Question

For the gaseous reaction: $K\left(g\right)+F\left(g\right)\to K^{+}\left(g\right)+F^{-}\left(g\right),$ $\Delta H$ was calculated to be $91.4\text{kJ/mol}$ under conditions where the cations and anions were prevented from combining with each other. The ionisation enthalpy of $K$ is $418\text{kJ/mol}$. What is the electron gain enthalpy $\left(\text{in kcal/mol}\right)$ of $F?$

• A. -78.13
• B. -326.6
• C. 326.6
• D. 78.13

Answer 1

The correct option is A -78.13

$\begin{array}{cc}K\left(g\right)+F\left(g\right)\to F^{-}\left(g\right)+K^{+}\left(g\right)& \Delta H=91.4\text{kJ/mol}\\ K\left(g\right)\to K^{+}\left(g\right)+e^{-}& I.E.=418\text{kJ/mol}\\ F\left(g\right)+e^{-}\to F^{-}\left(g\right)& {\Delta }_{eg}H=\Delta H-I.E.=91.4-418=-326.6\text{kJ/mol}=-78.13kcal/mol\\ & \\ & \end{array}$