# Example of calculation of order of reaction from the graph

Mayank Ranka
4 months ago
There are several simple ways of measuring a reaction rate. For example, if a gas was being given off during a reaction, you could take some measurements and work out the volume being given off per second at any particular time during the reaction.

A rate of 2 cm3s-1is obviously twice as fast as one of 1 cm3s-1.

Note: Read cm3s-1as "cubic centimetres per second".

However, for this more formal and mathematical look at rates of reaction, the rate is usually measured by looking at how fast theconcentration of one of the reactantsis falling at any one time.

For example, suppose you had a reaction between two substancesAandB. Assume that at least one of them is in a form where it is sensible to measure its concentration - for example, in solution or as a gas.

For this reaction you could measure the rate of the reaction by finding out how fast the concentration of, say,Awas falling per second.

You might, for example, find that at the beginning of the reaction, its concentration was falling at a rate of 0.0040 mol dm-3s-1.

Note: Read mol dm-3s-1as "moles per cubic decimetre (or litre) per second".

This means that every second the concentration ofAwas falling by 0.0040 moles per cubic decimetre. This rate will decrease during the reaction asAgets used up.