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CAIE A-Level Chemistry 8.2.2 Boltzmann Energy Distributions

Practise sketching Boltzmann distributions and using the area beyond activation energy to compare reaction rates.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • draw the curve from the origin with one peak and an asymptotic tail while preserving total area
  • for a higher temperature, lower and shift the peak right so the two curves cross once beyond the peak
  • compare the area at or above EA to identify which temperature and activation energy gives the faster rate

8.2.2—Sketch and use the Boltzmann distribution question 1

[Maximum number: 2]

Ammonia, NH3\mathrm{NH}_{3}, is manufactured from nitrogen and hydrogen by the Haber process.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92 kJ mol−1\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

The Haber process is usually carried out at a temperature of approximately 400∘C400^{\circ} \mathrm{C} in the presence of a catalyst. Changing the temperature affects both the rate of production of ammonia and the yield of ammonia.

The Boltzmann distribution for a mixture of nitrogen and hydrogen at 400∘C400^{\circ} \mathrm{C} is shown. Ea represents the activation energy for the reaction.

Figure for Question 8.2.2—Sketch and use the Boltzmann distribution question 1 — CAIE A-Level Chemistry AS

Using the same axes, sketch a second curve to indicate the Boltzmann distribution at a higher temperature.

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