Estimate Δ f H° [Pyridine(l)] (in kJ/mol) from the given data: Data: Δ f H° [CH 2 =N-CH 3 (g)] = +50, [CH 3 -NH-CH 3 (g)] = -20, [cyclohexene(g)] = -40, [cyclohexane(g)] = -156, [piperidine(g)] = -48

Question

Estimate Δ f H° [Pyridine(l)] (in kJ/mol) from the given data: Data: Δ f H° [CH 2 =N-CH 3 (g)] = +50, [CH 3 -NH-CH 3 (g)] = -20, [cyclohexene(g)] = -40, [cyclohexane(g)] = -156, [piperidine(g)] = -48 kJ/mol. Magnitude of resonance energy of pyridine = 120 kJ/mol. ΔH vap, 298 K [Pyridine(l)] = 38 kJ/mol.

TestHub · Accepted Answer

C=N hydrogenation: -20 - 50 = -70 kJ. C=C hydrogenation: -156 - (-40) = -116 kJ. Hypothetical Kekule pyridine (2 C=C, 1 C=N) + 3H 2 → piperidine: ΔH = 2(-116) + (-70) = -302 kJ. Δ f H(Kekule, g) = -48 + 302 = +254 kJ/mol. Subtract resonance energy: 254 - 120 = 134 kJ/mol for pyridine(g). Pyridine(l) = 134 - 38 = 96 kJ/mol. The answer is 0096.

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