ΔH_f of C₂H₂, C₂H₄ and C₂H₆ respectively are +226, +56 and -84 kJ/mole. The difference between heats of hydrogenation of ethyne and ethene forming ethane is (in kilo joules)

Heat of Hydrogenation Problem

Given Data:

• ΔH_f(C₂H₂) = +226 kJ/mol
• ΔH_f(C₂H₄) = +56 kJ/mol
• ΔH_f(C₂H₆) = -84 kJ/mol

Reactions:

Reaction 1: C₂H₂ + 2H₂ → C₂H₆, ΔH₁ = heat of hydrogenation

Reaction 2: C₂H₄ + H₂ → C₂H₆, ΔH₂ = heat of hydrogenation

Solution Method:

Subtracting equation (2) from equation (1):

C₂H₂ + H₂ → C₂H₄, ΔH_r = ΔH₁ - ΔH₂

Calculation:

ΔH_r = ΔH_f(C₂H₄) - [ΔH_f(C₂H₂) + ΔH_f(H₂)]

ΔH_r = 56 - [226 + 0] = 56 - 226 = -170 KJ

Final Answer:The difference between the heats of hydrogenation of ethyne and ethene forming ethane is -170 kJ.