It is important that chemists can control the rate of chemical reactions to ensure that processes are both economically viable (they will result in a good yield of products and profits for the company) and safe (the reaction does not progress too quickly potentially causing explosions).
The rate of a chemical reaction is proportional to concentration of reactants present. As reactants are used up during the process, the rate will decrease, and the reaction slows down.
By monitoring a chemical reaction and making measurements on how volume, concentration or mass change, the rate can be calculated.
The graph below shows how the rate in a chemical reaction changes as the reaction proceeds.
The average rate of reaction can be calculated by considering how the mass changes over a fixed period of time. This will give a rate measured in grams per second (g s-1) using the formula:
The relative rate of reaction is the rate at any one particular point in time.
This could not be measured using the results of an experiment, but since the rate of the reaction is proportional to time, relative rate can be given by the formula:
For example, the relative rate of a reaction at 20 seconds will be 1/20 or 0.05 s-1, while the average rate of reaction over the first 20 seconds will be the change in mass over that period, divided by the change in time.
Note that the units of relative rate are s-1 as no measurable change is being observed, whereas for average rate the unit used depends on the measurable quantity.
In the above graph, since a change in mass is measured in grams and a change in time is measured in seconds (in this example), the unit of rate would be grams per second (g s-1).
Similarly, if a change in concentration is measured (in mol l-1), then rate will have the unit moles per litre per second (mol l-1 s-1).
If a change in volume is measured (in cubic centimetres, cm3), the unit of rate would be centimetres cubed per second (cm3 s-1).