 In parts A and B of this video, we're going to look at how you can measure and analyze rates of reaction experimentally, and in doing so, learn something about the reaction. First, a quick definition of rate. The rate of a reaction is how fast it proceeds, its speed. Most of the time when you're studying rates, you will measure the concentrations of reactants and products in moles per litre, and because you're looking at how fast the reactants are being used up, or how fast the products are being produced, the speed of your reaction is therefore measured as the changing concentration per time. This means the units are moles per litre per time, and the time unit can be whatever's convenient, seconds, minutes, or hours or days if it's a slow reaction. This assumes, however, that you're able to measure concentrations in your reaction. If that's difficult, you might instead measure the mass of a reactant or product, like if it's a gas. In this case, you can also express rates as grams per second or moles per second. Second, a couple of notes about stoichiometry. You're going to need your stoichiometry skills in kinetics in order to predict, measure, and calculate amounts and concentrations of reactants and products. If you know the rate at which one of your chemicals is being used up, stoichiometry can tell you how fast the others are being used or produced. For instance, in this hypothetical reaction we've got here, A plus 3B goes to 2C. Say A is being used up at 0.01 moles per litre per second. You can see from the stoichiometry that 3Bs are used for every 1A. That means B must be used up three times as fast as A. So the rate at which B is consumed is 3 times 0.01 equals 0.03 moles per litre per second. And you can see that 2Cs are produced for each A that's used up. So the rate at which C is produced is 2 times A, which is 0.02 moles per litre per second. However, knowing the stoichiometry of a reaction can't help you predict the rate from scratch. This you have to find out by experiment. For instance, we had to measure the rate of consumption of A in the example above, before we could work out anything about the rates of B and C.