A food manufacturer stores freshly pressed fruit juice in a cold warehouse at 2 °C rather than at room temperature (20 °C). The juice spoils more slowly at the lower temperature because the chemical reactions that cause spoilage are slower. Explain, using the Maxwell-Boltzmann distribution, why the spoilage reactions are slower at 2 °C than at 20 °C.

Edexcel A-Level Chemistry (9CH0) — 9.1 Rates and the Maxwell-Boltzmann distribution · Explain · 5 marks · View as Markdown

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

A food manufacturer stores freshly pressed fruit juice in a cold warehouse at 2 °C rather than at room temperature (20 °C). The juice spoils more slowly at the lower temperature because the chemical reactions that cause spoilage are slower.

Model answer (5 marks)

At 2 °C the average kinetic energy of the juice molecules is lower than at 20 °C, so the particles move more slowly.

The Maxwell–Boltzmann distribution therefore shifts to the left; a smaller proportion of molecules have energy equal to or greater than the activation energy of the spoilage reactions.

Consequently, fewer collisions have sufficient energy to overcome the activation barrier, so the number of successful collisions per unit time is reduced.

In addition, because the molecules move more slowly, the overall collision frequency is lower at 2 °C.

Thus the rate of the spoilage reactions is lower at 2 °C, and the juice spoils more slowly.

Examiner tips

  • Use the phrase "shift to the left" for the distribution. Mention both fewer high‑energy molecules and lower collision frequency. Show the link between activation energy and Maxwell–Boltzmann. Conclude with the effect on reaction rate.
  • common_mistakes
  • :
  • Saying only that molecules move slower without linking to the distribution. Forgetting to mention the reduced number of high‑energy molecules. Using the wrong temperature units or not specifying the shift direction.

Mark scheme (5 marks)

  1. At the lower temperature (2 °C), the particles have less kinetic energy (on average) / particles move more slowly
  2. The Maxwell-Boltzmann distribution shifts so that fewer particles have energy greater than or equal to the activation energy at 2 °C compared to 20 °C
  3. Fewer successful collisions occur per unit time / fewer collisions have enough energy to result in a reaction
  4. The frequency of collisions is also lower at 2 °C because particles move more slowly
  5. Therefore the rate of the spoilage reaction is lower at 2 °C, so the juice takes longer to spoil

Key terms in this question

Maxwell-Boltzmann distribution

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