Atomic StructureHard

Question

The formation of O2-(g) starting from O(g) is endothermic by 639 kJ mol-1 . If electron gain enthalpy of O(g) is –141 kJ mol-1 , the second electron gain enthalpy of oxygen would be :-

Options

A.-780 kJ mol-1
B.+ 780 kJ mol-1
C.+ 498 kJ mol-1
D.- 498 kJ mol-1

Solution

O(g) + 2e- → O2(g) ; ᐃH = 639 kJ mol-1          .......(i)
O(g) + e- → O-(g) ; ᐃH = - 141kJ mol-1           ......(ii)
Subtracting equation (ii) from equation (i)
O-(g) + e-  → O2-(g) ; ᐃH = 639 + 141 = 780 kJ mol-1.

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