University13 min

The steady-state approximation: deriving a rate law cleanly

Set d[I]/dt ≈ 0 without slipping on a sign or a factor of 2, and recover step by step the rate law of H₂ + Br₂, of order 3/2 at the start.

The essentials

1

Statement of the approximation

For a highly reactive intermediate I: d[I]dt≈0\dfrac{d[\ce{I}]}{dt} \approx 0 that is, sum of the rates forming I == sum of the rates consuming it. This does not say [I]=0[\ce{I}] = 0: [I][\ce{I}] is small and changes very slowly compared with the reactants.
2

When it holds

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3

Writing an intermediate's balance

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4

Chain reaction

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5

The approximation contains the other two

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