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Reaction Kinetics: Question 10

Syllabus 8.1, 8.2

Structured AS 6 marks

Hydrogen gas and iodine vapour react together in a sealed, rigid container at constant temperature: H2(g)+I2(g)→2HI(g)\text{H}_2\text{(g)} + \text{I}_2\text{(g)} \rightarrow 2\text{HI(g)}

(a) Explain, in terms of collision theory, why increasing the pressure inside the container (by reducing its volume, with the same amounts of each gas present and no change in temperature) increases the rate of this reaction. [3]

(b) A second, identical mixture of H2(g)\text{H}_2\text{(g)} and I2(g)\text{I}_2\text{(g)}, at the original pressure, is instead heated to a higher temperature. State two distinct ways in which this temperature rise increases the rate of reaction that the pressure increase in part (a) does not, explaining each briefly. [3]

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Worked solution

Part (a): Effect of increasing pressure

Reducing the volume of the container, while keeping the same number of moles of H2\text{H}_2 and I2\text{I}_2, increases the number of gas particles present in each unit of volume. This means H2\text{H}_2 and I2\text{I}_2 particles are, on average, closer together, so they collide with each other more frequently per second.

Because the temperature has not changed, the Maxwell–Boltzmann energy distribution of the particles is unaffected. The same fraction of collisions has energy ≥EA\ge E_A as before. Since a fixed proportion of a now-larger number of collisions per second is successful, the rate increases as pressure increases.

Part (b): Two ways heating increases rate beyond what pressure does

1. Heating increases the fraction of particles with sufficient energy to react. Raising the temperature shifts and broadens the Maxwell–Boltzmann distribution towards higher energies. Because the distribution has a long high-energy tail, even a modest temperature rise causes a large increase in the fraction of particles with energy ≥EA\ge E_A. Increasing pressure alone does not do this, it leaves the energy distribution, and therefore the fraction of successful collisions, completely unchanged.

2. Heating also increases the average speed (and hence collision frequency) of the particles. As temperature rises, particles move faster on average, so, separately from any change in concentration or pressure, they collide with each other more often. This gives a further (though comparatively small) increase in collision frequency, on top of the much larger effect described above.

Together, these two effects mean that a given temperature rise typically increases rate far more than an equivalent increase in pressure, because pressure affects only collision frequency, while temperature affects both collision frequency and the fraction of collisions that are energetically successful.

Final answers

  • (a) A smaller volume raises the concentration of gas particles, increasing collision frequency; since the fraction of collisions with E≥EAE\ge E_A is unchanged (constant temperature), more collisions per second are successful, so rate increases.
  • (b) Heating (1) increases the fraction of particles with E≥EAE \ge E_A via the Boltzmann distribution shift (pressure cannot do this), and (2) further increases collision frequency by increasing average particle speed.