What is electron gain enthalpy?

Electron gain enthalpy is the enthalpy change when an electron is added to a gaseous atom or ion.

For oxygen: $\mathrm{O(g) + e^- \rightarrow O^-(g)}$ This is the first electron gain enthalpy, and it is given as: $\Delta_{eg}H_1 = -141\ \text{kJ mol}^{-1}$

Second electron gain enthalpy

The second electron is added to an already negatively charged ion: $\mathrm{O^-(g) + e^- \rightarrow O^{2-}(g)}$

Why is this different from the first?

In the second step, an electron is being added to a species that already has a negative charge. So the incoming electron experiences strong electrostatic repulsion from the negatively charged ion $\mathrm{O^-}$.

Because energy must be supplied to overcome this repulsion, the process is endothermic.

Sign of second electron gain enthalpy

Endothermic process means: $\Delta_{eg}H_2 > 0$

So the second electron gain enthalpy of oxygen is positive.

Checking options

• A: more negative than the first → Incorrect
• B: almost the same as the first → Incorrect
• C: negative, but less negative than the first → Incorrect
• D: a positive value → Correct

Final answer

$\boxed{\text{D: a positive value}}$