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 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 HO---H

H---C---H 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