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OCR A Level Chemistry A – Module 3 Periodic Table and Energy

3.2.2
Reaction Rates

Section 3.2.2 covers the effect of concentration and pressure on rate in terms of collision frequency, calculating rates from the gradients of graphs, the techniques used to follow a reaction (mass, gas volume, time), catalysts and their economic and environmental importance, homogeneous and heterogeneous catalysis, and the Boltzmann distribution used to explain the effects of temperature and of a catalyst.

Exam Paper
Paper 1 & 3
H432/01 and H432/03
Specification Points
3.2.2 (a) – (g)
7 learning outcomes
Topic Parts
4 pages
Revision notes available
Exam Board
OCR A
H432 (2015 onwards)

Revision Notes

Work through 3.2.2 Reaction Rates in a structured sequence.

Collision theory
Rates of
Reaction
1
Available

Collision Theory and Rates of Reaction

Collision theory: rate of reaction, activation energy, effective collisions and the effects of concentration, pressure, surface area and temperature.

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Gradients
Measuring
Rates
2
Available

Measuring and Calculating Rates

Measuring rates: mass, gas volume and clock methods, rate = 1/time, tangents and initial rate on concentration–time graphs.

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Energy spread
Boltzmann
Distribution
3
Available

The Maxwell–Boltzmann Distribution

The Boltzmann distribution: shape, activation energy, the effect of temperature and the effect of a catalyst on the proportion of molecules that can react.

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Lower Eₐ
Catalysts
4
Available

Catalysts and Reaction Profiles

Catalysts: lower activation energy, enthalpy profile diagrams with the intermediate, homogeneous and heterogeneous catalysis, and the economic and environmental benefits.

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Specification Coverage

3.2.2 Reaction Rates – OCR A Level Chemistry A

The following OCR A learning outcomes state what candidates should be able to do. Wording is taken from the OCR A Level Chemistry A (H432) specification.

3.2.2 Reaction rates

3.2.2(a)
the effect of concentration, including the pressure of gases, on the rate of a reaction, in terms of frequency of collisions
3.2.2(b)
calculation of reaction rate from the gradients of graphs measuring how a physical quantity changes with time
3.2.2(c)
explanation of the role of a catalyst:
i
in increasing reaction rate without being used up by the overall reaction
ii
in allowing a reaction to proceed via a different route with lower activation energy, as shown by enthalpy profile diagrams
3.2.2(d)
catalysts:
i
explanation of the terms homogeneous and heterogeneous catalyst
ii
explanation that catalysts have great economic importance and benefits for increased sustainability by lowering temperatures and reducing energy demand from combustion of fossil fuels, with resulting reduction in CO₂ emissions
3.2.2(e)
the techniques and procedures used to investigate reaction rates including the measurement of mass, gas volumes and time
3.2.2(f)
qualitative explanation of the Boltzmann distribution and its relationship with activation energy
3.2.2(g)
explanation, using Boltzmann distributions, of the qualitative effect on the proportion of molecules exceeding the activation energy and hence the reaction rate, for:
i
temperature changes
ii
catalytic behaviour