Determine the value of the first ionization potential of potassium atom if $∆\mathrm{H}$ for the given gaseous phase reaction: 

$\mathrm{K}\left(\mathrm{g}\right)+\mathrm{F}\left(\mathrm{g}\right)⟶{\mathrm{K}}^{+}\left(\mathrm{g}\right)+{\mathrm{F}}^{-}\left(\mathrm{g}\right)$

is 0.82 eV and the first electron affinity of F is -3.48 eV.

Relation between the change in enthalpy of a reaction, $∆\mathrm{H},$ ionization energy and electron affinity is:

$∆\mathrm{H}=\mathrm{Ionization} \mathrm{energy}+\mathrm{Electron} \mathrm{affinity}$

Substitute the given values.

$\begin{array}{ccc}0.82 \mathrm{eV}& =& \mathrm{Ionization} \mathrm{energy}+(-3.48 \mathrm{eV})\\ \mathrm{Ionization} \mathrm{energy}& =& 4.30 \mathrm{eV}\end{array}$