Question

Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.

Answer 1

Answer (Short)

The second electron gain enthalpy of oxygen is expected to be positive, i.e., energy must be supplied.

Stepwise Process in LaTeX

First electron addition: O(g) + e^- -> O^-(g) This process is exothermic (electron gain enthalpy is negative) because a neutral atom attracts the incoming electron.

Second electron addition: O^-(g) + e^- -> O^{2-}(g) Here, an electron is added to an already negatively charged ion.

Reason

• O^- already has extra electron density and carries a negative charge.
• The incoming electron (also negative) experiences strong electrostatic repulsion from the O^- ion.
• To force this second electron onto O^-, energy must be supplied to overcome this repulsion.

Therefore, the second electron gain enthalpy: \[ \Delta_{\text{eg}}H_2(\text{O}) > 0 \] i.e., it is positive (endothermic), whereas the first electron gain enthalpy is negative (exothermic).