This is a complete revision guide for AQA GCSE Triple Science Chemistry Paper 2 (specification code 8462), covering Topics 4.6 to 4.10: Rate and Extent of Chemical Change, Organic Chemistry, Chemical Analysis, Chemistry of the Atmosphere and Using Resources. This guide is for Triple Science students only. A separate Combined Science guide will follow. Content labelled Higher Tier only is for Higher students. Foundation students can skip those sections.
Do not read this passively. After each section, cover the page and try to recall the key points from memory. Then practise with past paper questions.
Work through each topic section by section. After reading, close the page and recall the key points from memory. Use the exam tips and common mistakes throughout to focus your technique, not just your content recall.
Topic 4.6: Rate and Extent of Chemical Change
Calculating rate of reaction
The rate of a chemical reaction measures how quickly reactants are used up or products are formed.
Quantity can be measured as mass in grams, volume in cm cubed, or moles (Higher Tier only). Units are g/s, cm cubed/s or mol/s.
Example: a reaction produces 60 cm cubed of gas in 30 seconds. Rate = 60 divided by 30 = 2 cm cubed per second.
On a rate graph, fast reactions produce steeper curves. When the graph levels off, a reactant has been completely used up and the reaction has stopped.
Higher Tier only: to find the rate at a specific point on a curve, draw a tangent touching the curve at that point and calculate its gradient using change in y divided by change in x. Always use two points on the tangent line, not on the curve itself.
When asked why a rate graph becomes horizontal, always write: "The limiting reactant has been completely used up so the reaction stops." Do not say the rate slows down. Once the graph is flat, the rate is zero.
Factors affecting rate of reaction
Five factors affect reaction rate: concentration, pressure (gases only), surface area, temperature and catalysts.
- Concentration: more particles per unit volume, more frequent collisions, faster reaction
- Pressure (gases only): particles pushed closer together, more collisions per second, faster reaction
- Surface area: smaller pieces have a larger surface area to volume ratio, more particles exposed, more collisions, faster reaction
- Temperature: increases collision frequency and collision energy, more particles exceed activation energy, faster reaction
- Catalysts: provide an alternative pathway with lower activation energy, speed up reaction without being used up
Collision theory
Particles react only when they collide and have enough energy to overcome the activation energy. This explains every factor above. For temperature questions in particular, always mention all four points: increased kinetic energy, increased collision frequency, more particles exceeding activation energy, more successful collisions. Simply writing "more collisions" typically earns only one mark.
Catalysts
Catalysts increase reaction rate but are not used up. Examples include iron in the Haber Process and enzymes in biological systems. Catalysts lower activation energy by providing an alternative reaction pathway. The overall energy change of the reaction is unchanged. The height of the energy barrier decreases, but the energy levels of reactants and products remain the same.
Catalysts lower the activation energy. They do not provide energy to reactants. They also do not change the position of equilibrium or the overall energy change of the reaction.
Required Practical 5: effect of concentration on rate of reaction
You need to know two methods.
Method 1: measuring gas produced. Example: marble chips and hydrochloric acid. Measure the volume of gas produced over time. Independent variable: concentration of acid. Dependent variable: volume of gas produced. Control variables: temperature, volume of acid, mass of marble chips.
Method 2: disappearing cross. Example: sodium thiosulfate and hydrochloric acid. Place a flask over a black cross, add the acid, start the timer and stop when the cross can no longer be seen. As concentration increases, the cross disappears faster. The amount of product formed may be identical between experiments. The correct answer is that the product formed in a shorter time, not that more product was made.
Reversible reactions and equilibrium
Some reactions can proceed in both directions, shown by the reversible arrow symbol. If the forward reaction is exothermic, the reverse reaction is endothermic, and the same amount of energy is transferred in both directions.
Equilibrium occurs when the forward and reverse reaction rates are equal. Concentrations remain constant but both reactions continue. The system is dynamic.
Higher Tier only (Le Chatelier's Principle): if a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.
- Increase reactant concentration: equilibrium shifts right, more products formed
- Decrease reactant concentration: equilibrium shifts left, fewer products formed
- Increase product concentration: equilibrium shifts left, more reactants formed
- Decrease product concentration: equilibrium shifts right, fewer reactants formed
- Increase temperature in an exothermic reaction: equilibrium shifts left, fewer products
- Decrease temperature in an exothermic reaction: equilibrium shifts right, more products
- Increase temperature in an endothermic reaction: equilibrium shifts right, more products
- Decrease temperature in an endothermic reaction: equilibrium shifts left, fewer products
- Increase pressure: equilibrium shifts towards the side with fewer gas molecules
- Decrease pressure: equilibrium shifts towards the side with more gas molecules
Always count the number of gas molecules on each side of the equation when answering pressure questions. This is where most marks are lost.
