Question
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:
JEE Main
Chemistry
Thermodynamics (C)
2026
JEE Main 2026 (24 January Shift 2)
JEE Main Chemistry Question (2026) — Solution
Options
- A. N _ A hc 250(4 y-6 x)
- B. hc 1000 ( y-6 x 4 )^ -1
- C. N _ A hc 250(y-6 x)
- D. N _ A hc (y- 6 x 4 )^ -1
Answer
A. N _ A hc 250(4 y-6 x)
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)
Related: Chemistry — Thermodynamics (C) · All PYQ Banks