Define the term average bond enthalpy.

Use the information from Table 10 of the Data Booklet to determine the standard enthalpy change for the complete combustion of ethanol.

\[{\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH(g)}} + {\text{3}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2C}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {\text{3}}{{\text{H}}_{\text{2}}}{\text{O(g)}}\]

The standard enthalpy change for the complete combustion of octane, \({{\text{C}}_{\text{8}}}{{\text{H}}_{{\text{18}}}}\), is \( - 5471{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\). Calculate the amount of energy produced in kJ when 1 g of ethanol and 1 g of octane is burned completely in air.

Ethanol can be oxidized using acidified potassium dichromate, \({{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}\), to form two different organic products.

\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH}}\xrightarrow[{{{\text{H}}^ + }}]{{{\text{C}}{{\text{r}}_{\text{2}}}{\text{O}}_7^{2 - }}}\) A \(\xrightarrow[{{{\text{H}}^ + }}]{{{\text{C}}{{\text{r}}_{\text{2}}}{\text{O}}_7^{2 - }}}\) B

State the structural formulas of the organic products A and B and describe the conditions required to obtain a high yield of each of them.

Deduce and explain whether ethanol or A has the higher boiling point.

Ethene can be converted into ethanol by direct hydration in the presence of a catalyst according to the following equation.

\[{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}{\text{(g)}} + {{\text{H}}_{\text{2}}}{\text{O(g)}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH(g)}}\]

For this reaction identify the catalyst used and state one use of the ethanol formed other than as a fuel.

State the name of one structural isomer of pentane.

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 made or broken in the gaseous molecule/state.

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

Bonds broken

\({\text{(1)(C}}–{\text{C)}} + {\text{(1)(O}}–{\text{H)}} + {\text{(5)(C}}–{\text{H)}} + {\text{(1)(C}}–{\text{O)}} + {\text{(3)(O=O)}}\)

\( = (1)(347) + (1)(464) + (5)(413) + (1)(358) + (3)(498) = 4728{\text{ (kJ)}}\);

Bonds formed

\({\text{(2}} \times {\text{2)(C=O)}} + {\text{(3}} \times {\text{2)(O}}–{\text{H)}}\)

\( = (4)(746) + (6)(464) = 5768{\text{ (kJ)}}\);

\(\Delta H = 4728 - 5768 =  - 1040{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}/ - 1040{\text{ kJ}}\);

Units needed for last mark.

Award [3] for final correct answer.

Award [2] for +1040 kJ.

\({M_{\text{r}}}{\text{(}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{OH)}} = 46.08/46.1\) and \({M_{\text{r}}}{\text{(}}{{\text{C}}_{\text{8}}}{{\text{H}}_{{\text{18}}}}{\text{)}} = 114.26/114.3\);

1ethanol produces 22.57 kJ and 1octane produces 47.88 kJ;

Accept values ranges of 22.5–23 and 47.8–48 kJ respectively.

No penalty for use of M_r = 46 and M_r = 114.

A: \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CHO}}\);

B: \({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH/C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{O}}_{\text{2}}}{\text{H}}\);

Accept either full or condensed structural formulas but not the names or molecular formulas.

A: distillation;

B: reflux;

ethanol/; \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH}}\)

hydrogen bonding (in ethanol);

Award second point only if the first is obtained.

(concentrated) \({{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}\)/(concentrated) phosphoric acid / \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\)/sulfuric acid;

dyes / drugs / cosmetics / solvent / (used to make) esters / (used in) esterification/disinfectant;

(2-)methylbutane / (2,2-)dimethylpropane;

The definition of average bond enthalpy given by most candidates was not complete in (a) (i). The word gaseous was missing and the fact that it is an average of values from bonds in similar compounds was very rarely mentioned.

In (ii) the calculation of the standard enthalpy change for the combustion of ethanol was done correctly by most candidates.

In (a) (iii) the amount of energy produced by 1g of ethanol and by 1g of octane was correctly calculated by some of the candidates.

Candidates gave correct formulas for the aldehyde and the carboxylic acid in (iv), but the conditions required to obtain a high yield were not correctly stated or were absent.

In (a) (v) most candidates correctly stated that ethanol would have a higher boiling point than ethanal because of the presence of hydrogen bonding in ethanol.

In (vi) the catalyst for the conversion of ethane into ethanol was not always identified.

In (b)(i) most candidates stated correctly that methylbutane would be a structural isomer of pentane.