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Question:

Using the data provided, calculate the multiple bond energy (kJ/mol) of a C≡C bond in C₂H₂. That energy is: (take the bond energy of a C−H bond as 350 kJ/mol)
2C(s) + H₂(g) → C₂H₂(g); ΔH = 225 kJ/mol
2C(s) → 2C(g); ΔH = 1410 kJ/mol
H₂(g) → 2H(g); ΔH = 330 kJ/mol

837

1165

865

815

Solution:

2C(s) + H₂(g) → C₂H₂(g); ΔH = 225 kJ/mol
2C(s) → 2C(g); ΔH = 1410 kJ/mol
H₂(g) → 2H(g); ΔH = 330 kJ/mol
From the first equation,
225 = [ΔH(2C(s) → 2C(g)) + ΔH(H₂(g) → 2H(g))] - [2 × BE(C−H) + 1 × BE(C≡C)]
225 = (1410 + 330) - (2 × 350 + BE(C≡C))
225 = 1740 - (700 + BE(C≡C))
BE(C≡C) = 1740 - 700 - 225
BE(C≡C) = 815 kJ/mol