This is a complete revision guide for AQA Combined Science Chemistry Paper 2 (specification code 8464), covering Topics 5.6 to 5.10: Rate and Extent of Chemical Change, Organic Chemistry, Chemical Analysis, Chemistry of the Atmosphere and Using Resources. This guide is written specifically for Combined Science students. If you are sitting Triple Science Chemistry, a separate guide is available. 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 in order. After reading a section, close the page and explain it from memory. Use the exam tips and common mistakes to sharpen your technique as well as your content.
Topic 5.6: Rate and Extent of Chemical Change
Calculating rate of reaction
The rate of a 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 80 cm cubed of gas in 40 seconds. Rate = 80 divided by 40 = 2 cm cubed per second.
On a rate graph, a steeper curve means a faster reaction. When the graph levels off, the reaction has stopped because a reactant has been completely used up.
Higher Tier only: to find the rate at a specific point, 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, not on the curve.
Write: "The limiting reactant has been completely used up, so the reaction stops." Do not write "the reaction slows down." Once the graph is flat, the rate is zero.
Factors affecting rate of reaction
| Factor | What happens | Why rate increases |
|---|---|---|
| Concentration | More particles in the same volume | More frequent collisions between reacting particles |
| Pressure (gases only) | Gas particles pushed closer together | More collisions per second |
| Surface area | Smaller pieces expose more particles | More collisions occur at the surface |
| Temperature | Particles gain kinetic energy | More frequent collisions and more particles have enough energy to react |
| Catalyst | Alternative reaction pathway provided | Activation energy is lowered so more collisions are successful |
Collision theory
Particles react only when they collide and have enough energy to overcome the activation energy. Increasing temperature increases kinetic energy, increases collision frequency, increases collision energy and increases the number of successful collisions. For temperature questions, always include all four points. Writing only "more collisions" typically earns just one mark.
Catalysts
Catalysts increase reaction rate, are not used up, lower activation energy and provide an alternative reaction pathway. Enzymes are biological catalysts. Catalysts do not provide energy, do not change the overall energy change of the reaction and are not consumed.
Required Practical 11: effect of concentration on rate of reaction
You need to know both methods.
Method 1: gas collection. 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, mass of marble chips, volume of acid.
Method 2: colour change or turbidity. Example: sodium thiosulfate and hydrochloric acid. Place a flask over a black cross, add the acid and start the timer. Stop when the cross can no longer be seen. Higher concentration means the cross disappears faster. The amount of product formed may be the same 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 go in both directions, shown by the reversible arrow symbol. If one direction is exothermic, the reverse is endothermic and the same amount of energy is transferred in each direction. Equilibrium occurs when the forward and reverse rates are equal, concentrations remain constant and both reactions continue. This is a dynamic equilibrium and only occurs in a closed system.
Higher Tier only (Le Chatelier's Principle): if conditions change, the equilibrium shifts to oppose the change.
Increase reactant concentration: shifts right
Decrease reactant concentration: shifts left
Increase product concentration: shifts left
Decrease product concentration: shifts right
Exothermic reaction: increase temperature shifts left, decrease shifts right
Endothermic reaction: increase temperature shifts right, decrease shifts left
Pressure: increase pressure shifts towards fewer gas molecules. Decrease pressure shifts towards more gas molecules. Always count gas molecules on both sides of the equation. This is where most equilibrium marks are lost.
Topic 5.7: Organic Chemistry
Crude oil, hydrocarbons and alkanes
Crude oil is a finite resource formed from ancient biomass, mainly plankton. It is a mixture of hydrocarbons, which contain only hydrogen and carbon. Most belong to the alkane homologous series with general formula CnH(2n+2). The first four alkanes are methane, ethane, propane and butane.
Fractional distillation
Crude oil is heated so most of it vaporises and enters a fractionating column where temperature decreases from bottom to top. Small molecules have low boiling points and condense near the top. Large molecules have high boiling points and condense near the bottom. Products include petrol, diesel, kerosene, LPG, lubricants, polymers and detergents.
