neutral solution has a pH of 7 because it has an equal concentration of hydrogen ions and hydroxide ions.] with sodium hydrogencarbonate, then a few drops of the mixture added to water so that the smell can be detected more effectively.

The first part of an ester’s name comes from the alcohol - it ends with the letters 'yl'. The second part of its name comes from the carboxylic acid - it ends with the letters 'oate'.

Here are three examples:

Name of alcohol	Name of carboxylic acid	Name of ester
Ethanol	Propanoic acid	Ethyl propanoate
Butanol	Methanoic acid	Butyl methanoate
Pentanol	Ethanoic acid	Pentyl ethanoate

Apart from ethyl ethanoate, you are not expected to name esters. However, you are expected to be able to recognise esters from their names or structural formulae. Remember to look for the –COO– group.