Reaction Order
Definition and meaning of Reaction Order in chemistry.
The reaction order describes how the concentration of a reactant mathematically influences the speed of a chemical reaction. It is defined as the exponent to which a reactant's concentration is raised in the rate law equation.
In more detail
The reaction order is a numerical value that describes how the concentration of a specific reactant affects the overall speed of a chemical reaction. In the field of chemical kinetics, the rate of a reaction is expressed mathematically using a rate law. The reaction order is the exponent to which a reactant's concentration is raised within that rate law equation.
For example, if doubling the concentration of a reactant causes the reaction rate to exactly double, the reaction is strictly first-order with respect to that specific chemical. If doubling the concentration causes the rate to quadruple, it represents a second-order relationship. The overall reaction order is calculated by simply adding together the individual orders of all the reactants involved in the experimental rate law.
Reaction orders are typically small positive integers, such as zero, one, or two. However, it is entirely possible for a reaction order to be a fraction or even a negative number in highly complex chemical pathways. A zero-order reactant is particularly interesting because changing its concentration has absolutely no impact on the speed of the chemical reaction, provided there is at least some of the reactant physically present to keep the process going.
It is a critical and unbreakable rule in chemistry that reaction orders cannot be reliably predicted just by looking at a balanced chemical equation. The coefficients in a standard chemical equation only represent the overall stoichiometry, not the step-by-step pathway the molecules actually take to react.
Therefore, reaction orders must always be determined through active laboratory experiments. By carefully measuring how the initial rate of a reaction changes when the starting concentrations of the reactants are systematically varied, scientists can deduce the exact mathematical relationship and identify the true reaction order.
Key facts
| Field | Physical Chemistry |
|---|---|
| Mathematical Definition | The exponent in the rate law equation |
| Determination Method | Must be determined experimentally |
| Zero-Order Meaning | Reaction rate is independent of concentration |
| Overall Order | The sum of all individual reactant orders |
| Relationship to Equation | Not reliably related to balanced equation coefficients |
If a rate law is Rate = k[A]<sup>2</sup>[B]<sup>1</sup>, the reaction is second-order with respect to A, first-order with respect to B, and third-order overall.
Frequently asked questions
Can a reaction order be a negative number?
Yes, a negative reaction order indicates that increasing the concentration of that specific substance actually slows down the rate of the reaction.
Why can't you just use the coefficients from the chemical equation?
Coefficients show the overall ratio of molecules reacting, but reaction order depends entirely on the slowest step (rate-determining step) of the underlying chemical mechanism.
What does a first-order reaction graph look like?
A graph of the natural logarithm of concentration versus time for a first-order reaction will yield a perfectly straight line.