| Date | November 2012 | Marks available | 2 | Reference code | 12N.2.sl.TZ0.2 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Define | Question number | 2 | Adapted from | N/A |
Question
The following equation represents a combustion reaction of propane, \({{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}{\text{(g)}}\) when the oxygen supply is limited.
\[{{\text{C}}_3}{{\text{H}}_8}{\text{(g)}} + {\text{3}}\frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{3CO(g)}} + {\text{4}}{{\text{H}}_2}{\text{O(g)}}\]
Define the term average bond enthalpy.
(i) Determine \(\Delta H\), the enthalpy change of the reaction, in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\), using average bond enthalpy data from Table 10 of the Data Booklet. The bond enthalpy for the carbon-oxygen bond in carbon monoxide, CO, is \({\text{1072 kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\).
(ii) The CO molecule has dative covalent bonding. Identify a nitrogen-containing positive ion which also has this type of bonding.
Markscheme
energy needed to break (1 mol of) a bond in a gaseous molecule/state/phase;
average calculated from a range of similar compounds / OWTTE;
Do not accept similar bonds instead of similar compounds.
M2 can be scored independently.
(i) Bonds breaking:
2 \( \times \) (C−C) + 8 \( \times \) (C−H) + 3.5 \( \times \) (O=O)
\( = (2)(347) + (8)(413) + (3.5)(498)\)
\( = {\text{5741(kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);
Bonds forming:
3 \( \times \) (CO) + 8 \( \times \) (O−H)
\( = (3)(1072) + (8)(464) = 6928{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);
Enthalpy change:
\((5741 - 6928 = ) - 1187{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);
Award [3] for correct final answer.
(ii) \({\text{NH}}_{\text{4}}^ + \)/ammonium / \({{\text{N}}_{\text{2}}}{\text{H}}_5^ + \) /hydrazinium / \({\text{C}}{{\text{H}}_{\text{3}}}{\text{NH}}_3^ + \) /methylammonium / methanaminium / \({{\text{H}}_{\text{2}}}{\text{NO}}_3^ + \) /nitrooxonium;
Examiners report
The definition of average bond enthalpy in part (a), proved challenging even though it has appeared on recent examination papers and very few scored two marks. A good number of candidates omitted gaseous and did not state that it is the energy needed to break 1 mol of a bond in a gaseous molecule and many did not understand that it is the average calculated from a range of similar compounds.
In Part (b) (i), the typical errors were using the incorrect bond enthalpies from the Data Booklet and using the sum of the bond enthalpies of bond forming (products) minus bond breaking (reactants) instead of the reverse. In Part (b) (ii), instead of NH4+, candidates identified a range of incorrect answers including \({\text{NH}}_3^ + \), NF, \({\text{C}}{{\text{N}}^ - }\), \({\text{NO}}_3^ - \), N2 and even NaCl, although the question asked for a nitrogen containing positive ion.
