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 combinig with each other. The ionisation potential of $$K$$ is $$4.3\ eV$$. Find electron affinity of $$F$$.
$$K+F\longrightarrow K^+ +F^- ;\quad \Delta H=19\ kcal =19\times 10^{3} \times 4.2\ J/ mole$$
$$I. E=4.3\ eV$$
$$E.A=?$$
We have,
$$\Delta H/ atom =\{I.E + (-E.A)\}/ atoms$$
$$\dfrac {4.2\times 19\times 10^3}{6.023\times 10^{23}}=(4.3-E.A)1.6\times 10^{-19}$$
$$\dfrac {4.2\times 19\times 10^{3}}{6.023\times10^{23}\times 1.6\times 10^{-19}}=4.3-E.A$$
$$4.3-E.A=0.828$$
$$E.A=4.3-0.828=3.47\ eV$$