| Marks available | 2 |
| Reference code | 09M.3.sl.TZ2.D3 |
Question
Ethanol, a depressant, is sufficiently volatile to pass into the lungs from the bloodstream. The roadside breathalyser test uses acidified potassium dichromate(VI) which reacts with any ethanol present in the breath and converts it to ethanoic acid.
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.
Markscheme
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 Cr2O72– \( \to \) 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;
Examiners report
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.
| Marks available | 1 |
| Reference code | 13M.3.sl.TZ1.D2 |
Question
Depressants can have different effects depending on their doses.
A breathalyser containing crystals of potassium dichromate(VI) can be used by the police to detect whether a driver has consumed alcohol.
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.
Markscheme
\({{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}\);
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.
Examiners report
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.
| Marks available | 1 |
| Reference code | 13M.3.sl.TZ2.D2 |
Question
Ethanol is a depressant.
The presence of ethanol in the breath can be detected by blowing into a “bag” through a tube containing acidified potassium dichromate(VI). The half-equation for the dichromate reaction is:
\[{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 – }{\text{(aq)}} + {\text{14}}{{\text{H}}^ + }{\text{(aq)}} + {\text{6}}{{\text{e}}^ – } \to {\text{2C}}{{\text{r}}^{3 + }}{\text{(aq)}} + {\text{7}}{{\text{H}}_2}{\text{O(l)}}\]
Describe the colour change observed when the dichromate ion reacts with the ethanol.
State the name of the organic product formed during the reaction.
In order to quantify exactly how much ethanol is present in the blood, a person may be required to give a blood sample or may be asked to blow into an intoximeter. Explain the chemistry behind the techniques for determining the ethanol content in a blood sample and by using an intoximeter.
Blood sample:
Intoximeter:
Ethanol may exert a synergistic effect when taken with other medicines. State the meaning of the term synergistic effect.
Markscheme
from orange to green;
ethanal/acetaldehyde/CH3CHO / ethanoic acid/acetic acid/CH3COOH;
Do not accept aldehyde / carboxylic acid.
Blood sample:
gas(-liquid) chromatography/GLC/GC / high pressure/performance liquid chromatography/HPLC;
No credit for just “chromatography”.
column separates the alcohol/ethanol from the other components in the blood / retention time identifies alcohol/ethanol / (the amount of alcohol/ethanol in the blood is) compared with a known sample / by measuring the area under the eluted peaks / OWTTE;
Intoximeter:
infrared spectroscopy/infrared light passed through;
the absorption of the C–H/C–O bond is measured (and compared with a calibrated sample) / OWTTE;
OR
fuel cell;
an electric current/voltage is generated (proportional to the concentration of alcohol/ethanol in the breath) / OWTTE;
enhances the effect/causes a stronger/different effect of another drug (present in the body at the same time);
Examiners report
This question produced significantly lower marks. In the first part candidates often confused moderate and high dose symptoms and the latter were often confused with chronic effects. The colour change and product were widely known though, as in the past, some students forgot that a colour change involves stating both the initial and final colour and some gave the class of compound (aldehyde/carboxylic acid) rather than the specific product from ethanol. Only a handful of students gained any of the marks for specific techniques used to assess blood alcohol levels, with IR methods being the best known. Many knew what a “synergistic effect” was, but many struggled to convey this in appropriate language rather than just quoting an example of this type of activity.
This question produced significantly lower marks. In the first part candidates often confused moderate and high dose symptoms and the latter were often confused with chronic effects. The colour change and product were widely known though, as in the past, some students forgot that a colour change involves stating both the initial and final colour and some gave the class of compound (aldehyde/carboxylic acid) rather than the specific product from ethanol. Only a handful of students gained any of the marks for specific techniques used to assess blood alcohol levels, with IR methods being the best known. Many knew what a “synergistic effect” was, but many struggled to convey this in appropriate language rather than just quoting an example of this type of activity.
This question produced significantly lower marks. In the first part candidates often confused moderate and high dose symptoms and the latter were often confused with chronic effects. The colour change and product were widely known though, as in the past, some students forgot that a colour change involves stating both the initial and final colour and some gave the class of compound (aldehyde/carboxylic acid) rather than the specific product from ethanol. Only a handful of students gained any of the marks for specific techniques used to assess blood alcohol levels, with IR methods being the best known. Many knew what a “synergistic effect” was, but many struggled to convey this in appropriate language rather than just quoting an example of this type of activity.
This question produced significantly lower marks. In the first part candidates often confused moderate and high dose symptoms and the latter were often confused with chronic effects. The colour change and product were widely known though, as in the past, some students forgot that a colour change involves stating both the initial and final colour and some gave the class of compound (aldehyde/carboxylic acid) rather than the specific product from ethanol. Only a handful of students gained any of the marks for specific techniques used to assess blood alcohol levels, with IR methods being the best known. Many knew what a “synergistic effect” was, but many struggled to convey this in appropriate language rather than just quoting an example of this type of activity.