Question

Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?

Answer 1

Balanced chemical equation often leads to incorrect order or rate law. e.g., the following reaction seems to be a tenth order reaction

$KCI{O}_3+6FeS{O}_4+3H_{2}S{O}_4\to KCl+3H_{2}O+3F{e}_2(S{O}_4{)}_3$

This is actually a second order reaction. Actually, the reaction is complex and occurs in several steps. The order of such reaction is determined by the slowest step in the reaction mechanism. The order is determined experimentally and is confined to the dependence of the observed rate of reaction on the concentration of reactants.