IB Chemistry home > Syllabus 2016 > Kinetics > Reaction mechanisms

Syllabus ref: 16.1

Mechanism means the process that the reactant particles must go through in order to arrive at the product particles.

We have already established that collisions are an essential feature of a reaction procedure. The mechanism explains which particles actually collide and the order in which the collisions occur.

Mechanisms

The term 'mechanism' refers to the actual processes taking place in a chemical reaction.

Normally a chemical reaction is expressed by a balanced equation. This tells us the relative amounts of reactants that react and products formed. It gives us no information about any intermediate species that may be formed, or any information about the actual collisions taking place.

In reality most reactions proceed via a series of steps involving intermediates, or high energy transition states. These steps, when put together, constitute the 'story' of the reaction - they are called the mechanism.

Example: The reaction between nitrogen(II) oxide and chlorine can be represented by the equation:

2NO(g) + Cl2(g) 2NOCl(g)

A suggested mechanism for this reaction has two steps:

1. NO(g) + Cl2(g) NOCl2(g)

2. NO(g) + NOCl2(g) 2NOCl(g)

You should notice that if all of the steps of a mechanism are added up, cancelling out species that appear on each side, it gives the stoichiometric equation:

NO(g) + Cl2(g) NOCl2(g)
NO(g) + NOCl2(g) 2NOCl(g)
2NO(g) + Cl2(g) 2NOCl(g)

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Why study the mechanism?

One of the aims of the chemist when studying kinetics is to come up with suggested mechanisms for reactions. By increasing the knowledge base of a specific reaction, scientists can then suggest ways of controlling or modifying the reaction.

This could lead to more efficient and cheaper processes for making novel and existing products in industry.

Industry, both on a large and small scale, is always on the lookout for ways of making things cheaper and more cost effective.

Understanding the mechanism of formation of a specific drug may help scientists to modify it in a beneficial way, or to come up with novel hitherto unconsidered methods of synthesis.


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The rate determining step

The steps of a mechanism may be fast or slow relative to one another. The slow step of a mechanism is the rate determing step.

It is called this because the slowest stage of any operation has a greater effect on the time taken for the operation to be completed.

Imagine a construction line with five different operational steps needed to make the final product. If one of the steps is very time consuming, then this part of the process will affect the time taken far more than the others.

It would be the rate determing step of the construction line.

In a chemical reaction exactly the same constraints apply. If one step of the mechanism is much slower than the others, then it will determine the overall rate of the reaction.

Example

1. NO(g) + Cl2(g) NOCl2(g) - fast

2. NO(g) + NOCl2(g) 2NOCl(g) - slow

When we conduct experiments to measure the rate of a reaction, we are really measuring the rate of the slowest step, the rate determining step.

Any fast steps in the mechanism of the reaction will have little influence on the overall rate.


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Molecularity

All of our rates investigations are used to follow the rate determing step (the slowest step) of a mechanism. The rate equation is solved for the reaction using the process described in section 6.21.

The final rate expression gives us the orders of the reaction with respect to the individual reactants.

The actual number of particles colliding in the rate determining step is called the molecularity.

The order of each component gives the molecularity of the rate determining step (or that of the rate determining step plus any steps that feed into the rate determining step).

Example: The reaction: 2NO(g) + Cl2(g) 2NOCl(g)

1. NO(g) + Cl2(g) NOCl2(g) - fast

2. NO(g) + NOCl2(g) 2NOCl(g) - slow

The molecularity of the rate determining step (slowest) is 2, but the rate expression for this reaction is:

Rate = k[NO]2[Cl2]1

Althought the slow step of the mechanism has only one particle of NO and one particle of NOCl2, the particle of NOCl2 is not one of the reactants, it has to be made in a prior step, step 1.

In step 1, NOCl2 is made from the collision between one NO and one Cl2 particle.

Therefore overall we have needed two NO particles and one Cl2 particle to carry out the slowest step.

In the above example the mechanism was used to explain the rate expression.

In kinetics studies this is, of course, usually the reverse; the rate expression is first determined by experiment and then mechanisms are proposed that fit the rate expression.


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Determining the mechanism

A series of experiments are carried out to determine the rate expression for the reaction under study. This will reveal the order of reaction with respect to the concentration of each component.

The next stage is a little more complex in that the order wth respect to each component concentration gives us the molecularity of the rate determining step.

The problem is there may be several possible mechanisms that emerge from a consideration of this slowest step. At this point the investigator will use his knowledge of chemistry to suggest the most likely mechanism based on chemical stabilities and other factors.

It is not suggested that a student be able to do this, however it should be possible to suggest mechanisms based on the following three facts:

1. The order of reaction from the rate expression gives us the number of particles of that type involved in the rate determing step (or used in producing species that appear in the rate determing step, but that do not appear in the stoichiometric equation)

2. Three particle collisions are deemed improbable.

3. The sum of all of the steps must add up to the stoichiometric equation.

Example: The following data were obtained for the reaction between nitrogen(II) oxide gas and hydrogen. Use this data to find the rate expression and suggest a mechanism for the reaction:

2H2(g) + 2NO(g) 2H2O(g) + N2(g)
Initial [NO]/mol dm-3
Initial [H2]/mol dm-3
Initial rate/mol dm-3 s-1
6 x 10-3
1 x 10-3
3.19 x 10-3
6 x 10-3
2 x 10-3
6.36 x 10-3
6 x 10-3
3 x 10-3
9.56 x 10-3
1 x 10-3
6 x 10-3
0.48 x 10-3
2 x 10-3
6 x 10-3
1.92 x 10-3
3 x 10-3
6 x 10-3
4.30 x 10-3

By inspection order wrt [NO] = 2, order wrt [H2] = 1, therefore the rate expression is: Rate = k[NO]2[H2]1

This suggests that two particles of NO are involved and one particle of H2, but, as three particle collisions are highly improbable, there must be an equilibrium step (fast) leading into a rate determining step.

Step 1: NO + NO N2O2 (fast)

Step 2: N2O2 + H2 H2O + N2O (slow)

Step 3: N2O + H2 H2O + N2 (fast)

There is no certainty that this is the actual mechanism of the reaction, but it does fulfill all of the requirements for a possible mechanism.


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