A student investigates how temperature affects the rate of a reaction between sodium thiosulfate solution and dilute hydrochloric acid. As the temperature of the sodium thiosulfate solution is increased from 20 °C to 60 °C, the reaction gets progressively faster. Explain, in terms of particles, why increasing the temperature causes the rate of reaction to increase.

OCR A-Level Chemistry B: Salters (H433) — 5.2 Energy · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The reaction between sodium thiosulfate solution and dilute hydrochloric acid produces a yellow sulfur precipitate. The student measures how long it takes for the precipitate to obscure a cross marked on paper beneath the flask. At 20 °C the cross disappears after 90 seconds; at 60 °C it disappears after only 12 seconds.

Model answer (5 marks)

Increasing the temperature gives the particles more kinetic energy.

This makes the particles move faster and collide with one another more often, so the frequency of collisions rises.

At the same time, a larger proportion of the particles possess energy equal to or greater than the activation energy required for the reaction.

Consequently, there are more successful collisions per unit time.

Therefore the rate of reaction increases as the temperature rises.

Examiner tips

  • Use the exact terms: kinetic energy, collision frequency, activation energy, successful collisions.
  • Show the chain: temperature → kinetic energy → faster motion & more collisions → higher proportion above Ea → more successful collisions → higher rate.
  • Keep the answer concise, 5 points, no extra words.

Common mistakes

  • Saying “more energy” without linking to kinetic energy or activation energy.
  • Confusing collision frequency with collision energy; both are needed for the full answer.
  • Using vague phrases like “the reaction speeds up” without explaining the particle basis.

Mark scheme (5 marks)

  1. Increasing temperature gives particles more kinetic energy
  2. Particles move faster / collide more often (higher frequency of collisions)
  3. A greater proportion of particles have energy equal to or greater than the activation energy
  4. Therefore there are more successful collisions per unit time
  5. So the rate of reaction increases (linking both reasons — frequency and proportion — to the faster rate)

Key terms in this question

rate of reaction

Related

More Energy questions

▶ Try answering this question with AI marking (free) →