The enthalpy of formation of ethane (C₂H₆) from ethylene by addition of hydrogen where the bond-energies of C−H, C−C, C=C, H−H are 414 kJ, 347 kJ, 615 kJ and 435 kJ respectively is ______ kJ.

The reaction: CH₂=CH₂ + H₂ → CH₃-CH₃

Bonds broken: 1 C=C (615 kJ) + 1 H-H (435 kJ) = 1050 kJ

Bonds formed: 1 C-C (347 kJ) + 2 C-H (2 × 414 = 828 kJ) = 1175 kJ

ΔH = Energy of bonds broken - Energy of bonds formed = 1050 - 1175 = -125 kJ

The enthalpy of formation is -125 kJ. The magnitude is 125 kJ.

The answer is 125.