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  Horxn for the reaction:

ΔHorxn = Σ (bonds broken) – Σ (bonds formed)

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

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

ΔHorxn = Σ [4(413) + 2(498)]  -  Σ [2(745) + 4(463)]

ΔHorxn =  (1652 J + 996 J)  -  (1490J + 1852J)

ΔHorxn = 2648J  -  3342 J

ΔHorxn =  - 694 J