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The process of converting O(g) into O²⁻(g) requires 603 kJ mol⁻¹ of energy. If the first electron affinity of O(g) is -141 kJ mol⁻¹, what is the value of the second electron affinity of oxygen?

1. 603 kJ mol⁻¹
2. -603 kJ mol⁻¹
3. -744 kJ mol⁻¹
4. +744 kJ mol⁻¹

Correct answer: +744 kJ mol⁻¹

Solution

Forming O2- from O requires the sum of both electron affinities = 603 kJ/mol. With EA1 = -141 kJ/mol, EA2 = 603 - (-141) = +744 kJ/mol. The second electron gain is endothermic (positive) due to electron-electron repulsion adding to the negative O- ion, so the answer is +744 kJ/mol.

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