Reaction Mechanisms - Deduction
- 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
- Propose a one step mechanism for the above reaction
- 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)
Examiner 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 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
Examiner Tip
Intermediates in the mechanism cannot appear as substances in the rate equationThis is why you substitute the N2O2 in the above example. Step 1 shows that 2NO molecules are required to form the necessary N2O2