Question

The heat of atomization of methane and ethane are $360\, kJ/mol$ and $620\, kJ/mol,$ respectively. The longest wavelength of light capable of breaking the C - C bond is (Avogadro number $= 6.02 \times 10^{23},\, h = 6.62 \times 10^{-34}\, J s$) :

• A. $1.49 \times 10^{3}\,nm$
• B. $2.48 \times 10^{3}\, nm$
• C. $2.48 \times 10^{4}\,nm$
• D. $1.49 \times 10^{4}\, nm$

Answer 1

Answer: (A)

$CH _{4} \rightarrow C +4 H \Delta H=360$

$4( C - H )=360$

$B . E .$ of $C - H =90\, kJ$

$C _{2} H _{6} \rightarrow 2 C +6 H \Delta H=620$

$1( C - C )+6( C - H )=620 \Rightarrow 1( C - C )+6 \times 90=620$

$B. E$ of $C - C =80\, kJ$

$E =\frac{h C}{\lambda}$

$\Rightarrow 80 \times 10^{3}=\frac{6.62 \times 10^{-34} \times 3 \times 10^{8}}{\lambda}$