Rate of reaction

AS Level Chemistry · Reaction kinetics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The rate of reaction is the change in concentration of a reactant or product per unit time. Particles can react only when they collide. Most collisions do nothing: the particles just bounce apart. These are non-effective collisions.

An effective collision is one that leads to reaction. The particles must collide with at least the activation energy and in the correct orientation.

Raising the concentration of a solution, or the pressure of a gas, puts more particles in each unit of volume. Collisions become more frequent, so effective collisions are more frequent and the rate goes up. As a reaction goes on, the reactants are used up and the rate falls.

Rules to remember

Common mistake

Students write 'more collisions' for higher concentration. Write 'more frequent collisions': it is the number of effective collisions per unit time that matters.

Worked example

On a graph of reactant concentration against time, the tangent drawn at 40 s passes through the points (0 s, 0.60 mol dm−3) and (100 s, 0.10 mol dm−3). What is the rate of reaction at 40 s?

  1. Rate at 40 s = size of the gradient of the tangent.
  2. Change in concentration = 0.60 − 0.10 = 0.50 mol dm−3; change in time = 100 − 0 = 100 s.
  3. Rate = 0.50 ÷ 100 = 5.0 × 10−3 mol dm−3 s−1.

Answer: 5.0 × 10−3 mol dm−3 s−1

Practice questions

Try each one, then open the answer.

1. Why does the rate of a reaction in solution decrease as the reaction proceeds?

  1. A
    the activation energy increases
  2. B
    the particles lose kinetic energy as they react
  3. C
    the catalyst is used up
  4. D
    the concentration of reactants falls, so there are fewer collisions per unit time
Show answer

Answer: D. As reactants are used up, fewer reactant particles are present per unit volume, so the collision frequency, and therefore the rate, falls.

2. Magnesium reacts with dilute acid and 48 cm3 of hydrogen is collected in the first 20 s. What is the average rate over this period?

  1. A
    0.42 cm3 s−1
  2. B
    2.4 cm3 s−1
  3. C
    24 cm3 s−1
  4. D
    960 cm3 s−1
Show answer

Answer: B. Rate = change in quantity ÷ time = 48/20 = 2.4 cm3 s−1. Dividing time by volume (0.42) or multiplying (960) are the common slips.

3. Why does increasing the concentration of a reactant in solution increase the rate of reaction?

  1. A
    the particles collide with more energy
  2. B
    the activation energy is lowered
  3. C
    a greater proportion of collisions have energy above EA
  4. D
    more particles are present per unit volume, so collisions happen more frequently
Show answer

Answer: D. Concentration changes how often particles meet, not how much energy they have. More collisions per second at the same temperature means more effective collisions per second.

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