Question

For the gaseous reaction, $K+F\to K^{+}{F}^{-},\Delta H$ was calculated to be 19 kcal under conditions where the cations and anions were prevented by electrostatic separation from combining with each other. The ionization potential of K is 4.3 eV atom. The electron affinity of F will be:

• A. 3.476 eV
• B. -3.476 eV
• C. 2.468 eV
• D. 3.96 eV

Answer 1

Answer: (A)

3.476 eV

For 1 mole, $\Delta H=19kcal=19\times 2.611\times {10}^{22}=4.96\times {10}^{23}$ eV/mol

For 1 atom, $\Delta H=\frac{4.96\times {10}^{23}}{6.023\times {10}^{23}}=0.824$ eV/atom

The electron affinity of F is $4.3-0.824=3.476$ eV.