Factors affecting Rate Constant (CIE A Level Chemistry)

• 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:

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

• 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

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