IGCSE Chemistry Rates and Equilibria Notes
Rates of reaction are all about how often particles collide with enough energy. Reversible reactions can reach a dynamic equilibrium where forward and backward rates are equal.
Key concepts
- Collision theory
- Reactions happen when particles collide with energy ≥ the activation energy in the correct orientation.
- Factors affecting rate
- Concentration, temperature, surface area, pressure (gases) and catalysts.
- Reversible reaction
- Products can react to reform reactants; shown by ⇌.
- Dynamic equilibrium
- In a closed system, forward and backward rates become equal; concentrations no longer change.
- Higher temperature → faster movement and more energetic collisions → faster rate.
- Catalysts speed up a reaction by providing an alternative pathway with lower activation energy; they are not used up.
- For an exothermic reaction, increasing temperature shifts equilibrium backward.
- Increasing pressure shifts equilibrium toward the side with fewer moles of gas.
Worked example
Explain why powdered calcium carbonate reacts faster with acid than lumps.
- 1Powder has a greater surface area than lumps for the same mass.
- 2More particles of CaCO₃ are exposed to acid particles.
- 3So the frequency of successful collisions is greater.
- 4Therefore the rate of reaction is faster.
Answer: Greater surface area → more collisions per second → faster rate.
Common mistakes
✗ Saying temperature 'gives energy for more collisions'.
✓ Temperature increases particle speed, which means more frequent AND more energetic collisions.
✗ Claiming a catalyst changes the position of equilibrium.
✓ Catalysts only speed up reaching equilibrium — the position of equilibrium stays the same.
✗ Ignoring 'closed system' for equilibrium.
✓ Dynamic equilibrium only occurs when nothing enters or leaves the system.
Quick check
Try these, then reveal the answers.
What is activation energy?Reveal answer
The minimum energy needed for a collision to lead to a reaction.
How does a catalyst affect activation energy?Reveal answer
It lowers it by providing an alternative pathway.
In N₂ + 3H₂ ⇌ 2NH₃, what happens when pressure is increased?Reveal answer
Equilibrium shifts to the right (fewer moles of gas), producing more ammonia.
Exam tips
- Structure rate answers: factor → frequency of collisions / energy of collisions → rate.
- For equilibrium, use Le Chatelier's principle: system shifts to oppose the change.
- Sketch rate graphs with the correct shape: steepest at the start, flattening as reactants run out.
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Frequently asked questions
What is dynamic equilibrium?+
A state in a closed system where the forward and backward reactions proceed at the same rate, so overall concentrations remain constant.
How does increasing concentration affect rate?+
More reactant particles in the same volume mean more frequent collisions and a faster rate.
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