Movement and Interactions: The Rate and Extent of Chemical Change — AQA Combined Science: Synergy
This topic covers reaction rates, collision theory, catalysts and enzymes, and reversible reactions and equilibrium.
Factors that affect reaction rates
The rate of reaction is how quickly reactants are used up or products formed. It can be measured by the amount of product formed or reactant used over time: $$\text{mean rate} = \frac{\text{quantity of product or reactant}}{\text{time}}$$ A steeper line on a rate graph means a faster rate; the line levels off when a reactant runs out. The gradient of a tangent gives the rate at a moment.
Collision theory
Reactions happen when particles collide with at least the activation energy and in the correct orientation. Anything that increases the frequency or energy of successful collisions increases the rate.
Effect of surface area, temperature, concentration and pressure
- Surface area — smaller pieces/powder → more frequent collisions → faster.
- Temperature — particles move faster, colliding more often and with more energy → much faster.
- Concentration / pressure — more particles in a given volume → more frequent collisions → faster.
Required practical: investigating how a factor (e.g. concentration or temperature) affects rate, e.g. the disappearing-cross method or measuring gas produced.
Activation energy
The activation energy is the minimum energy colliding particles need to react. Catalysts lower it.
Bond breaking and bond forming (Higher Tier)
Breaking bonds is endothermic; forming bonds is exothermic. The overall energy change is the difference between energy used breaking bonds and energy released forming them.
Catalysts and enzymes
A catalyst speeds up a reaction without being used up, by providing a pathway with a lower activation energy. Enzymes are biological catalysts.
Reversible reactions and dynamic equilibrium
Some reactions are reversible (shown by ⇌). If the forward reaction is exothermic, the reverse is endothermic by the same amount. In a closed system, a reversible reaction reaches dynamic equilibrium — forward and reverse reactions occur at the same rate, so concentrations stay constant (but the reactions continue).
Factors affecting the position of equilibrium (Higher Tier)
By Le Chatelier's principle, a system shifts to oppose a change:
- More reactant → shifts towards products.
- Higher temperature → shifts in the endothermic direction.
- Higher pressure (gases) → shifts towards the side with fewer gas molecules.
Exam tips
- Explain every rate change using collision theory.
- A catalyst lowers the activation energy; it is not used up.
- Equilibrium is dynamic and needs a closed system.
- For Le Chatelier questions, state the change then which way equilibrium shifts and why.