IB Chemistry HL

Revision Notes

16.1.5 Mechanism Problems

Mechanism Problems

  • Chemical kinetics can only suggest a reaction mechanism, they cannot prove it
    • However, they can be used to disprove a proposed mechanism
  • Elementary steps are the steps involved in a reaction mechanism
    • For example, in the following general reaction:

A + B → C + D

    • The elementary steps could involve the formation of an intermediate:

Elementary step 1: A → R + D

Elementary step 2: R + B → C

  • It is important that the elementary steps for a proposed mechanism agree with the overall stoichiometric equation
    • For example, combining the 2 elementary steps above gives the overall stoichiometric equation

A + R + B → R + C + D

A + B → C + D

Worked Example

Sulfur dioxide reacts with oxygen to form sulfur trioxide

  1. Propose a one step mechanism for the above reaction
  2. The above reaction is catalysed by the formation of nitrogen dioxide from nitrogen monoxide. Propose a two step mechanism for this reaction.

Answers

Answer 1: A one step reaction mechanism is simply the overall stoichiometric equation

Therefore, the correct answer is 2SO2 + O2 → 2SO3

Answer 2: One of the two elementary steps for this two step mechanism can be taken from the question:

Elementary step 1: 2NO + O2 → 2NO2

The second elementary step must involve the reaction of the nitrogen dioxide formed with sulfur dioxide:

Elementary step 2: NO2 + SO2 → NO + SO3 (or 2NO2 + 2SO2 → 2NO + 2SO3)

Exam Tip

It is important that you check that the equations you are proposing for a reaction mechanism.

They must add up to the overall stoichiometric equation, otherwise the proposed mechanism is wrong.

Predicting the reaction mechanism

  • The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction
    • This shows the individual reaction steps which are taking place
  • For example, nitrogen dioxide (NO2) and carbon monoxide (CO) react to form nitrogen monoxide (NO) and carbon dioxide (CO2)
    • The overall reaction equation is:

NO2 (g) + CO (g) → NO (g) + CO2 (g)

    • The rate equation is:

Rate = k [NO2]2

  • From the rate equation, it can be concluded that the reaction is zero-order with respect to CO (g) and second-order with respect to NO2 (g)
  • This means that there are two molecules of NO2 (g) involved in the rate-determining step and zero molecules of CO (g)
    • This means that in terms of molecularity, the rate determining step is bimolecular
  • A possible reaction mechanism could therefore be:

Step 1:

   2NO2 (g) → NO (g) + NO3 (g)                   slow (rate-determining step)

Step 2:

   NO3 (g) + CO (g) → NO2 (g) + CO2 (g)     fast

Overall:

    2NO2 (g) + NO3 (g) + CO (g) → NO (g) + NO3 (g) + NO2 (g) + CO2 (g)

   =     NO2 (g) + CO (g) → NO (g) + CO2 (g)

Exam Tip

It is important that the elementary steps for a proposed mechanism also agree with the experimentally determined rate equation

The rate equation and the overall reaction must be related, i.e. the correct chemical species involved

Remember: There is no direct link between the orders in the rate equation and the stoichiometry of the overall equation

However, the rate equation can be derived directly from the rate determining step and its stoichiometry

Predicting the reaction order & deducing the rate equation

  • The order of a reactant and thus the rate equation can be deduced from a reaction mechanism if the rate-determining step is known
  • For example, the reaction of nitrogen oxide (NO) with hydrogen (H2) to form nitrogen (N2) and water

2NO (g) + 2H2 (g) → N2 (g) + 2H2O (l)

  • The reaction mechanism for this reaction is:

Step 1:

   NO (g) + NO (g) → N2O2 (g)                      fast

Step 2:

   N2O2 (g) + H2 (g) → H2O (l) + N2O (g)     slow (rate-determining step)

Step 3:

   N2O (g) + H2 (g) → N2 (g) + H2O (l)           fast

  • The second step in this reaction mechanism is the rate-determining step
  • The rate-determining step consists of:
    • N2O2 which is formed from the reaction of two NO molecules
    • One H2 molecule
  • The reaction is, therefore, second order with respect to NO and first order with respect to H2
  • So, the rate equation becomes:

Rate = k [NO]2 [H2]

  • The reaction is, therefore, third order overall

Exam Tip

Intermediates in the mechanism cannot appear as substances in the rate equation

This is why you substitute the N2O2 in the above example. Step 1 shows that 2NO molecules are required to form the necessary N2O2

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