Influence of Temperature on Rate Constants

6 INFLUENCE OF TEMPERATURE ON RATE CONSTANTS 2.6.1 Arrhenius Equation 

From the earliest studies of reaction rates, it has become evident that they are profoundly influenced by temperature. Harcourt (1867) established a well-known rule of thumb that the rates of many reactions approximately doubled for each 10 rise in temperature. The early studies of van t Hoff (1884) and Arrhenius (1889) form the starting point for all modem theories of the temperature dependence of equilibrium and rate constants. Van t Hoff and Arrhenius attempted to find quantitative relationships between the temperature and equilibrium and rate constants by comparing kinetic observations with the known properties of these constants. 

Any equilibrium constant K varies with the absolute temperature T in accordance with the van t Hojf law  

In order to assess properly the meaning of the activation energy, Eq. (2.40) must be written in the exponential form  

The exponential term is often called a Boltzmann term, because, according to Boltzmann s theory, the distribution of energies among molecules, the number of molecules in a mixture that have an energy in excess of is proportional to q-ejrt therefore, interpret the Arrhenius equation to mean that 

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