Topic 4.7: Organic Chemistry
Crude oil, hydrocarbons and alkanes
Crude oil is a finite resource formed from the remains of ancient plankton buried under sediment over millions of years. It is a mixture of hydrocarbons. Hydrocarbons contain only hydrogen and carbon atoms. Most hydrocarbons in crude oil belong to the alkane homologous series.
Alkanes have the general formula CnH(2n+2). The first four are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). A homologous series shares the same general formula, similar chemical properties and a gradual change in physical properties.
Alkanes are saturated and contain only single bonds. Alkenes are unsaturated and contain a C=C double bond. Confusing these is one of the most common errors in organic chemistry questions.
Fractional distillation
Crude oil is heated so most of it vaporises. Vapours enter a fractionating column where temperature decreases from bottom to top. Hydrocarbons condense at different heights according to their boiling points. Small molecules have low boiling points and condense near the top. Large molecules have high boiling points and condense near the bottom. Fractions are used to produce petrol, diesel, kerosene, LPG, lubricants, solvents, polymers and detergents.
Properties of hydrocarbons
As molecular size increases: boiling point increases (stronger intermolecular forces), viscosity increases (thicker liquid) and flammability decreases (smaller hydrocarbons ignite more easily).
Complete combustion (plentiful oxygen) produces carbon dioxide and water. Incomplete combustion (limited oxygen) produces carbon monoxide and water (as well as carbon particles, soot). Carbon monoxide is toxic. Always include water as a product in combustion equations.
Cracking and alkenes
Large hydrocarbons are less useful. Cracking breaks them into smaller, more useful molecules including shorter alkanes and alkenes. Two methods: catalytic cracking (catalyst, high temperature) and steam cracking (steam, very high temperature).
Alkenes contain a C=C double bond and have the general formula CnH2n. The first four are ethene (C2H4), propene (C3H6), butene (C4H8) and pentene (C5H10). Because of the double bond, alkenes are more reactive than alkanes.
The bromine water test identifies alkenes. Alkenes decolourise bromine water: orange to colourless. Alkanes do not react, so the orange colour remains.
AQA specifically uses the word colourless, not clear. Writing "orange to clear" may not gain the mark. Learn this wording exactly.
Reactions of alkenes
Alkenes undergo addition reactions. Atoms add across the double bond, which becomes a single bond. Addition of hydrogen produces an alkane. Addition of water produces an alcohol. Addition of a halogen produces a haloalkane. When asked why alkenes react, mention the C=C double bond as the reactive functional group.
Alcohols
Alcohols contain the OH functional group. The first four are methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH) and butanol (C4H9OH). Reactions include combustion (alcohol plus oxygen gives carbon dioxide and water), reaction with sodium (produces hydrogen gas and a salt) and oxidation (ethanol can be oxidised to ethanoic acid).
Fermentation produces ethanol from glucose using yeast, warm temperature and anaerobic conditions. Glucose gives ethanol plus carbon dioxide.
Carboxylic acids
Carboxylic acids contain the COOH functional group. Examples: methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (C2H5COOH). They are acidic, dissolve in water and react with carbonates to produce a salt, water and carbon dioxide.
Ester formation: alcohol plus carboxylic acid gives an ester plus water. Example: ethanol plus ethanoic acid gives ethyl ethanoate plus water. Esters are used in perfumes and flavourings.
Polymers
In addition polymerisation, monomers with a double bond join together to form a polymer and no other product forms. The repeating unit contains the same atoms as the monomer. In drawing questions: break the double bond, draw single bonds extending outside the brackets, place brackets around the repeating unit and add n outside the brackets. The double bond must not remain in the polymer structure.
Both LDPE and HDPE are made by addition polymerisation of ethene. LDPE is produced at high temperature and pressure, giving many branched chains that cannot pack closely, making it softer and more flexible. HDPE is produced at lower temperature and pressure with a catalyst, giving fewer branches, so chains pack closely, making it stronger and more rigid.
