Q.What is an order of a reaction ?

For a rate law $rate=k[A]^x[B]^y\ldots$, obtained purely from experimental data, the order of the reaction is defined as the sum $x+y+\ldots$ of the individual concentration exponents.

For a general reaction with rate law $rate=k[A]^x[B]^y$ (where $x$ and $y$ are experimentally determined exponents, called the order of the reaction with respect to A and B respectively), the overall order of the reaction is the algebraic sum of these individual orders: $\text{order}=x+y$. Crucially, the order of a reaction is an experimentally measured quantity — determined by studying how the rate actually changes with concentration — and it may or may not be equal to the stoichiometric coefficients of the reactants in the balanced chemical equation (this only coincides for elementary/single-step reactions). The order can be zero, a positive integer, a negative number, or even a fraction, and is a purely empirical description of how the rate depends on concentration, dis …