State the oxidation and reduction half-equations that occur in the breathalyser when ethanol is present in the breath.

Oxidation:

Reduction:

Describe the colour change that occurs to the acidified dichromate(VI) if ethanol is present in the breath.

Police use the intoximeter, an infrared spectrophotometer to confirm a roadside breathalyser test. Explain how the amount of ethanol is determined from the infrared spectrum.

Oxidation:

\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{OH}} + {{\text{H}}_{\text{2}}}{\text{O}} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}} + {\text{4}}{{\text{H}}^ + } + {\text{4}}{{\text{e}}^ - }\);

Reduction:

\({\text{C}}{{\text{r}}_{\text{2}}}{\text{O}}_7^{2 - } + {\text{14}}{{\text{H}}^ + } + {\text{6}}{{\text{e}}^ - } \to {\text{2C}}{{\text{r}}^{3 + }} + {\text{7}}{{\text{H}}_{\text{2}}}{\text{O}}\);

Accept balanced equation with molecular formulas.

If both equations are wrong, award [1] for C₂H₅OH \( \to \) CH₃COOH and Cr₂O₇^2– \( \to \) 2Cr³⁺.

If correct equations are used but oxidation and reduction reversed, award [1].

orange to green;

peak at \({\text{2950 c}}{{\text{m}}^{ - 1}}\) / absorption occurs due to C–H bonds in ethanol;

No mark for absorption due to just ethanol, or O–H bond in ethanol (water vapour in breath also contributes).

intensity / height of peak / absorption / amount of transmittance depends on amount of ethanol / compare absorption to standard / reference/control sample / sample containing no alcohol;

Very few candidates gave correct equations for oxidation and reduction in a breath analyzer.

Most candidates described correctly the color change.

The way how a breath analyzer works seemed clear for most candidates.