Kinetics: Rate of Reaction, Order of Equation

Edited by Jamie (ScienceAid Editor)


Rate Equations

The rate of reaction is the change of concentration of a substance in a given time. Whether that be reactants disappearing or products appearing; the rate of reaction is affected by the temperature. However, the chemical equation does not tell us how fast things happen, for this we use a rate equation.

Was this helpful? Yes | No | I need help

rate equation

[A] means the concentration of A, k is the rate constant and m and n are the order of the reaction. the values of m and n can only be found by experimentation and have nothing to do with the moles of substance. The addition of m and n gives you overall order.

Was this helpful? Yes | No | I need help

Orders of Reaction

In a zero order reaction, the rate=k since anything to the power of 0 is 1. Therefore the rate of reaction does not change over time and the [A] (for example) changes linearly.

In a first order reaction, the rate and concentration are proportional. This means that if the concentration is doubled, the rate will double.

And finally, in a second order reaction, if the concentration is doubled, the rate will increase by a factor of 4 (22). The speed at which the [A] changes is much faster in a second order reaction.

Determining the Rate

As we said above, the orders of a reaction can only be found by using experimental data, so now you will learn how to do that.

example of determining the rate equation

Here we need to find m and n in the equation: rate = k[A]m[B]n.

In order to do this you need to compare individual experiments. Look at experiment 1 and then experiment 2. [A] is doubled and [B] is the same, so we can deduce the order with respect to A. The rate increases by a factor of 4 which is 22 so m is 2.

Was this helpful? Yes | No | I need help

Now we do the same thing for n. If you compare experiments 2 and 3, the initial [B] is doubled, the initial rate stays the same so n is 0. Therefore the overall equation is: rate = k[A]2[B]0.

The overall order is 2, and this can be seen when comparing experiments 1 and 4, both concentrations are trebled, and the rate increases by a factor of 9.

Units of k

The units of k (the rate constant) vary according to the overall order of the equation. Fortuneately it follows an easy to follow pattern, so remembering the below table should be very easy.

Overall order (n+m) Units of k
0 mol dm-3s-1
1 s-1
2 mol-1 dm3 s-1
3 mol-2 dm6 s-1
4 mol-3 dm9 s-1


ScienceAid welcomes all comments. If you do not want to be anonymous, register or log in. It is free.

Featured Author
166 Articles Started
1,267 Article Edits
42,900 Points
Jamie is a featured author with ScienceAid. Jamie has achieved the level of "Captain" with 42,900 points. Jamie has started 166 articles (including this one) and has also made 1,267 article edits. 8,700 people have read Jamie's article contributions.
Jamie's Message Board
Jamie: Hi, my name is Jamie.
Jamie: Can I help you with your problem about "Kinetics: Rate of Reaction, Order of Equation"?

Article Info

Categories : Physical

Share this Article:

Thanks to all authors for creating a page that has been read 79 times.

Do you have a question not answered in this article?
Click here to ask one of the writers of this article

Thank Our Volunteer Authors.

Would you like to give back to the community by fixing a spelling mistake? Yes | No