The breathalyser, one of the earliest tests, uses the reaction between ethanol and acidified potassium dichromate(VI). Ethanol is first oxidized to ethanal.

Deduce the half-equation for the reaction of ethanol to ethanal.

Outline why the colour changes from orange to green.

Explain how the ethanol concentration in the breath can be measured by an intoximeter using infrared absorption.

\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH(aq)}} \to {\text{C}}{{\text{H}}_3}{\text{CHO(aq)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}} + {\text{2}}{{\text{e}}^ - }\);

Accept equilibrium sign, e for e– and different representations of organic compounds (such as \({C_2}{H_6}O\) and \({C_2}{H_4}O\)).

Ignore state symbols.

Do not accept \(C{H_3}C{H_2}OH + [O] \to C{H_3}CHO + {H_2}O\) (since half-equation requested).

(orange) dichromate(VI) \({\text{ion/C}}{{\text{r}}_{\text{2}}}{\text{O}}_{\text{7}}^{2 - }{\text{/Cr(VI)/C}}{{\text{r}}^{6 + }}\) is reduced/converted to (green) chromium(III) \({\text{ion/Cr(III)/C}}{{\text{r}}^{3 + }}\);

absorption (of IR at \({\text{2950 c}}{{\text{m}}^{ - 1}}\)) by C–H bond;

(IR) absorption increases with/proportional to concentration / (IR) intensity compared to an empty/control cell;

Accept area under peak/size of peak (on IR spectrum) can be used to measure ethanol concentration.

The ability to construct a redox half-equation connecting a given starting material and product was disappointingly rare. The colour change was better explained though many students failed to specifically identify the species involved or their oxidation states. An encouraging number of students were aware of the role of the C-H bond absorption in IR based intoximeters, though frequently mention of the importance of the absorption intensity was omitted.

The ability to construct a redox half-equation connecting a given starting material and product was disappointingly rare. The colour change was better explained though many students failed to specifically identify the species involved or their oxidation states. An encouraging number of students were aware of the role of the C-H bond absorption in IR based intoximeters, though frequently mention of the importance of the absorption intensity was omitted.

The ability to construct a redox half-equation connecting a given starting material and product was disappointingly rare. The colour change was better explained though many students failed to specifically identify the species involved or their oxidation states. An encouraging number of students were aware of the role of the C-H bond absorption in IR based intoximeters, though frequently mention of the importance of the absorption intensity was omitted.