D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
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Influence of the surface of contact on the velocity. Additional material.

Heterogeneous reactions have two possible modes of course: kinetic and diffusive. This is due to the fact that such reactions strongly depend on the processes of inflow of reagents and removal of products from the area of reaction.

Winnie-the-Pooh is capable to eat 1 spoon of jam in a second, i.e. the speed of eating of jam makes 1 spoon/sec. But if the Rabbit will not have time to bring it to him speed of eating of jam will decrease regardless of capabilities of Winnie-the-Pooh: at speed of supply of jam at 1 jam-jar in a minute (= 30 spoons for 60 seconds = 0,5 spoon/sec) observable speed of eating of jam will be equal to speed of its supply, i.e. 0,5 spoon/sec. And so with chemical reactions.

If reagents approach to each other more quickly, than react, a limiting stage is the reaction itself, and the kinetic mode is observed.

If chemical reaction proceeds more quickly, than reagents approach to each other, observable speed of reaction is determined not by its mechanism, but by speed of diffusion of reagents to a place of reaction. It is the diffusion mode.

Before investigation of kinetics of chemical reaction, it is necessary to be convinced, that it proceeds in a kinetic mode.



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