# how to determine order of a reaction?tell me in detail.

SAGAR SINGH - IIT DELHI
878 Points
13 years ago

Orders of reaction are always found by doing experiments. You can't deduce anything about the order of a reaction just by looking at the equation for the reaction.

So let's suppose that you have done some experiments to find out what happens to the rate of a reaction as the concentration of one of the reactants, A, changes. Some of the simple things that you might find are:

One possibility: The rate of reaction is proportional to the concentration of A

That means that if you double the concentration of A, the rate doubles as well. If you increase the concentration of A by a factor of 4, the rate goes up 4 times as well.

You can express this using symbols as:

Writing a formula in square brackets is a standard way of showing a concentration measured in moles per cubic decimetre (litre).

You can also write this by getting rid of the proportionality sign and introducing a constant, k.

Another possibility: The rate of reaction is proportional to the square of the concentration of A

This means that if you doubled the concentration of A, the rate would go up 4 times (22). If you tripled the concentration of A, the rate would increase 9 times (32). In symbol terms:

Generalising this

By doing experiments involving a reaction between A and B, you would find that the rate of the reaction was related to the concentrations of A and B in this way:

This is called the rate equation for the reaction.

The concentrations of A and B have to be raised to some power to show how they affect the rate of the reaction. These powers are called the orders of reaction with respect to A and B.

All the best.