Question

For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow why?

Answer 1

Threshold energy is the minimum energy required by the colliding particles to have an effective collision and result into a reaction. For certain reactions it may happen that large number of molecules colliding may possess the energy greater than threshold energy value but it will not result into a past reaction because there is one more factor responsible for this and that is orientation factor. Orientation factor is that factor which will determine the relative orientations of the colliding molecules which will result into a faster reaction or a slower reaction.

(Refer to Image)

As it can be seen by the above diagram that the relative orientation of the molecule affects the interaction of the colliding molecules so the rate of reaction will be different in two cases.