Rate Equation and Reaction Mechanism - Kinetics

In this video we want to discuss the relationship between the kinetics rate equation and reaction mechanism.

The reaction mechanism is the detailed breakdown of the overall reaction showing each elementary step that is taking place in that reaction.

One of these elementary steps is the slowest step and it determines the rate of the overall reaction. Therefore the slow step is also referred to as the rate determining step.

The rate determining step is directly related to the rate equation, where the order of reaction with respect to a reactant is the number of that reactant involved in the slow step.

Take for instance the following rate equation where order with respect to A and B is 1 and 2 respectively. This means that in the slow step, 1 A and 2 B are involved hence we can write out the slow step as follows:

rate equation and mechanism rate equation and slow step

Example 1

Let's try to write out the rate equation given the mechanism as shown:

rate equation and mechanism example 1

From the slow step there is 1 X and 1 Y as reactants. Therefore order with respect to X and Y are both 1 and the rate equation can be written as follows:

rate = k[X][Y]

Example 2

Let's have another example:

rate equation and mechanism example 2

From the slow step there is 1 AB and 1 B as reactants. Therefore order with respect to AB and B are both 1.

The rate equation can first be expressed as:

rate = k[AB][B]

However AB is an intermediate since it is formed in the first step but removed in the second step.

Therefore we need to rewrite the intermediate AB in terms of the reactants that forms AB in step 1.

Since AB is formed from 1 A and 1 B, we can write it as first order with respect to both A and B.

The rate equation will now look like this:

rate = k[A][B][B] = k[A][B]2

So we can conclude the order of reaction with respect to A is 1 and with respect to B is 2.

For the detailed step-by-step discussion on the relationship between rate equation and reaction mechanism, check out this video!

Topic: Kinetics, Physical Chemistry, A Level Chemistry, Singapore

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