CIE A Level Chemistry

Revision Notes

Syllabus Edition

First teaching 2020

Last exams 2024

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5.6.6 Factors affecting Rate Constant

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Effect of Temperature on the Rate Constant

  • At higher temperatures, a greater proportion of molecules have energy greater than the activation energy
  • Since the rate constant and rate of reaction are directly proportional to the fraction of molecules with energy equal or greater than the activation energy, then at higher temperatures:
    • The rate constant increases
    • The rate of reaction increases

  • The relationship between the rate constant and the temperature is given by the following equation:

Effect of Temperature on the Rate Constant equation 1

ln k = natural logarithm of the rate constant

A = constant related to the collision frequency and orientation of the molecules

Ea = activation energy (joules, J)

R = gas constant (8.31 J K-1 mol-1)

T = temperature (kelvin, K)

  • A varies only a little bit with temperature, it can be considered a constant
  • Ea and R are also constants

Effect of Temperature on the Rate Constant equation 2

  • The equation shows that an increase in temperature (higher value of T) gives a greater value of ln k (and therefore a higher value of k)
  • Since the rate of the reaction depends on the rate constant (k) an increase in k also means an increased rate of reaction

5.2.5 Arrhenius sketch of ln k against 1_T_2

The graph of ln k over 1/T is a straight line with gradient -Ea/R

Exam Tip

You are not required to learn this equation however it is helpful in understanding the effects of temperature on the rate constant.

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