Please explain why order of a reaction differs from the value of the sum of the powers of the concentration terms involving in the equation for forward reaction rate as per the law of mass action.

jessu

If I understand you correctly, the reason is that reactions can be multistep. If a reaction procedes through a series of steps, and one step is slower than the rest, that step can dominate the reaction rate.

Thank you for ur reply.You are right.This explanation is given in the text book.But confusion is what should be the equation for equilibrim constant for the chemical changes occuring in multiple steps.Will this be equilibrium constant for the rate determining step?

If there is one step that is truly rate determining, yes. Often, things are more complicated and several steps contribute to the rate order.