Why are the rates of some reactions insensitive to temperature

Some rare reactions occur at a rate that appears to be insensitive to temperature. Such reactions are extremely rapid, and are termed diffusion-controlled reactions. 

If the activation energy is extremely small - of the order of 1 kJ mol-1 or so - then all the energy necessary to overcome the activation energy is available from the solvent, etc., so reaction occurs immediately the reactants collide. 

In fact, the only kinetic limitation to such a reaction is the speed at which they move through solution before the collision that forms product. This rate is itself dictated by the speed of diffusion (which is not generally an efficient form of transport). The rate of reactants colliding is, therefore, said to be diffusion controlled . Typically, diffusion-controlled processes in which Ea is tiny involve radical intermediates. 

If a second-order reaction is diffusion controlled, then its rate constant has a magnitude of about 1010 or even 1011 dm3 mol-1 s-1. 

25 The rate of a reaction is said to be diffusion controlled because its activation energy is 1.4 Id mol-1. The rate constant of reaction is 4.00 x 1010 dm3 mol-1 s 1 at 298 K. Show that the rate constant is effectively the same at 330 K. 

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