Order of a Reaction

Defining order

In the general rate law

the exponents $x$ and $y$ describe how sensitive the rate is to a change in the concentration of $A$ and of $B$ individually — $x$ is the order of the reaction with respect to $A$, and $y$ the order with respect to $B$. Their sum, $x + y$, is the overall order of the reaction:

The sum of the powers of the concentration terms in the rate law expression is called the order of that chemical reaction.

Order is not restricted to whole numbers — it can be $0$, $1$, $2$, $3$, or even a fraction. A zero-order reaction is one whose rate does not change with the concentration of the reactants at all.

Why order needs a mechanism behind it

A balanced chemical equation is really just an accounting statement of reactants and products; it almost never shows what actually happens step by step, because most reactions do not go to completion in a single step. A reaction that does occur in one single step is called an elementary reaction. When a reaction instead proceeds through a sequence of elementary reactions — called its mechanism — before yielding the final products, it is called a complex reaction.

Two everyday patterns illustrate this:

• Consecutive reactions — for example, the oxidation of ethane to CO₂ and H₂O does not happen in one leap; it passes through a series of intermediate stages in which alcohol, then an aldehyde, and then an acid are formed along the way.
• Reverse and side reactions — for example, nitration of phenol does not give a single clean product; it yields both ortho-nitrophenol and para-nitrophenol side by side.

The units of the rate constant

For the general reaction $aA + bB \rightarrow cC + dD$ with rate law $\text{Rate} = k[A]^x[B]^y$ and overall order $n = x + y$, the rate constant can be isolated as

$k = \frac{\text{Rate}}{[A]^{x}[B]^{y}} = \frac{\text{concentration}}{\text{time}} \times \frac{1}{(\text{concentration})^{n}} \quad (\text{where } [A]=[B])$ …