Reaction mechanisms
• Single step: elementary reaction
o But many reactions don’t take place in a single step (non-elementary reactions)
but occur as a sequence of steps à Elementary steps.
o The slowest E.S in a reaction mechanism: Rate-determining step (RDS)
§ Has the highest activation energy.
§ Determines the rate at which the reaction can proceed.
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Example: NO2(g) + CO(g) à NO(g) + CO2(g)
o Step 1 NO2(g) + NO2(g) à NO3(g) + NO(g) slow
o Step 2 NO3(g) + CO(g) à NO(g) + CO2(g) fast
§ Step 1 is the slow step à RDS.
§ Overall rate is dependent on the RDS.
§ The rate expression is deduced from the RDS à rate = k[NO2]2
§ Second order reaction with respect to the NO2.
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A reaction mechanism is only a theory about how a reaction takes place; can’t be
proved. A reaction mechanism must meet the following requirements to be considered
plausible:
o The elementary steps must add together to give the overall balanced equation
for the reaction.
o The reaction mechanism must be consistent with the experimentally determined
rate expression.
NO2 and NO3 on each side can be cancelled, which makes the overall equation balanced.
o The NO3 is a reaction intermediate (appears in both steps but not in the overall
equation).
Experimental data shows that the reaction is second order with respect to NO2.
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Example (slowest step isn’t the first): 2NO(g) + 2H2(g) à N2(g) + 2H2O(g)
o Step 1 NO(g) + NO(g) ⇌ N2O2(g) fast
o Step 2 N2O2(g) + H2(g) à N2O(g) + H2O (g) slow
o Step 3 N2O(g) + H2(g) à N2(g) + H2O(g) fast
The second step is the slowest = RDS
But because N2O2 is produced in step 1, [N2O2] in step 2 depends on the [NO] in step 1.
o rate = k[NO]2[H2]
o This corresponds to the experimental data for the reaction; the orders of
reaction are determined to be second order with respect to NO and first order
with respect to H2.