Energy involved to ionise O2(g) to O2−(g) is 639kJ mol−1. If the ionisation enthalpy of O−(g) is 141kJ mol−1. The second electron gain enthalpy of oxygen would be:
A
−780kJ mol−1
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B
780kJ mol−1
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C
498kJ mol−1
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D
−498kJ mol−1
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Solution
The correct option is B780kJ mol−1 Given: O(g)+2e−→O2−(g);ΔH=639kJ mol−1...(i) O−(g)→O(g)+e−;ΔH=141kJ mol−1...(ii)
Adding equation (i) and (ii), we get O−(g)+e−→O2−(g);ΔH=780kJ mol−1...(iii)