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6.1.3Collision theory and activation energy

AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Rate of reaction

Practise Collision theory and activation energy. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Complete the sentence.
Chemical reactions can only happen when reacting particles ________ with each other with enough energy.
collide

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy5 marks
(a) Complete the sentence.
Chemical reactions can only happen when reacting particles ________ with each other with enough energy.[1]
(b) What is the name for the minimum amount of energy that particles must have to react?[1]
(c) Why does increasing the concentration of an acid increase the rate of its reaction with a metal?
Tick (✓) one box.[1]
  • The acid particles move faster.
  • There are more acid particles in the same volume, so collisions are more frequent.
  • The activation energy is lower.
  • Each collision has more energy.
(d) Increasing the temperature increases the rate of a reaction.
Give two reasons why, in terms of particles.[2]
Show the answer and mark scheme
(a) Answer: collide
  • collide
(b) Answer: Activation energy
  • activation energy
(c) Answer: There are more acid particles in the same volume, so collisions are more frequent.
(d) Answer: Particles move faster so collide more often; collisions are more energetic, so more particles have at least the activation energy.
  • particles move faster, so they collide more frequently
  • collisions are more energetic / more particles have energy greater than or equal to the activation energy
Question 2Medium6 marks
A student compared a single 2 cm cube of calcium carbonate with another 2 cm cube that had been cut into eight 1 cm cubes.
(a) Calculate the surface area to volume ratio of the single 2 cm cube.[2]
(b) Calculate the total surface area of the eight 1 cm cubes.[1]
(c) Both samples were added to equal volumes of the same hydrochloric acid.
Explain why the eight small cubes reacted faster than the single cube.[3]
Show the answer and mark scheme
(a) Answer: 3 : 1
  • surface area = 6 × 2 × 2 = 24 (cm2) and volume = 2 × 2 × 2 = 8 (cm3)
  • (24 : 8 =) 3 : 1
(b) Answer: 48 cm2
  • 48 (cm2)
(c) Answer: The small cubes have a larger surface area to volume ratio (6 : 1 instead of 3 : 1), so more particles are exposed to the acid and collisions are more frequent.
  • the small cubes have a larger surface area to volume ratio (6 : 1 compared with 3 : 1)
  • so more calcium carbonate particles are exposed to the acid
  • so there are more frequent collisions
Question 3Hard7 marks
A company produces a gas by reacting lumps of a solid with an acid at room temperature.
(a) What is meant by a successful collision?[1]
(b) The company wants to increase the rate of production of the gas.
Explain how the company could change the conditions to increase the rate of reaction.
Use collision theory in your answer.[6]
Show the answer and mark scheme
(a) Answer: A collision between reacting particles with at least the activation energy, so that they react.
  • a collision between reacting particles with energy greater than or equal to the activation energy (so that a reaction happens)
(b) Answer: See indicative content.
  • increase the temperature: particles move faster, so they collide more frequently
  • at a higher temperature collisions are more energetic, so a greater proportion have energy greater than or equal to the activation energy
  • use smaller lumps or a powder: larger surface area to volume ratio, so more particles are exposed and collisions are more frequent
  • increase the concentration of the acid: more acid particles in the same volume, so collisions are more frequent
  • add a catalyst: provides a different pathway with a lower activation energy, so more collisions are successful
  • changing the pressure has no effect because the reactants are not gases
  • each change gives more successful collisions per second, so a faster rate

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