The longest wavelength of light capable of breaking a single (C-CI) bond in Cl₂ is if

$\mathrm{Cl}-\mathrm{Cl}\left(\mathrm{g}\right)⟶2\mathrm{Cl}\left(\mathrm{g}\right); \mathrm{\Delta H}=242 \mathrm{kJ} {\mathrm{mol}}^{-1}$

$\begin{array}{c}\mathrm{\Delta H}=242\mathrm{kJ}{\mathrm{mol}}^{-1}\\ =242\times {10}^3\mathrm{J}{\mathrm{mol}}^{-1}=\frac{242\times {10}^3\mathrm{J}}{{\mathrm{N}}_0}\mathrm{J}{\text{ molecule }}^{-1}\\ =\frac{242\times {10}^3\mathrm{J}}{6.02\times {10}^{23}}\mathrm{J}\text{ per }(\mathrm{Cl}-\mathrm{Cl})\text{ bond }\end{array}$

$\text{ Also }\mathrm{\Delta H}=\mathrm{hv}=\frac{\mathrm{hc}}{\mathrm{\lambda }}$

$\begin{array}{c}\therefore \mathrm{\lambda }=\frac{\mathrm{hc}}{\mathrm{\Delta H}}=\frac{6.62\times {10}^{-34}\mathrm{Js}\times 3\times {10}^8{\mathrm{ms}}^{-1}\times 6.02\times {10}^{23}}{242\times {10}^3\mathrm{J}}\\ =4.94\times {10}^{-7}\mathrm{m}=494\times {10}^{-9}\mathrm{m}=494\mathrm{nm}\end{array}$