The enthalpy of formation of liquid ethyl alcohol in kJ/mol, using the following data (in kJ/mol):
Heat of vaporization of liquid ethyl alcohol = 43.
Heat of formation of gaseous atoms from the elements in their standard states: H, 218; C, 715; O, 249.
Average bond energies: C-H, 412; C-C, 348; C-O, 360; O-H, 463.

Question

The enthalpy of formation of liquid ethyl alcohol in kJ/mol, using the following data (in kJ/mol):

Heat of vaporization of liquid ethyl alcohol = 43.

Heat of formation of gaseous atoms from the elements in their standard states: H, 218; C, 715; O, 249.

Average bond energies: C-H, 412; C-C, 348; C-O, 360; O-H, 463.

TestHub · Accepted Answer

Atomize the elements for C 2 H 5 OH: 2C(s) + 3H 2 (g) + 1/2O 2 (g) → 2C(g) + 6H(g) + O(g): 2(715) + 6(218) + 249 = 2987 kJ. Bonds in C 2 H 5 OH(g): 5 C-H + 1 C-C + 1 C-O + 1 O-H = 5(412) + 348 + 360 + 463 = 3231 kJ released. Δ f H(g) = 2987 - 3231 = -244 kJ/mol; liquid: -244 - 43 = -287 kJ/mol. This is option (d).

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