Higher Tier only: condensation polymerisation uses monomers with two functional groups and produces a small molecule, usually water. Examples include polyesters and proteins. Amino acids contain both NH2 and COOH groups and join by condensation polymerisation to form polypeptides and proteins.
Thermosoftening polymers soften when heated and can be reshaped. Thermosetting polymers do not melt when heated and cannot be reshaped because strong cross-links form between polymer chains. DNA is a naturally occurring polymer made from nucleotide monomers.
Topic 4.8: Chemical Analysis
Pure substances and formulations
In chemistry, a pure substance is a single element or a single compound. Pure substances melt at one specific temperature and boil at one specific temperature. Impure substances melt and boil over a range of temperatures. This is different from the everyday use of the word pure, which does not mean the same thing in chemistry.
A formulation is a mixture designed as a useful product where each component has a specific purpose and must be present in carefully measured quantities. Examples include medicines, fuels, fertilisers, paints, cleaning products and foods. If a question mentions a designed mixture or specific proportions for a manufactured product, the answer is usually formulation.
Chromatography
Chromatography separates mixtures and helps identify substances. The stationary phase is the paper. The mobile phase is the solvent. As the solvent moves up the paper, substances travel at different speeds, separating the mixture. One spot on a chromatogram indicates a pure substance. Multiple spots indicate a mixture. Substances at the same height as a known reference are likely to be the same substance.
Required Practical 6: paper chromatography
Draw a pencil line near the bottom of chromatography paper (pencil, not pen, because pencil is insoluble and does not dissolve into the solvent). Place a spot of the sample on the line. Put the paper in solvent ensuring the solvent level is below the pencil line. Allow the solvent to move up the paper. Remove before the solvent reaches the top. Mark the solvent front immediately. Allow to dry.
If the sample dissolves directly into the solvent, the separation will not work correctly. Always use the same solvent when comparing Rf values, because Rf values change with different solvents.
Example: substance moves 4 cm, solvent moves 8 cm. Rf = 4 divided by 8 = 0.50. Always measure from the origin line, not from the bottom of the paper.
Gas tests
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Burning splint | Squeaky pop |
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Bubble through limewater | Turns cloudy (milky) |
| Chlorine | Damp litmus paper | Bleaches white |
Hydrogen gives a squeaky pop. Oxygen relights a glowing splint. Do not confuse these two.
Identifying ions
Flame tests:
| Ion | Flame colour |
|---|---|
| Lithium | Crimson |
| Sodium | Yellow |
| Potassium | Lilac |
| Calcium | Orange-red |
| Copper | Green |
Method: clean a nichrome wire with hydrochloric acid, dip into the sample, place in a blue Bunsen flame and observe the colour.
Sodium hydroxide tests for metal ions:
| Ion | Result with NaOH |
|---|---|
| Aluminium | White precipitate that dissolves in excess NaOH |
| Calcium | White precipitate that remains |
| Magnesium | White precipitate that remains |
| Copper(II) | Blue precipitate |
| Iron(II) | Green precipitate |
| Iron(III) | Brown precipitate |
Only aluminium hydroxide dissolves in excess sodium hydroxide. This distinguishes aluminium from calcium and magnesium, which both give white precipitates that remain.
Carbonate test: carbonates react with acid to produce a salt, water and carbon dioxide. Test the gas with limewater: it turns cloudy.
Halide ions: add dilute nitric acid then silver nitrate solution. Chloride gives a white precipitate, bromide gives a cream precipitate, iodide gives a yellow precipitate.
Sulfate ions: add dilute hydrochloric acid then barium chloride solution. A white precipitate forms if sulfate ions are present.
Always add the acid first in halide and sulfate tests to remove interfering ions before adding silver nitrate or barium chloride. One test alone may not identify a compound completely, which is why multiple tests are used in Required Practical 7.
Instrumental methods are faster, more accurate and more sensitive than chemical tests and can detect tiny amounts of substances. Flame emission spectroscopy identifies metal ions and can detect mixtures and measure concentration, making it more useful than a standard flame test. The key words for instrumental methods are: accurate, sensitive and rapid.
Topic 4.9: Chemistry of the Atmosphere
Composition of the atmosphere
The modern atmosphere has remained relatively stable for around 200 million years. It contains approximately 78% nitrogen, 21% oxygen, around 1% argon and other noble gases, and about 0.04% carbon dioxide. Water vapour is present in varying amounts. Carbon dioxide makes up only 0.04%, not the larger figure that students often assume.
