Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.8 Mechanism from rate equation-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.8 Mechanism from rate equation- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.8 Mechanism from rate equation- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
Key Concepts:
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
11.8 Deducing a Reaction Mechanism
A reaction mechanism can be deduced using both the rate equation and the overall stoichiometric equation by proposing steps that are consistent with experimental data.
Key Idea
- Overall equation shows total reactants and products
- Rate equation shows species in rate-determining step
- Mechanism must satisfy both
Method
- Step 1: Write overall (stoichiometric) equation
- Step 2: Identify rate equation
- Step 3: Determine species in rate-determining step
- Step 4: Propose slow step matching rate equation
- Step 5: Add fast steps to give overall equation
Key Rules
- Slow step must match rate equation
- Mechanism must sum to overall equation
- Intermediates are formed and then consumed
- Intermediates do not appear in overall equation
- Fast steps complete the reaction
Important Insight
- If a species appears in the overall equation but not in the rate equation → it is not in the slow step
- If a species appears in rate equation but not overall → it may be an intermediate or catalyst
Example 1
Overall equation:
\( \mathrm{A + 2B \rightarrow C} \)
Rate equation:
\( \mathrm{rate = k[A][B]} \)
Deduce a possible mechanism.
▶️ Answer/Explanation
Rate equation shows A and B in slow step
Step 1 (slow): \( \mathrm{A + B \rightarrow X} \)
Step 2 (fast): \( \mathrm{X + B \rightarrow C} \)
X is an intermediate
Steps add to give overall equation
Example 2
Overall equation:
\( \mathrm{2NO + O_2 \rightarrow 2NO_2} \)
Rate equation:
\( \mathrm{rate = k[NO]^2} \)
Suggest a mechanism consistent with the data.
▶️ Answer/Explanation
Rate equation depends only on NO → O₂ not in slow step
Step 1 (slow): \( \mathrm{NO + NO \rightarrow N_2O_2} \)
Step 2 (fast): \( \mathrm{N_2O_2 + O_2 \rightarrow 2NO_2} \)
N₂O₂ is an intermediate
Mechanism matches rate equation and overall reaction