5.1.6 Temperature & Rate Constants

• The following general reaction and rate equation will be used to discuss the effect of temperature on the rate constant, k:

A + B → C + D

Rate of reaction = k[A][B]

• The rate equation shows that rate of reaction depends on the rate constant, k, and the concentration of the reactants
  • As the rate of reaction increases the rate constant will increase

• Increasing the temperature of a reaction increases the rate of a chemical reaction
  • Remember: this does not necessarily increase the yield of a chemical reaction depending on whether a reaction is endothermic or exothermic according to Le Châtelier’s principle

• Therefore, increasing the temperature also increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged

• An exponential relationship between the rate of reaction and temperature is observed when seen on a graph:

Relationship between temperature and rate constant, k

• The graph shows that the rate of reaction roughly doubles with an increase of 10 ^oC
  • This general relationship does not apply to all reactions
  • Also, it is not necessarily every 10 ^oC, the rate may double every 9 °C or 11 °C
  • The number of degrees needed to double the rate also changes gradually as temperature increases

• When the temperature of a reaction mixture increases, there are two contributing factors to the increased rate and rate constant:
  • Increasing temperature causes the particles to move around faster resulting in more frequent collisions
  • Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction

The Maxwell-Boltzmann distribution curve at T ^oC and when the temperature is increased by 10 ^oC

• Therefore, an increase in temperature causes an increased rate of reaction due to:
  • There being more effective collisions as the particles have more kinetic energy, making them move around faster
  • A greater proportion of the molecules having kinetic energy greater than the activation energy

• The increase in proportion of molecules having kinetic energy greater than the activation has a greater effect on the rate of reaction than the increase in effective collisions