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JEE Main 2015 · 10 Apr · Shift 1, Chemistry question 35

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 = 6.02×10236.02\times10^{23}, h=6.62×10−34h=6.62\times10^{-34} J s) :

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Answer and worked solution

Official answer: A

Ethane has 6 C-H and 1 C-C bonds; from methane, C-H =90=90 kJ/mol. So C-C =620−540=80=620-540=80 kJ/mol, i.e. 1.33×10−191.33\times10^{-19} J per bond.
λ=hcE=6.62×10−34×3×1081.33×10−19≈1.49×10−6 m=1.49×103 nm.\lambda=\frac{hc}E=\frac{6.62\times10^{-34}\times3\times10^8}{1.33\times10^{-19}}\approx1.49\times10^{-6}\text{ m}=1.49\times10^3\text{ nm}.

Topics:Bond enthalpyPhotoelectric effect and spectra
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