To calculate the order of reaction for a reactant:

- Write the general rate expression for the reaction.
- Write the general rate expression for each experiment.
- Take the ratio of the rate expressions from two different experiments. (eg. Divide Rate
_{2}by Rate_{1}.) - Cancel out the rate constant and any concentrations that are the same in the two experiments.
- Put in the data and use logs to find the order.

For example, given the following experimental results, what is the order of reaction for the reactant A and what is the rate law for this reaction?

Expt | [A] | Init Rate |
---|---|---|

#1 | 1.5 | 4.2 |

#2 | 2.0 | 6.2 |

The general rate law for the reaction is rate=k[A]^{x}.

The rate expressions for each experiment are:

rate_{1}=k[A]_{1}^{x} and rate_{2}=k[A]_{2}^{x} or 4.2 = k(1.5)^{x} and 6.2 = k(2.0)^{x}

Dividing rate_{2} by rate_{1} gives:

6.2/4.2 = (2/1.5)^{x} and taking the ln of both sides gives:

ln(6.2/4.2) = x[ln(2/1.5)] or ln(1.48) = x[ln(1.33)]

x = ln(1.48)/ln(1.33) = 1.37 and rate = k[A]^{1.37}

Use the first experiment to find k:

k = 4.2/(1.5)^{1.37} = 4.2/1.743 = 2.4 and

rate = (2.4)[A]^{1.37}