Properties of hydrocarbons
As molecular size increases: boiling point increases, viscosity increases and flammability decreases. 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. Carbon monoxide is produced by incomplete combustion, not complete combustion.
Cracking and alkenes
Cracking breaks large hydrocarbons into smaller, more useful molecules including alkanes and alkenes. Methods include catalytic cracking and steam cracking. Alkenes are more reactive than alkanes because they contain a C=C double bond. The bromine water test identifies alkenes: alkenes turn bromine water from orange to colourless. Alkanes produce no change. Write colourless, not clear. AQA specifically expects the word colourless.
Topic 5.8: Chemical Analysis
Pure substances and formulations
In chemistry, a pure substance is a single element or compound. Pure substances melt at one specific temperature and boil at one specific temperature. Impure substances melt and boil over a range. This is different from the everyday meaning of the word pure.
A formulation is a carefully designed mixture where each component has a specific purpose. Examples include medicines, fuels, paints, fertilisers and foods.
Chromatography
Chromatography separates mixtures using a stationary phase (paper) and a mobile phase (solvent). A pure substance produces one spot. A mixture produces multiple spots. Substances at the same height as a known reference are likely to be the same.
Required Practical 12: paper chromatography
Draw a pencil line (not pen, because pencil is insoluble) near the bottom of the chromatography paper. Add the sample spot on the line. Place the paper in solvent ensuring the solvent level is below the pencil line. Allow the solvent to rise, remove the paper before the solvent reaches the top and mark the solvent front immediately. Calculate Rf values by measuring 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 |
| Chlorine | Damp litmus paper | Bleaches white |
Hydrogen gives a squeaky pop. Oxygen relights a glowing splint. These are commonly confused — do not mix them up.
Topic 5.9: Chemistry of the Atmosphere
Composition of the atmosphere
The modern atmosphere contains approximately 80% nitrogen, approximately 20% oxygen and small amounts of carbon dioxide, water vapour and noble gases. Carbon dioxide makes up only a very small fraction, around 0.04%.
The Earth's early atmosphere
The early atmosphere formed from volcanic activity and contained carbon dioxide, water vapour, nitrogen, methane and ammonia. There was little or no oxygen. Evidence is limited because the atmosphere formed around 4.6 billion years ago and very few rocks from that period remain.
How oxygen increased and carbon dioxide decreased
Photosynthetic algae appeared around 2.7 billion years ago. Photosynthesis released oxygen into the atmosphere. As plants evolved, oxygen levels gradually increased. Carbon dioxide decreased through photosynthesis, the formation of carbonate rocks and the formation of fossil fuels.
Greenhouse gases and the greenhouse effect
The main greenhouse gases are water vapour, carbon dioxide and methane. The greenhouse effect keeps Earth warm enough for life. Without it, Earth would be too cold. Problems occur when the effect intensifies due to increased greenhouse gas concentrations from human activity.
Burning fossil fuels. Deforestation reduces photosynthesis so less CO2 is removed from the atmosphere.
Cattle farming. Landfill sites (decomposition of waste produces methane).
Effects of global climate change include rising sea levels, habitat loss, more extreme weather and reduced biodiversity. A carbon footprint is the total greenhouse gas emissions over a product's life cycle. Ways to reduce it include renewable energy, improved efficiency, reducing waste and reducing deforestation.
Atmospheric pollutants
| Pollutant | How it is produced | Effects |
|---|---|---|
| Carbon monoxide (CO) | Incomplete combustion when oxygen supply is limited | Toxic gas that reduces the blood's ability to carry oxygen |
| Soot (carbon particulates) | Incomplete combustion when oxygen is insufficient | Lung damage and global dimming |
| Sulfur dioxide (SO2) | Sulfur impurities in fuels burning in oxygen | Acid rain and respiratory problems |
| Oxides of nitrogen (NOx) | Nitrogen and oxygen react at high temperatures in engines and power stations | Acid rain and respiratory problems |
| Carbon dioxide (CO2) | Complete combustion of carbon-containing fuels | Greenhouse gas contributing to climate change |
Sulfur dioxide causes acid rain, not global warming. Carbon monoxide is produced by incomplete combustion, not complete combustion.
