The Rate and Extent of Chemical Change — AQA Combined Science: Trilogy
This unit covers how fast reactions happen (rate), what affects rate, and reversible reactions and equilibrium.
Rate of reaction
The rate of reaction is how quickly reactants are used up or products are formed. It can be found by measuring:
- the amount of product formed over time (e.g. volume of gas using a gas syringe), or
- the amount of reactant used up over time (e.g. loss of mass on a balance).
$$\text{mean rate} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}$$
On a graph of product against time, the steeper the line, the faster the rate. The line levels off when the reaction finishes (a reactant runs out). The gradient of a tangent gives the rate at a particular moment.
Collision theory
Reactions happen when particles collide with enough energy (the activation energy) and in the correct orientation. The rate depends on the frequency and energy of collisions. Anything that increases the number of successful collisions per second increases the rate.
Factors affecting rate
- Concentration (of solutions) / pressure (of gases) — more particles in a given volume means more frequent collisions, so faster rate.
- Temperature — particles move faster, so they collide more often and with more energy, sharply increasing the rate.
- Surface area — breaking a solid into smaller pieces (or powder) increases the surface area, giving more frequent collisions and a faster rate.
- Catalysts — speed up reactions without being used up.
Required practical: investigating how a factor (e.g. concentration of sodium thiosulfate, or temperature) affects the rate of reaction, using a method such as timing how long a cross takes to disappear, or measuring gas produced.
Catalysts
A catalyst increases the rate of a reaction without being used up in the reaction. It works by providing a different reaction pathway with a lower activation energy. Catalysts are specific to particular reactions and are important in industry because they reduce energy costs. Enzymes are biological catalysts.
Reversible reactions
Some reactions are reversible — the products can react to reform the reactants. This is shown with the ⇌ symbol.
A + B ⇌ C + D
If the forward reaction is exothermic, the reverse reaction is endothermic by the same amount of energy.
Example
Heating hydrated copper sulfate (blue) drives off water to form anhydrous copper sulfate (white); adding water reverses it: hydrated CuSO₄ ⇌ anhydrous CuSO₄ + water
Equilibrium
When a reversible reaction occurs in a closed system, it reaches a dynamic equilibrium: the forward and reverse reactions are happening at the same rate, so the concentrations of reactants and products stay constant (but the reactions don't stop).
Le Chatelier's principle (Higher Tier)
If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change:
- Concentration: increasing a reactant's concentration shifts the equilibrium towards the products.
- Temperature: increasing temperature shifts the equilibrium in the endothermic direction; decreasing it shifts towards the exothermic direction.
- Pressure (gases): increasing pressure shifts the equilibrium towards the side with fewer gas molecules.
This lets industrial chemists choose conditions to maximise the yield of a desired product.
Exam tips
- Be able to describe how to measure rate by gas volume or mass loss, and to calculate mean rate.
- Always explain rate changes using collision theory — frequency and/or energy of collisions.
- A catalyst lowers the activation energy; it is not used up and does not change the products.
- For equilibrium, stress that it is dynamic (both reactions continue) and needs a closed system.
- For Le Chatelier questions, state the change and then which way the equilibrium shifts and why.