 Let's have a look at another example. In this reaction two bromine atoms are combining to form a diatomic bromine molecule. An experiment has been performed where the initial concentration of bromine atoms is varied systematically and the rate of reaction is measured. As you can see for this reaction when the bromine atom concentration doubles the rate quadruples. You can see we go from 0.1 to 0.2 and the rate goes from 1.2 to 4.8 moles per litre per second. If you look at the first and third experiments you can see that when the concentration triples the rate is multiplied by 9 and from the first and fourth experiments you can see that when the concentration quadruples the rate is multiplied by 16. If you line all this up you can clearly see that the rate is not proportional simply to the concentration of bromine atoms but instead to the square of their concentration. And if we want to convert that to a proper equation we need the rate constant as before so it becomes rate equals K Br squared.