What controls how fast a reaction goes and how far it proceeds?
Calculating and measuring rate of reaction, the factors affecting rate and collision theory, the action of catalysts, and reversible reactions and dynamic equilibrium including Le Chatelier's principle.
A focused answer to the AQA GCSE Combined Science: Trilogy Rate and extent of chemical change topic, covering measuring rate of reaction, collision theory and the factors affecting rate, catalysts, and reversible reactions and dynamic equilibrium.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
AQA wants you to calculate and measure the rate of a reaction, explain the factors affecting rate using collision theory, describe how catalysts work, and explain reversible reactions and dynamic equilibrium including the effect of changing conditions.
Measuring rate
Common methods to follow a reaction include measuring the volume of gas given off with a gas syringe, recording the loss of mass on a balance as gas escapes, or timing how long a solution takes to turn cloudy (for example sodium thiosulfate and acid, where a cross under the flask disappears). To find the rate at a single instant from a curved graph, draw a tangent at that point and calculate its gradient.
Collision theory and factors affecting rate
- Concentration (of solutions) or pressure (of gases): more particles in the same volume means collisions are more frequent, so the rate increases.
- Surface area: breaking a solid into smaller pieces (or a powder) exposes more particles at the surface, so collisions are more frequent and the rate increases.
- Temperature: particles move faster (more frequent collisions) and have more energy, so a greater proportion of collisions have at least the activation energy; both effects increase the rate.
- Catalyst: provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful; the catalyst is not used up and can be reused. Enzymes are biological catalysts.
Reversible reactions and equilibrium
In a reversible reaction the products can react to reform the reactants, shown by the symbol with two half-arrows. If the forward reaction is exothermic, the backward reaction is endothermic by the same amount. In a closed system (nothing enters or leaves), the reaction reaches dynamic equilibrium, where the forward and backward reactions are still happening but at the same rate, so the amounts of reactants and products stay constant.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksIn an experiment, 48 cubic centimetres of gas is produced in the first 30 seconds of a reaction. Calculate the mean rate of reaction in cubic centimetres per second, and explain why the rate is fastest at the start.Show worked answer →
A Chemistry Paper 1 rate calculation. Method: mean rate cubic centimetres per second. For the explanation: at the start the concentration of the reactants is highest, so there are the most frequent collisions per second and the rate is fastest; as the reaction proceeds the reactants are used up, the concentration falls, collisions become less frequent and the rate slows, which is why the gradient of the graph is steepest at the start. Markers reward the correct rate with units, and the link from high concentration to frequent collisions.
AQA 20214 marksIn the reversible reaction used to make ammonia, the forward reaction is exothermic. Predict and explain the effect on the equilibrium yield of ammonia of increasing the temperature.Show worked answer →
A Chemistry Paper 1 equilibrium question using Le Chatelier's principle. Reward: increasing the temperature shifts the position of equilibrium in the endothermic direction to oppose the change (to take in the added heat). Because the forward reaction is exothermic, the backward reaction is endothermic, so raising the temperature shifts the equilibrium backwards, towards the reactants, and the yield of ammonia falls. Markers credit the statement of Le Chatelier's principle, identifying the backward reaction as endothermic, and the correct prediction that the yield decreases. A strong answer notes the trade-off that a higher temperature still speeds up the rate even though it lowers the yield.
Related dot points
- Exothermic and endothermic reactions and their everyday uses, reaction profiles and activation energy, and the energy changes involved in breaking and making bonds.
A focused answer to the AQA GCSE Combined Science: Trilogy Energy changes topic, covering exothermic and endothermic reactions and their uses, reaction profiles and activation energy, and the energy changes when bonds are broken and made.
- The reactivity series and metal extraction, the reactions of acids with metals, bases and carbonates, neutralisation and the pH scale, making soluble salts, and electrolysis of molten compounds and aqueous solutions.
A focused answer to the AQA GCSE Combined Science: Trilogy Chemical changes topic, covering the reactivity series and metal extraction, reactions of acids, neutralisation and the pH scale, making salts, and electrolysis of molten and aqueous compounds.
- Conservation of mass and balanced equations, relative formula mass, the mole and Avogadro's number, calculating amounts and masses in reactions, limiting reactants, and concentration of solutions.
A focused answer to the AQA GCSE Combined Science: Trilogy Quantitative chemistry topic, covering conservation of mass, relative formula mass, the mole, calculating reacting masses, limiting reactants, and the concentration of solutions.
- Crude oil, hydrocarbons and the alkanes, fractional distillation and the uses of fractions, the properties of hydrocarbons and combustion, and cracking to produce alkenes and more useful products.
A focused answer to the AQA GCSE Combined Science: Trilogy Organic chemistry topic, covering crude oil and hydrocarbons, the alkanes, fractional distillation and the uses of fractions, combustion, and cracking to make alkenes.
- The three types of bonding (ionic, covalent and metallic), how to represent them, the states of matter and changes of state, and how the structures of ionic compounds, small molecules, giant covalent structures, polymers and metals explain their properties.
A focused answer to the AQA GCSE Combined Science: Trilogy Bonding and structure topic, covering ionic, covalent and metallic bonding, the states of matter, and how the structure of ionic compounds, simple molecules, giant covalent structures, polymers and metals explains their properties.