Question

The first electron gain enthalpy of oxygen is \( -141 \, \text{kJ mol}^{-1} \), its second electron gain enthalpy is:

• A. A positive value
• B. Less negative value than the first
• C. Maximum negative value than the first
• D. Same value as that of the first

Hint:

Second electron gain enthalpy is always positive because adding an electron to a negatively charged ion requires energy to overcome repulsion.

Answer 1

The Correct Option is A

Step 1: Understand first electron gain enthalpy.
First electron gain enthalpy is the energy change when an electron is added to a neutral atom:
\[ O + e^- \rightarrow O^- \]
This process releases energy, hence it is negative (\(-141\,\text{kJ mol}^{-1}\)).

Step 2: Consider second electron addition.
Second electron gain enthalpy involves adding an electron to an already negatively charged ion:
\[ O^- + e^- \rightarrow O^{2-} \]

Step 3: Analyze electrostatic repulsion.
The incoming electron experiences strong repulsion from the negatively charged ion.
This requires energy to overcome repulsion.

Step 4: Nature of second electron gain enthalpy.
Since energy is required, the process is endothermic.
Hence, second electron gain enthalpy is positive.

Step 5: Compare with first electron gain enthalpy.
First electron gain enthalpy is negative (exothermic), while second is positive (endothermic).

Step 6: Interpretation.
Addition of electron to an anion is always energetically unfavorable due to repulsion.

Step 7: Final conclusion.
\[ \boxed{\text{A positive value}} \]