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

Step-by-step solution

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 = hc and = hc E : = N_A hc 250(4y - 6x) (in SI units)