CHM 2045C Module Seven Sample Exam Answer

Module 7 Part I: Heat of Reaction from Bond Energies Answer

Methane burns in oxygen to produce heat for homes by the following reaction:

CH₄ + 2 O₂ è CO₂ + 2 H₂O

First Draw the Dot Structure of each reactant and product to determine how many of each bond was made or broken

H

| + O==O H—O---H

H---C---H à O==C==O +

| + O==O H---O---H

H

If the average bond energies in kJ/mol are:

O-O 146

O=O 498

O-H 463

C-C 346

C-H 413

C-O 358

C=O 745

H-H 436

Calculate ∆H^o_rxn for the reaction:

ΔH^o_rxn = Σ (bonds broken) – Σ (bonds formed)

Then substitute the values for each bond broken and each bond formed into the equation:

ΔH^o_rxn = Σ [4(C---H) bonds + 2 (O==O) bonds broken] - Σ [2(C==O) bonds + 4(H---O) bonds formed]

ΔH^o_rxn = Σ [4(413) + 2(498)] - Σ [2(745) + 4(463)]

ΔH^o_rxn = (1652 J + 996 J) - (1490J + 1852J)

ΔH^o_rxn = 2648J - 3342 J

ΔH^o_rxn = - 694 J