Topic 5.10: Using Resources
Sustainable development and resources
Sustainable development means meeting today's needs without preventing future generations from meeting theirs. Finite resources include crude oil, coal, natural gas and metal ores. Renewable resources include solar, wind and biomass.
Potable water
Potable water is water that is safe to drink. It contains low levels of dissolved salts and low levels of microbes. Potable water is not chemically pure because it contains dissolved substances. Most potable water in the UK comes from fresh water sources such as rivers, lakes and groundwater. Treatment involves choosing an appropriate source, filtering to remove solids and sterilising to kill microorganisms using chlorine, ozone or ultraviolet light.
Potable water means safe to drink. Pure water means only H2O molecules are present. Potable water contains dissolved salts and gases, so it is not chemically pure. Confusing these two is a very common exam error.
Desalination
In parts of the world where fresh water is limited, desalination converts sea water into potable water. Two methods are required by AQA.
Sea water is heated until it evaporates. The vapour is collected and condensed. Dissolved salts are left behind because they do not evaporate.
Sea water is forced through partially permeable membranes under high pressure. Water molecules pass through but dissolved salts do not.
Both methods require large amounts of energy, making desalination expensive and less sustainable than using natural fresh water. If asked why desalination is not used everywhere, write: "Desalination requires large amounts of energy, making it expensive." Do not simply write "it costs a lot." Always link the cost to the energy requirement.
Required Practical 13: analysis and purification of water
Measure pH using universal indicator or a pH probe. Test for dissolved solids by evaporating the water and measuring the residue left behind. 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.
Waste water treatment
Stages of waste water treatment: screening and grit removal, sedimentation, anaerobic digestion of sludge and aerobic biological treatment of effluent.
Alternative methods of extracting metals (Higher Tier only)
As high-grade copper ores become scarcer, alternative biological methods are used to extract copper from low-grade ores with less environmental damage than traditional mining.
Plants are grown on land containing low-grade copper ore. They absorb copper compounds through their roots, are then harvested and burned. The ash contains copper compounds from which copper is extracted by electrolysis or displacement using scrap iron.
Advantages: less landscape damage, useful for low-grade ores, can be used on contaminated land. Disadvantages: very slow, weather-dependent, needs large areas of land.
Bacteria are added to low-grade copper ore and produce leachate solutions containing copper compounds. Copper is then extracted from the solution by displacement with iron or by electrolysis.
Advantages: less destructive than mining, can use very low-grade ores, fewer heavy machines. Disadvantages: very slow, bacteria need controlled conditions, leachate solutions may be toxic.
Life Cycle Assessment and recycling
A Life Cycle Assessment (LCA) considers raw material extraction, manufacture, use during the product's lifetime and disposal. Transport must be considered at every stage. When evaluating an LCA, always mention energy use, resource use, waste produced, transport and the reliability of available data. Some impacts such as habitat destruction and biodiversity loss are difficult to quantify, meaning LCAs are not completely objective.
The three Rs: reduce (use fewer resources), reuse (use products again) and recycle (process waste into new products). Benefits include conserving finite resources, saving energy, reducing landfill and reducing environmental damage.
Required practicals checklist for Paper 2
- Required Practical 11: effect of concentration on rate of reaction (gas collection and colour change methods)
- Required Practical 12: paper chromatography and Rf values
- Required Practical 13: 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 provide energy: they lower activation energy, they do not provide it
- Saying equilibrium means reactions stop: it is dynamic, both reactions continue
- Forgetting to count gas molecules when answering pressure and equilibrium questions
- Writing "orange to clear" for the bromine water test: the correct word is colourless
- Using the everyday definition of pure: in chemistry, pure means a single element or compound only
- Measuring Rf from the bottom of the paper instead of 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
- Forgetting that potable water is not chemically pure
- Forgetting to link desalination costs to the energy requirement
- Forgetting that distillation removes dissolved salts because salts do not evaporate
