DP Chemistry (last assessment 2024)

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Question 19M.3.hl.TZ2.27

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Date May 2019 Marks available [Maximum mark: 5] Reference code 19M.3.hl.TZ2.27
Level hl Paper 3 Time zone TZ2
Command term Describe, Suggest Question number 27 Adapted from N/A
27.
[Maximum mark: 5]
19M.3.hl.TZ2.27

The presence of alcohol in the breath can be detected using a breathalyzer.

(a)

Describe how a fuel cell breathalyser works.

[3]

Markscheme

Any three of:
ethanol «in breath» is oxidized «to ethanoic acid»  [✔]
electrons pass through external circuit/meter [✔]
«to cathode where» O2 is reduced [✔]
current is proportional to alcohol concentration [✔]

 

Note: Accept equations for oxidation of ethanol or reduction of oxygen.

Examiners report

While many scored the first marking point, full marks were rarely seen. Many candidates mixed up this and a dichromate breathalyser.

(b)

Alcohol levels in the breath can also be determined using IR spectroscopy.

Suggest, giving a reason, which bond’s absorbance is most useful for detecting ethanol in breath.

Bond: 

Reason:

[2]

Markscheme

Bond:
C–O
OR
C–H  [✔]

Reason:
cannot use O–H bonds as in water «found in breath» [✔]

 

Note: Accept “C–O/C–H «bonds in molecules in breath» most likely to be in ethanol”.

Do not apply ECF here.

Examiners report

Most candidates incorrectly identified O-H, failing to realise it is unsuitable due to its abundant presence in the breath.

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