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