D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
Table of content

Influence of temperature on the velocity of chemical reaction.

The velocities of the most chemical reactions increases with the rise of temperature. The temperature affects the number and the energy of collisions. The influence of the temperature on the number of molecule collisions could be shown by the model.

In a first approximation the influence of temperature on the velocity of reactions is defined by the rule of Vant-Hoff (it was stated by J.H.Vant-Hoff, who discovered it by analysing a great number of reactions):

In a temperature range from 0oC up to 100oC a 10-degree rise of temperature leads to 2-4 time increase of the velocity of chemical reaction

where g - the temperature coefficient varying from 2 to 4.

Do you want to know why the rule of Vant-Hoff is only the first approximation?

The explanation to this rule was given by S. Arrhenius, who stated that just the strongest collisions between the molecules of reagents lead to reaction. Only the molecules with an excess of kinetic energy are capable to provoke chemical reaction.

S.Arrhenius calculated the part of active (i.e. bringing to reaction) impacts of the reacting particles - a, which depends on temperature: a = exp(-E/RT). From this, he deduced the equation of Arrhenius:

k = koe-E/RT

where ko and E depend on the character of reagents. E is the energy which is necessary for the molecules to interact, referred to as the energy of activation.

Do you want to know, why?


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