DP Chemistry (last assessment 2024)

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Question 19N.3.hl.TZ0.c

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Date November 2019 Marks available [Maximum mark: 2] Reference code 19N.3.hl.TZ0.c
Level hl Paper 3 Time zone TZ0
Command term Explain Question number c Adapted from N/A
c.
[Maximum mark: 2]
19N.3.hl.TZ0.c

Steroids are lipids with a steroidal backbone. The structure of cholesterol is shown in section 34 of the data booklet.

Explain how redox chemistry is used to measure the ethanol concentration in a breathalyser.

[2]

Markscheme

ALTERNATIVE 1
oxidizing agent/«acidified» potassium dichromate(VI) converts ethanol to ethanoic acid ✔
colour change «from orange to green» is measured/analysed «using photocell» ✔

ALTERNATIVE 2
ethanol is oxidized to ethanoic acid «at anode and oxygen is reduced to water at cathode» ✔
current/voltage/potential is measured «by computer»
OR
current/voltage/potential is proportional to ethanol concentration ✔

NOTE: Accept names or formulas for reagents.
Accept “«acidified» dichromate/Cr2O72−” for “K2Cr2O7”.
Award [1 max] for "Cr(VI) going to Cr(III) AND colour changing/colour changing from orange to green".
Do not penalize incorrect oxidation state notation here.
Accept "EMF" for "voltage".

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