MathsGradeUp logoMathsGradeUp
ChemistryInternational GCSEEdexcel & Cambridge IGCSEYears 10–11

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.

Practise this topic free Ask Study Coach

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.

  1. 1Powder has a greater surface area than lumps for the same mass.
  2. 2More particles of CaCO₃ are exposed to acid particles.
  3. 3So the frequency of successful collisions is greater.
  4. 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.

Practise more questions with Study Coach feedback

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.

Related practice

Practise this topic

Exam-style questions with instant Study Coach feedback.

Flashcards

Quick-fire recall to lock in the key facts.

Worksheets

Printable practice you can work through offline.

Past papers

Full papers with official mark-scheme marking.

Study Coach

Ask for a step-by-step explanation any time.

All Chemistry

Every Chemistry topic across every exam board.

Related topics

Rates of ReactionStoichiometry

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is dynamic equilibrium?Full glossary

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.

Create a free account to practise rates and equilibria with Study Coach feedback

Try 5 questions now, get instant mark-scheme marking and ask the Study Coach to explain anything. Free during our early launch period — no credit card required.

Create free account Try the Study Coach

Related on MathsGradeUp

Keep studying

Practise it

Enquire now