Calculate the enthalpy of the following homogeneous gaseous reaction:
HCHO + 1/2 O₂ → HCOOH
from the following data:
Bond energies (kJ/mol): C-H = 413; C=O = 605; C-O = 358; O=O = 498; O-H = 459;
Magnitude of resonance energy (kJ/mol): HCOOH = 120.

Question

Calculate the enthalpy of the following homogeneous gaseous reaction:

HCHO + 1/2 O₂ → HCOOH

from the following data:

Bond energies (kJ/mol): C-H = 413; C=O = 605; C-O = 358; O=O = 498; O-H = 459;

Magnitude of resonance energy (kJ/mol): HCOOH = 120.

TestHub · Accepted Answer

Bonds broken (HCHO: 2 C-H, 1 C=O; plus 1/2 O=O): 2(413) + 605 + 249 = 1680 kJ. Bonds formed (HCOOH: 1 C-H, 1 C=O, 1 C-O, 1 O-H): 413 + 605 + 358 + 459 = 1835 kJ. ΔH = 1680 - 1835 = -155 kJ. Resonance stabilizes the product by 120 kJ, so ΔH = -155 - 120 = -275 kJ. This is option (b).

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