CAIE A-Level Chemistry 26.1.1 Rate and Mechanism Terms

Practise using rate-equation, order, rate constant, half-life, intermediate and rate-determining-step terms.

Exam points

define reaction order as the exponent of a reactant concentration in the experimental rate equation
define overall order as the sum of all concentration exponents and k as the rate constant
identify the rate-determining step as the slowest mechanism step and an intermediate as formed then consumed

[Maximum number: 1]

A four-step mechanism is suggested for the reaction between hydrogen peroxide and iodide ions in an acidic solution.

$step $1 \quad \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{I}^{-} \rightarrow \mathrm{IO}^{-}+\mathrm{H}_{2} \mathrm{O}$$

$step $2 \mathrm{H}^{+}+\mathrm{IO}^{-} \rightarrow \mathrm{HIO}$$

$step $3 \mathrm{HIO}+\mathrm{I}^{-} \rightarrow \mathrm{I}_{2}+\mathrm{OH}^{-}$$

$step $4 \mathrm{OH}^{-}+\mathrm{H}^{+} \rightarrow \mathrm{H}_{2} \mathrm{O}$$

Step 1 is the rate-determining step.

State what is meant by the term rate-determining step.

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