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Question 19N.3.sl.TZ0.2

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Date November 2019 Marks available [Maximum mark: 6] Reference code 19N.3.sl.TZ0.2
Level sl Paper 3 Time zone TZ0
Command term Deduce, Describe, Determine, Justify, List Question number 2 Adapted from N/A
2.
[Maximum mark: 6]
19N.3.sl.TZ0.2

Ethanol was electrolysed at different voltages. The products at the anode, ethanoic acid, ethanal and carbon dioxide, were collected and analysed.

The percentages of products obtained using three different catalysts mounted on a carbon anode, platinum (Pt/C), platinum and ruthenium alloy (PtRu/C) and platinum and tin alloy (PtSn/C) are shown.

Chemical yields of ethanoic acid, ethanal and carbon dioxide as a function of voltage for
oxidation of 0.100 mol dm−3 ethanol at Pt/C, PtRu/C and PtSn/C anodes at 80°C.


[Source: Product Distributions and Efficiencies for Ethanol Oxidation in a Proton Exchange Membrane Electrolysis Cell, Rakan M. Altarawneh and Peter G. Pickup, Journal of the Electrochemical Society, 2017, volume 164, issue 7, http://jes.ecsdl.org/. Distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/)]

(a(i))

Describe the effect of increasing the voltage on the chemical yield of:

Ethanal using Pt/C:

Carbon dioxide using PtRu/C:

[2]

Markscheme

Ethanal using Pt/C:
decreases ✔

Carbon dioxide using PtRu/C:
«generally» increases AND then decreases ✔

NOTE: Accept “no clear trend/pattern” OR “increases and decreases” OR “increases, reaches a plateau and «then» decreases” for M2.

(a(ii))

Determine the change in the average oxidation state of carbon. 

From ethanol to ethanal:

From ethanol to carbon dioxide:

[2]

Markscheme

From ethanol to ethanal:
−2 to −1
OR
+1/increases by 1 ✔

NOTE: Do not accept “2− to 1−”.

From ethanol to carbon dioxide:
−2 to +4
OR
+6/increases by 6 ✔

NOTE: Do not accept “2− to 4+”.

Do not penalize incorrect notation twice.

Penalize incorrect oxidation state value of carbon in ethanol once only.

(a(iii))

List the three products at the anode from the least to the most oxidized.

[1]

Markscheme

ethanal < ethanoic acid < carbon dioxide ✔

NOTE: Accept formulas.
No ECF from 2aii calculations.

(b)

Deduce, giving your reason, which catalyst is most effective at fully oxidizing ethanol.

[1]

Markscheme

Pt/platinum/PtC AND highest yield of CO2 «at all voltages» ✔

NOTE: ECF from 2aiii.

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