Question

The energy required to brake one mole of $Cl-Cl$ bonds in $C{l}_2$ is $242kjmo{l}^{-1}$. The longest wavelength of light capable of breaking single $Cl-Cl$ bond is:
$(c=3\times {10}^{8}m{s}^{-}1$ and $N_A=6.02\times {10}^{23}mo{l}^{-1})$

• A. $494nm$
• B. $594nm$
• C. $640nm$
• D. $700nm$

Answer 1

The correct option is A $494nm$

$C{l}_2\to 2Cl$
$\Delta H=+242kJ/mol$
the energy required to dissociate 1 molecule of $C{l}_2$
$E=\frac{242\times {10}^3}{6.022\times {10}^{23}}=4.0186\times {10}^{-19}J$
The Planks expression:
$E=hf=\frac{hc}{\lambda }$
$\therefore \lambda =\frac{hc}{E}$
$\lambda =\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{4.0186\times {10}^{-19}}m$
$\lambda =4.949\times {10}^{-7}m$
$\lambda =495nm\approx 494nm$