Does Increasing the Concentration of a Reactant Speed up an Experiment?

Dave - A reaction is normally to do with reacting two different substances together, so a type of molecule, we'll call molecule A and molecule B. If we just think about one molecule of molecule A, if you have a great big vat with only a few molecules of B in it, molecule A is going to wonder around gently throughout this huge vat, and because there's hardly any molecules of B in there, it's almost never going to meet a molecule B. So, it's very rarely going to react, so it's going to have a very, very slow rate of reaction. If you've got loads and load of molecule B, so billions and billions in there, it's hardly going to have to travel any distance at all. So the time it takes for a single molecule of A to react is going to be much, much less. So the greater concentration of B there is, the higher the rate of reaction. Similarly of course, the more of A there are, the more times this is happening at once, so the rate of reaction is going to increase. So, in general, the total rate of reaction is related to the concentration of molecule A times the concentration of molecule B.