Define the term average bond enthalpy.

Deduce the balanced chemical equation for the complete combustion of butan-1-ol.

Determine the standard enthalpy change, in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\), for the complete combustion of butan-1-ol, using the information from Table 10 of the Data Booklet.

Based on the types of intermolecular force present, explain why butan-1-ol has a higher boiling point than butanal.

energy required to break (1 mol of) a bond in a gaseous molecule/state;

Accept energy released when (1 mol of) a bond is formed in a gaseous molecule/state / enthalpy change when (1 mol of) bonds are formed or broken in the gaseous molecule/state.

average values obtained from a number of similar bonds/compounds / OWTTE;

\({\text{C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_3}{\text{OH(l)}} + {\text{6}}{{\text{O}}_2}{\text{(g)}} \to {\text{4C}}{{\text{O}}_2}{\text{(g)}} + {\text{5}}{{\text{H}}_2}{\text{O(l)}}\);

Allow C₄H₉OH or C₄H₁₀O for CH₃(CH₂)₃OH.

Ignore state symbols.

Bonds broken:

(6)(O=O) \( + \) (3)(C–C) \( + \) (1)(O–H) \( + \) (1)(C–O) \( + \) (9)(C–H) /

\(\left( {(6)(498) + (3)(347) + (1)(464) + (1)(358) + (9)(413) = } \right){\text{ }}8568{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

Bonds formed:

(8)(C=O) \( + \) (10)(O–H) / \(\left( {(8)(746) + (10)(464) = } \right){\text{ }}10608{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

\(\Delta H{\text{ = }}(8568 - 10608 = ){\text{ }} - 2040{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

Award [3] for correct final answer.

Award [2] for +2040 (kJ\(\,\)mol^–1).

hydrogen bonding in butan-1-ol;

stronger than dipole-dipole attractions in butanal;

Accept converse argument.

Do not penalize dipole-dipole bonding instead of dipole-dipole attractions.

Again this definition proved very challenging even though it has appeared on recent examination papers and very few scored both marks. Gaseous was often omitted and few stated that the average values are obtained from a number of similar bonds (again similar was often omitted).

In part (b) many of the better candidates were able to write the correct balanced combustion reaction. Some had an incorrect coefficient for oxygen and others wrote incorrect products which were often hydrocarbons.

In part (c) there were some fully correct responses, but many did lose marks. Common mistakes included using the O–O bond energy value instead of O=O. Others mixed up the signs.

In part (d) it was pleasing that nearly all candidates knew that hydrogen bonding occurs in butan-1-ol, but only the best students mentioned the dipole-dipole interactions in butanal. Generally butanal was described as having van der Waal’s or dispersion forces.