How do you write the rate of expression? how do you find the order of the reaction?

Answer 1

Rate = k [A]^a [B]^b [C]^c etc. for however many reactants there are.

The order of the reaction as regards to each reactant is determined experimentally, usually by holding the concentrations of all other reactants constant and see how it affects the rate. For example, if it was A + B + C ---> D + E, and we hold [B] and [C] constant but double [A] and see the rate double, then the reaction is first order with respect to A. If we hold [A] and [C] constant and double [B] and see the rate quadruple, then the reaction is second order with respect to B. Finally, if we hold [A] and [B] constant and double C and see no effect on the rate, then it would be zero order with respect to C. The rate expression would then be:

Rate = k [A] [B]^2

and the overall reaction order would be third.