Rate-Law Expression
Definition and meaning of Rate-Law Expression in chemistry.
A rate-law expression is a mathematical equation showing how the reaction rate depends on reactant concentrations and a rate constant. It has the form Rate = k[A]^m[B]^n and must be determined experimentally.
In more detail
In this expression, k is the rate constant, [A] and [B] are the concentrations of reactants, and m and n are the reaction orders determined from experimental data. These orders cannot be predicted from stoichiometric coefficients; they reflect the actual reaction mechanism. The overall reaction order (m + n) shows how the reaction rate responds to concentration changes. Rate laws are fundamental to chemical kinetics for understanding reaction mechanisms and predicting how reactions behave under different conditions.
Key facts
| General form | Rate = k[A]^m[B]^n (k is rate constant; m, n are reaction orders) |
|---|---|
| Experimentally determined | Cannot be predicted from the balanced equation |
| Temperature dependence | The rate constant k generally increases with temperature (Arrhenius behavior); a few reactions show the reverse trend |
| Field | Physical Chemistry |
For the reaction 2NO + O2 → 2NO2, experiments show the rate law is Rate = k[NO]^2[O2], indicating second-order dependence on NO and first-order on O2.
Frequently asked questions
Why must the rate law be determined experimentally?
The rate law depends on the reaction mechanism and elementary steps, not the overall stoichiometry. Only experiments can reveal how concentrations affect the rate.
What does 'second-order in a reactant' mean?
The reaction rate is proportional to the square of that concentration. If you double the concentration, the rate quadruples (2^2 = 4).