Question

For the gaseous reaction, \[\ce{K_{(g)} + F_{(g)} -> K^+_{ (g)} + F^-_{ (g)}}\], ΔH was calculated to be 19 kcal/mol under conditions where the cations and anions were prevented by electrostatic separation from combining with each other. The ionisation energy of K is 4.3 eV. The electron affinity of F is ______. (in eV)

Options

• 3.48

• 5.97

• 6.28

• 7.42

Answer 1

For the gaseous reaction, \[\ce{K_{(g)} + F_{(g)} -> K^+_{ (g)} + F^-_{ (g)}}\], ΔH was calculated to be 19 kcal/mol under conditions where the cations and anions were prevented by electrostatic separation from combining with each other. The ionisation energy of K is 4.3 eV. The electron affinity of F is 3.48.

Explanation:

\[\ce{K -> K^+ + e^-}\], ΔE₁ = 4.3 eV

\[\ce{F + e^- -> F^-}\], ΔE₂ = − EeV

`19.0/23.06` = ΔE₁ − ΔE₂ = 4.3 − E

0.82 = 4.3 − E

E = 3.48