Rate equation

This is an equation which shows how the various reactant concentrations affect the rate of a particular reaction. It is not possible to work out the rate equation for a reaction just by looking at the chemical equation for the reaction. The rate equation is worked out following experimental work being done on the reaction. We may be looking at a chemical reaction involving just two chemicals:

A + B ⇒ C + D

Until we do some experiments we are unaware of how each of the reactants A and B might affect the rate of reaction. It may be that one of them has no effect at all on the rate of reaction, and so would not appear in the rate equation. The other likely results that we see at "A" level are a linear relationship (you double the concentration and the rate doubles) and a square relationship (you double the concentration and the rate quadruples). There are various ways of studying the effect that each reactant has on the overall rate of reaction, and you will investigate some of these in the practical work. Having analyzed our experimental results we can write the rate equation for the particular reaction concerned. It may look like:

Rate = k[B]

In this case, the reactant A did not affect the rate, and reactant B affected the rate in a linear manner. k is a constant known as the rate constant.

You should be able to work out the units of the rate constant. They depend on the overall order of the reaction. Try to work out the correct units for k in a zero order, first order and second order reaction. Check your answers here .