Consider results for a rate of reaction experiment between X and Y.

In these three runs the initial rate of reaction has been measured. The orders of reaction can be determined by comparing these runs.

Before performing run D, it is possible to calculate its initial rate of reaction.

We could use any of the runs A, B or C. Let's look at run C.

This is the rate equation for the reaction. Notice that the rate of reaction is first order with respect to X and second order with respect to Y.

Rearrange the equation to find the value of the rate constant, k.

Substitute into the equation the values obtained for run C.

Calculate the value of k.

It is 0.005 mol–2 dm6 s–1.

Section Two explains how to work out the units of k.

Now we know the value of the rate constant, k, we can calculate the initial rate of reaction for run D.

Substitute into the equation the value of k and the two concentrations for run D.

The initial rate of reaction in run D would be 0.04 mol dm–3 s–1. You can easily verify this by comparing runs A or B with run D.

Let's look at how to work out the units for the rate constant.

Consider a reaction which is first order overall.

This equation shows how to calculate the value of the rate constant, k.

The units for the rate of reaction are mol dm–3 s–1.

The units for concentration are mol dm–3.

We can cancel out the mol dm–3 at the top and bottom, leaving units of s–1.

The rate constant for a first order reaction has the unit s–1.

There is an easier way to do this. Subtract the order of reaction from 1. This gives you a factor to multiply the numbers in mol dm–3.

In this case it is zero, so the concentration units can be removed leaving s–1.

For a second order reaction the factor is –1.

This changes mol dm–3 to mol–1 dm3, which gives us units for k of mol–1 dm3 s–1.

For a third order reaction the factor is –2.

This changes mol dm–3 to mol–2 dm6, which gives us units of k of mol–2 dm6 s–1.

For a fourth order reaction the factor is –3.

This changes mol dm–3 to mol–3 dm9, which gives us units of k of mol–3 dm9 s–1.

The rate constant for a given reaction depends upon the temperature. It is only constant at a particular temperature.

As the temperature increases, so does the rate constant.

At higher temperatures, there are more frequent collisions between reactant particlesand a higher proportion of particles have the activation energy or more.