The Earth's early atmosphere
The early atmosphere formed mainly from gases released by volcanoes. It contained large amounts of carbon dioxide, water vapour and nitrogen, along with small amounts of methane and ammonia. There was little or no oxygen. Evidence is limited because the atmosphere formed approximately 4.6 billion years ago and very few rocks from that period remain, so scientists rely on indirect evidence.
How oxygen increased and carbon dioxide decreased
Around 2.7 billion years ago, algae evolved and began photosynthesis, producing oxygen. Over time oxygen accumulated in the atmosphere, enabling animal life to develop.
Carbon dioxide decreased for three reasons: photosynthesis removed it from the atmosphere; it dissolved in oceans where marine organisms used carbon compounds to build shells and skeletons, forming limestone and carbonate rocks; and carbon became trapped in fossil fuels. When AQA asks for two reasons, use photosynthesis and the formation of carbonate rocks.
Greenhouse gases and the greenhouse effect
Greenhouse gases include water vapour, carbon dioxide and methane. The greenhouse effect: the Sun emits short-wavelength radiation which the Earth absorbs; the Earth emits long-wavelength infrared radiation; greenhouse gases absorb some of this and warm the atmosphere. The greenhouse effect is not harmful in itself. Without it, Earth would be too cold for life. Problems occur when the effect becomes stronger due to increased greenhouse gas concentrations.
Burning fossil fuels (coal, oil, gas). Deforestation reduces photosynthesis, so less CO2 is removed from the atmosphere.
Cattle farming. Rice farming (waterlogged conditions). Landfill sites (decomposition of waste).
Effects of global climate change include rising sea levels (melting ice and thermal expansion), habitat loss, changes in weather patterns (more droughts, storms and floods) and reduced biodiversity. These four effects cover most AQA questions.
A carbon footprint is the total greenhouse gases released throughout the life cycle of a product, service or event. Reducing it requires renewable energy, improved energy efficiency, carbon capture and storage, and reduced deforestation. Reduction is difficult because of cost, existing infrastructure relying on fossil fuels, and economic and political priorities.
Atmospheric pollutants
| Pollutant | Source | Effects |
|---|---|---|
| Carbon monoxide | Incomplete combustion | Toxic, reduces oxygen transport in blood |
| Sulfur dioxide | Sulfur impurities burning in fuels | Acid rain, respiratory problems |
| Oxides of nitrogen | High engine temperatures | Acid rain, respiratory problems |
| Particulates | Incomplete combustion | Lung damage, global dimming |
Carbon monoxide is produced by incomplete combustion (limited oxygen), not complete combustion. Sulfur dioxide causes acid rain, not global warming. Always link pollutants to how they form, not just what they are.
Topic 4.10: Using Resources
Sustainable development and resources
Sustainable development means meeting the needs of the present without compromising the ability of future generations to meet their own needs. Finite resources include crude oil, coal, natural gas and metal ores. Renewable resources include solar energy, wind energy, biomass and sustainably managed timber.
Potable water
Potable water is water that is safe to drink. It has low levels of dissolved salts and low levels of microbes. It is not chemically pure. Most drinking water in the UK comes from rivers, lakes or groundwater. Treatment involves filtering to remove solids and sterilising to kill microorganisms using chlorine, ozone or ultraviolet light.
Desalination removes dissolved salts from seawater using distillation or reverse osmosis. It requires large amounts of energy, which is its main disadvantage. Waste water from homes, agriculture and industry is treated through screening and grit removal, sedimentation, anaerobic digestion of sludge and aerobic biological treatment of effluent.
Required Practical 8: analysis and purification of water
Measure pH using universal indicator or a pH probe. Test for dissolved solids by evaporating water and measuring the residue. Purify water by distillation: heat the sample, collect the vapour and condense it. Distillation removes dissolved salts because salts do not evaporate with the water.
Alternative methods of extracting metals (Higher Tier only)
Plants absorb metal compounds from the soil. The plants are harvested and burned. The metal is extracted from the ash. Advantages: less environmental damage, useful for low-grade ores. Disadvantage: slow process.
Bacteria produce leachate solutions containing metal ions. Metals are then extracted by electrolysis or displacement. Advantages: less destructive than mining. Disadvantages: slow, may produce toxic solutions.
