Rate Equations and Determining Reaction Order
The Rate Equation and Reaction Mechanisms
Definition
Rate equation
The rate equation describes the relationship between the rate of a chemical reaction and the concentrations of the reactants involved.
It takes the form:
$$\text{Rate} = k[A]^m[B]^n$$
where:
- Rate: Speed of the reaction, measured in $\text{mol dm}^{-3}$.
- $k$: Rate constant, dependent on temperature and catalysts.
- $[A]$ and $[B]$: Molar concentrations of the reactants.
- $m$ and $n$: The orders of reaction with respect to each reactant.
What Do the Orders of Reaction Mean?
Definition
Order of a reaction
The order of a reaction describes how the concentration of a reactant influences the rate.
- If $m = 0$ changing $[A]$ has no effect on the rate (zero-order).
- If $m = 1$, doubling $[A]$ doubles the rate (first-order).
- If $m = 2$, doubling $[A]$ quadruples the rate (second-order).
- The overall order of a reaction is the sum of the exponents: $$\text{Overall Order} = m + n$$
Orders of reaction are determined experimentally and are not always linked directly to the stoichiometric coefficients in the balanced equation.
Why Must Rate Equations Be Determined Experimentally?
- The rate equation reflects the rate-determining step in a reaction mechanism, the slowest step that limits the overall reaction speed.
- Since reaction mechanisms can involve complex steps that aren't obvious from the overall balanced chemical equation, experiments are necessary to determine the correct orders of reaction.
Experimental Methods to Determine the Rate Equation
- The rate equation is typically determined by measuring how the rate changes with varying concentrations of reactants.
- Common methods include:
