The heat of atomisation of methane and ethane are ' x ' kJ mol^ -1 and ' y ' kJ mol^ -1 respectively. The longest wavelength (λ) of light capable of breaking the C-C bond can be expressed in SI unit as:

Question

Quantrex Academy · Accepted Answer

Correct answer: A. N_ A hc 250(4 y-6 x)

Heat of atomization: CH₄ has x kJ/mol (4 C-H bonds), C₂H₆ has y kJ/mol (1 C-C + 6 C-H bonds)
Let C-H bond energy = a, C-C bond energy = b
Then: x = 4a and y = b + 6a
Solving: a = x/4, so b = y - 6x/4 = (4y - 6x)/4 kJ/mol
Converting to J/mol: b = (4y - 6x) × 250 J/mol
Using

E = \frac{hc}{\lambda}

and

\lambda = \frac{hc}{E}

:

\lambda = \frac{N_A hc}{250(4y - 6x)}

(in SI units)

The heat of atomisation of methane and ethane are ' x ' kJ mol^ -1...

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