State the chemical formula for potassium dichromate(VI).

Describe the colour change observed during its reaction with ethanol.

State the oxidation number of chromium in the product.

Deduce the full balanced chemical equation for the redox reaction of ethanol with acidified potassium dichromate(VI).

State the name of the organic product formed.

An intoximeter is used to determine an accurate value for the concentration of ethanol in the breath. Explain one technique used for the detection of ethanol in an intoximeter.

${{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}$;

orange to green;

Allow yellow instead of orange.

+3/III;

Do not allow incorrect notation such as 3+ or 3.

${\text{3C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} + {\text{2C}}{{\text{r}}_2}{\text{O}}_7^{2 - } + {\text{16}}{{\text{H}}^ + } \to {\text{3C}}{{\text{H}}_3}{\text{C}}{{\text{O}}_2}{\text{H}} + {\text{4C}}{{\text{r}}^{3 + }} + {\text{11}}{{\text{H}}_2}{\text{O}}$

correct formulas of ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH}}$ and ${\text{C}}{{\text{r}}_{\text{2}}}{\text{O}}_{\text{7}}^{{\text{2}} - }{\text{/}}{{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}$ as reactants and

${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{O}}_{\text{2}}}{\text{H/C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ and ${\text{C}}{{\text{r}}^{3 + }}$ as products;

full balanced chemical equation;

M2 can only be scored if M1 is correct.

Allow full balanced chemical equation to produce ethanal,

${\text{3C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} + {\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - } + {\text{8}}{{\text{H}}^ + } \to {\text{3C}}{{\text{H}}_3}{\text{CHO}} + {\text{2C}}{{\text{r}}^{3 + }} + {\text{7}}{{\text{H}}_2}{\text{O}}$.

Accept full or condensed structural formulas.

ethanoic acid;

Allow acetic acid.

Allow ethanal/acetaldehyde.

infrared (spectroscopy)/IR;

CH characteristic band (at ${\text{2950 c}}{{\text{m}}^{ - 1}}$) for ethanol / C–H bonds in ethanol absorb at certain frequency/wavelength;

Do not award M2 for CH characteristic band if however wavenumber range/value is given for OH (eg, 3200–3600 cm^–1 or value in between or even 2500–3300 cm^–1).

area under peak used to measure concentration (of ethanol);

Accept “size of” instead of “area under”.

Do not accept “height” instead of “area under”.

OR

fuel cell;

ethanol converts/oxidized to ${\text{C}}{{\text{O}}_{\text{2}}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$;

(energy released converted to) voltage/potential difference (which is) proportional to/can be used to measure concentration (of ethanol);

Allow potential instead of potential difference.

Only about half the candidates gave the correct formula for potassium dichromate(VI).

Most candidates knew the colour change.

The oxidation number was often given using incorrect notation (3+ or 3) failing to score the mark.

The redox equation was challenge except for the strongest candidates.

About half the candidates gave the correct product for the oxidation of ethanol.

Part (c) was poorly answered. About half of the candidates scored one mark for recognizing that the intoximeter used IR radiation. Few candidates gained a second mark for recognizing that the absorption by C-H bonds is used to determine ethanol concentration. It was rare to see an answer mentioning the area under the peak or using the fuel cell in the intoximeter.