Temperature Effect on Rate – IGCSE Chemistry Definition

Increasing the temperature of a reaction system increases the rate of reaction. This is the most powerful of the four rate factors because temperature affects both the frequency and the energy of collisions between reactant particles.

Collision theory explanation

At higher temperatures, reactant particles have more kinetic energy and move faster. This produces two effects. First, the particles collide more frequently because they cover more distance per second. Second, and more importantly, a greater proportion of the collisions have energy equal to or exceeding the activation energy. This means a larger fraction of all collisions are successful, leading to a significantly faster rate.

At IGCSE level, candidates should mention both effects for full marks. However, examiners place particular emphasis on the second effect --- the increased proportion of particles exceeding the activation energy --- because this has a larger impact on the rate than the modest increase in collision frequency.

Quantitative rule of thumb

As a rough guide, increasing the temperature by 10 degC approximately doubles the rate of many reactions. This is because even a small temperature increase significantly shifts the energy distribution, bringing many more particles above the activation energy threshold.

Practical example

Marble chips react with hydrochloric acid more quickly at 40 degC than at 20 degC. The same mass of marble and the same concentration of acid produce the same total volume of carbon dioxide, but the gas is collected much faster at the higher temperature.

Exam context

Paper 4 questions almost always require the collision theory explanation when asking about temperature and rate. A common question format provides a graph showing two experiments at different temperatures and asks candidates to explain the difference.

Worked exam question

Explain, in terms of particles, why increasing the temperature increases the rate of a chemical reaction. (3 marks)

At a higher temperature, particles have more kinetic energy and move faster [1]. They collide more frequently [1]. A greater proportion of the collisions have energy equal to or greater than the activation energy, so more collisions are successful [1].

Common mistakes

The most common error is saying only “particles move faster.” This earns at most one mark. Candidates must also explain why faster movement leads to more successful collisions. Saying “particles have more energy so they react faster” is too vague. The answer must link temperature to kinetic energy, then to collision frequency, and then to the proportion of collisions exceeding the activation energy.