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Ozone reacts with oxygen atoms in the presence of chlorine atoms by the two-step mechanism: Step (i): O3(g) + Cl(g) -> O2(g) + ClO(g), rate constant ki = 5.2 x 10⁹ L mol⁻¹ s⁻¹ Step (ii): ClO(g) + O(g) -> O2(g) + Cl(g), rate constant kii = 2.6 x 10¹⁰ L mol⁻¹ s⁻¹ What is the closest value of the rate constant for the overall reaction O3(g) + O(g) -> 2O2(g)?

1. 3.1 x 10¹⁰ L mol⁻¹ s⁻¹
2. 2.6 x 10¹⁰ L mol⁻¹ s⁻¹
3. 5.2 x 10⁹ L mol⁻¹ s⁻¹
4. 1.4 x 10²⁰ L mol⁻¹ s⁻¹

Correct answer: 5.2 x 10⁹ L mol⁻¹ s⁻¹

Solution

The slowest step controls the overall rate; since ki (5.2 x 10⁹) is smaller than kii (2.6 x 10¹⁰), step (i) is rate-determining and the overall rate constant is approximately 5.2 x 10⁹ L mol⁻¹ s⁻¹.

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