Q.What are the characteristics of order of a reaction ?

Order of reaction is a purely experimental quantity derived from the rate law, not the balanced equation, can take any numerical value including zero or a fraction, and for multi-step mechanisms is fixed by the slowest step.

1. Experimentally determined: Order of a reaction cannot be predicted merely by looking at the stoichiometric (balanced) chemical equation; it must be determined experimentally by studying how the rate of reaction varies with the concentration of each reactant (e.g. by the method of initial rates, or graphically). Order is a purely empirical quantity found from the actual rate law of the reaction.

2. Can be zero, whole number, or fractional: Unlike molecularity (which must be a small positive whole number, 1, 2, or 3), the order of reaction can be zero (rate independent of a reactant's concentration, as in some surface-catalysed/heterogeneous reactions), a positive whole number (1, 2, 3, ...), or even a fraction (e.g. order $3/2$), and in principle can also be negative with respect to a particular species (if increasing its concentration decreases the rate, as in some inhibited reactions).

3. Defined from the rate law, not stoichiometry: If the experimentally established rate law is $\text{Rate} = k[A]^x[B]^y$, then the order with respect to $A$ is $x$, with respect to $B$ is $y$, and the overall order of the reaction is $x+y$. These exponents ($x$, $y$) are NOT necessarily equal to the stoichiometric coefficients of $A$ and $B$ in the balanced equation — they must come from experiment.

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