Calculate Δ H for C₂H₄(g) + H₂(g) → C₂H₆(g) using bond enthalpies: C=C = 610 kJ/mol, H-H = 436 kJ/mol, C-H = 413 kJ/mol,
Bonds broken: C=C (610) + H-H (436) = 1046 kJ. Bonds formed: C-C (346) + 2 × C-H (2 × 413 = 826) = 1172 kJ. Δ H = 1046 - 1172 = -126 kJ/mol.
Ref: NCERT Class 11 Chemistry > Chapter 5: Thermodynamics > Topic: Thermodynamic Terms - System Surroundings and Types of Systems
