Factors affecting Rate Constant (Cambridge (CIE) A Level Chemistry) : Revision Note

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:

• Where:

  • ln k = natural logarithm of the rate constant

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

  • E_a = 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

• E_a and R are also constants

• A graph of ln k against  gives a line with an equation of the form y = mx + c

• Where:

  • y = ln k 

  • x = 

  • m =  (the gradient)

  • c = ln A (the y-intercept)

• 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

Example graph of ln k over 1/T 

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