A-level Chemistry/AQA/Module 4/Kinetics
Order and rate equations
The reaction is investigated by running several series of experiments. In each series just one of the reactants is varied so that we may observe the effect on the rate of reaction. Here there are two reactants A and B. The units of rate are change of concentration divided by time (e.g. moles per litre by seconds). For any one reactant the change in the rate of the reaction is categorised as:
- Zero order
- Any increase in the concentration of the reactant has no effect on the rate of the reaction.
- First order
- The rate of reaction increases in direct proportion to the increased concentration of the reactant. This can be observed by comparing runs 1 and 2. The concentration of A remains constant whilst the concentration of B is doubled (x2). Consequently the rate of the reaction is doubled from 0.8 to 0.16. We would say that this reaction is first order with respect to B.
- Second order
- The rate of the reaction increases in proportion to the square of the concentration change. This can be observed by comparing runs 1 and 3. The concentration of B remains constant whilst the concentration of A is doubled. Consequently the rate of the reaction is quadrupled (x4) from 0.8 to 0.32. This reaction is second order with respect to A.
|Run||Initial concentration of A||Initial concentration of B||Initial rate|
It should be made clear that rates can only be determined by experiment.
Constructing Rate Equations
Rate equations allow us to predict the rate of a reaction under different conditions. They take the form
- [A] is the concentration of reactant A (in moldm-3) and a is the order of reaction with respect to A
- [B] is the concentration of reactant B (in moldm-3) and b is the order of reaction with respect to B
- k is the Rate Constant. This depends on the reaction itself - how inherently fast it is - temperature, and any catalysts.
The rate equation can be rearranged in the normal manner to find any one of the required variables. At A level chemistry this is often finding k given data like above. The overall order of the reaction is the sum of the indexes a+b ..etc.
Rate determining step
Mechanisms can have one step or a series of steps in which each step can have a different rate. The overall rate is decided be the step with the slowest rate known as the rate determining step.