There are different ways to determine the rate of a reaction. The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.
The mean rate of reaction can be calculated using either of these two equations:
The change in mass of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, Mr.
The units for rate are usually g/s or g/min.
The change in volume of a reactant or product can be followed during a reaction. This method is useful when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.
The units for rate are usually cm3/s or cm3/min.
The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.
The gradient of the line is equal to the rate of reaction:
The rate of a chemical reaction can also be measured in mol/s.
For example, if two moles of a product were made during ten seconds, the average rate of reaction would be 2 ÷ 10 = 0.2 mol/s.