Home · IGCSE Chemistry 0620 · Chemical reactions · Collision theory and catalysts · Practice
IGCSE · Practice questions · Collision theory and catalysts

Explain it with collisions.

Six original Cambridge-style questions on collision theory: activation energy, frequency of collisions, the effect of temperature and how catalysts speed up reactions. Extended content.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Frequency and energy.

01
Recall
[2 marks]

(Extended) State the two conditions needed for a collision between particles to result in a reaction.

  • The particles must collide. ✓
  • The collision must have energy greater than or equal to the activation energy. ✓
02
Explain
[2 marks]

(Extended) Use collision theory to explain why hydrochloric acid of concentration 2 mol/dm3 reacts faster with calcium carbonate than acid of concentration 1 mol/dm3.

  • There are more acid particles in the same volume of the more concentrated acid. ✓
  • So collisions between acid particles and the calcium carbonate are more frequent. ✓
03
Explain
[3 marks]

(Extended) Use collision theory to explain why increasing the temperature increases the rate of a reaction.

  • The particles gain kinetic energy and move faster. ✓
  • So they collide more frequently. ✓
  • A greater proportion of collisions have energy greater than or equal to the activation energy, so there are more successful collisions per second. ✓
04
Explain
[2 marks]

(Extended) Explain how a catalyst increases the rate of a reaction.

  • It provides an alternative reaction pathway with a lower activation energy. ✓
  • So a greater proportion of collisions have enough energy to react, and the frequency of successful collisions increases. ✓
05
Explain
[2 marks]

(Extended) Explain why flour dust in the air of a flour mill can explode, although a heap of flour only burns slowly.

  • Dust particles are very small, so the surface area in contact with oxygen is very large. ✓
  • Collisions between flour and oxygen particles are therefore extremely frequent, so the reaction is very fast (explosive). ✓
06
Analysis
[3 marks]

(Extended) Sketch reaction pathway diagrams for an exothermic reaction with and without a catalyst, and label the activation energy for each.

  • Reactants at a higher energy level than products, joined by a curve with a hill. ✓
  • The catalysed curve has a lower hill between the same reactant and product levels. ✓
  • Each Ea labelled as an arrow from the reactants level to the top of its own hill. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.