Question

The electron gain enthalpy (Cl₂,H) of chlorine is -3.7 eV mol^-1. How much energy (in kcal mol^-1) is released when 71 g of chlorine atoms are completely converted into Cl^- ions in gaseous state? (1 eV = 23 kcal)

• A. 1.072
• B. 10.72
• C. 17.02
• D. 85.1

Hint:

Electron gain enthalpy is energy released per mole. Convert eV to kcal using the given conversion factor, and adjust for the number of moles if specified.

Answer 1

The Correct Option is D

Electron gain enthalpy of Cl is -3.7 eV per atom. 
For 1 mole of Cl atoms (35.5 g), energy released = $3.7 \times 23 = 85.1$ kcal mol$^{-1}$ (since 1 eV = 23 kcal). 
Given mass = 71 g, which is $\frac{71}{35.5} = 2$ moles of Cl atoms. 
Total energy for 2 moles = $2 \times 85.1 = 170.2$ kcal. 
However, the question asks for energy per mole: 85.1 kcal mol$^{-1}$ matches option (4).