For most reactions the concentration of the reactants affects the rate of reaction.
A + B → Products
rate = k [A]n [B]m
If doubling [A] doubles the reaction rate, then it is first order.
rate = k[A]1[B]0
If doubling [B] quadruples the reaction rate, then it is second order.
rate = k[A]0[B]2
If doubling [A] octuples the reaction rate, then it is third order.
rate = k[A]3[B]0
| Overall Order | Units |
|---|---|
| O | mol l-1 s-1 | 1 | s-1 |
| 2 | mol-1 l s-1 |
| 3 | mol-2 l2 s-1 |
Most reactions are believed to proceed by a series of steps rather than just one.
This series of steps is know as the reaction mechanism.
The slowest step in a reaction is known as the rate determining step.
Because the slowest step will determine the overall rate of the reaction, we only care about the slow step.
When writing the mechanism for the slow step, the order of each reactant becomes it coefficient.