Question
Consider the following data: _ f H ^ (methane, g)=- X kJ mol ^ -1 Enthalpy of sublimation of graphite = Y kJ mol ^ -1 Dissociation enthalpy of H _ 2 = Z kJ mol ^ -1 The bond enthalpy of C - H bond is given by :
JEE Main
Chemistry
Thermodynamics (C)
2026
JEE Main 2026 (21 January Shift 2)
JEE Main Chemistry Question (2026) — Solution
Options
- A. -X+Y+Z 4
- B. X+Y+4 Z 2
- C. X+Y+2 Z 4
- D. X+Y+Z
Answer
C. X+Y+2 Z 4
Step-by-step solution
For methane formation: C(g) + 2 H _2(g) CH _4(g). Using enthalpy balance: _f H° = Bonds broken - Bonds formed . Breaking: sublimation of C (Y) and 2 H-H bonds (2Z); Forming: 4 C-H bonds. Energy equation: -X = Y + 2Z - 4B where B is C-H bond enthalpy. Solving: 4B = X + Y + 2Z, so B = X + Y + 2Z 4 .
Related: Chemistry — Thermodynamics (C) · All PYQ Banks