alleryStart learning →What rate law does a unimolecular elementary step obey?
Arate = k, so it is zero order
Brate = k[A], so it is first order
Crate = k[A]^2, so it is second order
Drate = k[A][B], so it is second order
Answer & Solution
Correct answer: B. rate = k[A], so it is first order
1. A unimolecular step involves the rearrangement of a single reactant species.
2. Doubling the concentration doubles the number of molecules per litre able to react.
3. The fraction with enough energy stays the same, so twice as many react and the rate doubles.
4. A rate directly proportional to concentration is first order, so rate = k[A].
5. Note that this deduction from the equation is allowed only because the step is elementary.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 12 "Kinetics", section 12.6 Reaction Mechanisms_
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