The Effect of Temperature on 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