Life Cycle Assessment and recycling
A Life Cycle Assessment (LCA) measures environmental impact across four stages: extracting raw materials, manufacturing and packaging, use during the product's lifetime, and disposal. Transport must be considered at every stage. Some impacts such as habitat destruction and biodiversity loss are difficult to quantify, meaning LCAs are not completely objective. When evaluating LCAs, always mention energy use, resource use, waste produced, transport and the reliability of available data.
The three Rs: reduce (use fewer resources), reuse (use products again, for example glass bottles) and recycle (process waste into new products). Benefits include conserving finite resources, saving energy, reducing landfill and reducing environmental damage.
Corrosion and alloys
Rusting requires both oxygen and water to be present. Barrier methods prevent air and water reaching the metal (paint, grease, plastic coating, electroplating). Sacrificial protection uses a more reactive metal that corrodes instead of the iron. Zinc on iron is galvanising. The key mark scheme point is that zinc is more reactive than iron and corrodes first, not simply that it blocks oxygen.
Alloys are mixtures of metals or a metal and another element. Bronze is copper and tin. Brass is copper and zinc. Steel types: high carbon steel is strong but brittle; low carbon steel is softer and easier to shape; stainless steel contains chromium and nickel and is hard and corrosion resistant. Aluminium alloys are strong with low density, used in aircraft manufacture.
The Haber Process
Nitrogen is obtained from air. Hydrogen is obtained from natural gas. Conditions: approximately 450 degrees Celsius, approximately 200 atmospheres pressure and an iron catalyst.
Higher temperature gives a faster reaction but lower ammonia yield. Lower temperature gives higher yield but the reaction is too slow. Higher pressure gives higher yield and faster reaction but requires expensive equipment. The iron catalyst provides an alternative pathway with lower activation energy, increasing rate without being used up and without changing the equilibrium position.
Higher Tier only: the forward reaction is exothermic, so increasing temperature shifts equilibrium left, producing less ammonia. Increasing pressure shifts equilibrium right because there are fewer gas molecules on the ammonia side (4 gas molecules become 2). Always count gas molecules when discussing pressure changes in equilibrium questions.
NPK fertilisers
NPK fertilisers are formulations containing nitrogen (N), phosphorus (P) and potassium (K). Nitrogen promotes leaf growth and protein synthesis, usually supplied as ammonium salts such as ammonium nitrate. Phosphorus promotes root growth, supplied as phosphate compounds. Potassium promotes flowering, fruit formation and disease resistance, supplied as potassium salts such as KCl or K2SO4. All fertiliser compounds must be soluble in water so they can be absorbed by root hair cells.
Ammonia from the Haber Process is used to make nitric acid and ammonium nitrate. Phosphate rock is insoluble, so it must be converted into soluble salts before plants can absorb it. It reacts with nitric acid to give calcium nitrate and phosphoric acid, with sulfuric acid to give single superphosphate, or with phosphoric acid to give triple superphosphate. Potassium chloride and potassium sulfate are mined and already soluble, so they can be mixed directly into fertiliser formulations.
Required practicals checklist for Paper 2
- Required Practical 5: investigating the effect of concentration on rate of reaction (gas collection and disappearing cross methods)
- Required Practical 6: paper chromatography to separate and identify coloured substances
- Required Practical 7: identification of ions using flame tests, sodium hydroxide, silver nitrate and barium chloride
- Required Practical 8: analysis and purification of water samples using distillation
Most common exam mistakes across all five topics
- Writing "more collisions" without explaining why there are more successful collisions
- Saying catalysts increase energy or change the equilibrium position
- Writing "orange to clear" for the bromine water test: the correct word is colourless
- Leaving the C=C double bond in a polymer drawing question
- Using the everyday meaning of pure: in chemistry, pure means a single element or compound only
- Measuring Rf from the bottom of the paper rather than the origin line
- Confusing hydrogen and oxygen gas tests: hydrogen gives a squeaky pop, oxygen relights a glowing splint
- Saying the early atmosphere contained lots of oxygen: there was little or no oxygen present
- Saying sulfur dioxide causes global warming: it causes acid rain and respiratory problems
- Saying carbon monoxide is produced by complete combustion: it is produced by incomplete combustion
- Saying zinc protects iron by blocking oxygen: the correct answer is that zinc is more reactive than iron and corrodes first
- Forgetting to count gas molecules on each side when answering Haber Process pressure questions
- Forgetting that phosphate rock is insoluble and must be converted before plants can absorb it
