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In a reaction relevant to upper-atmosphere chemistry, H2O and O combine bimolecularly to form two OH radicals. For this forward reaction at 500 K, deltaH = 72 kJ and the activation energy Ea(forward) = 77 kJ/mol. Calculate the activation energy for the reverse reaction (recombination of 2 OH radicals to give H2O and O) at 500 K.

1. 5 kJ mol⁻¹
2. 149 kJ mol⁻¹
3. 77 kJ mol⁻¹
4. 72 kJ mol⁻¹

Correct answer: 5 kJ mol⁻¹

Solution

Using deltaH = Ea(forward) - Ea(reverse), the reverse activation energy is Ea(reverse) = Ea(forward) - deltaH = 77 - 72 = 5 kJ/mol.

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