A level Chemistry · Edexcel 9CH0 · Topic 16B
Reaction mechanisms
Key points
- A mechanism is a series of steps; the slowest is the rate-determining step (RDS).
- The rate equation includes only species involved in or before the RDS; a species in the overall equation but zero order reacts after the RDS.
- Intermediates form in one step and are used up in a later step; a catalyst may appear in the rate equation.
- SN1 (tertiary halogenoalkanes): rate = [RX]; the RDS is the C–X bond breaking to form a carbocation.
- SN2 (primary halogenoalkanes): rate = [RX][OH⁻]; one step via a transition state.
- Propanone–iodine in acid: rate = [CH₃COCH₃][H⁺], zero order in I₂, so I₂ reacts after the RDS.
- When proposing a mechanism, make sure the steps add up to the overall equation and match the orders.
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Most reactions happen in a series of simple steps called a mechanism. The rate equation cannot prove a mechanism, but it is strong evidence: it tells you which particles collide in the slowest step, and any proposed mechanism must agree…
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