15. The Rates of Chemical Reactions   Previous PageNext Page
       Molecular Collisions and Reaction

This temperature dependence of collision frequency is almost negligible in comparison with the exponential temperature dependence of the e-Ea/RT term, which gives the fraction of the colliding molecules with energy greater than Ea.

The collision theory tells us how the experimental Rate constant should vary with temperature: There should be a straight-line relationship between the logarithm of k and the reciprocal temperature, 1/T, with a negative slope proportional to the activation energy, Ea

If we measure k at several temperatures and plot ln k against 1/T, we obtain an Arrhenius plot (fig 11 opposite). (This kind of plot is named after the Swedish chemist who first developed the theory. )

This is the most direct means of obtaining the activation energy of a reaction, and is convincing evidence for the entire picture of activation-energy barriers.

In order that the reverse reaction occurs, the same energy level for the reaction intermediates must be achieved by the collision of product molecules. If product molecules are more stable than reactants (DH0 negative), then the energy of activation will be greater for the reverse reaction than the forward.


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