DP Chemistry (first assessment 2025)

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Question 23M.2.HL.TZ2.3

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Date May 2023 Marks available [Maximum mark: 12] Reference code 23M.2.HL.TZ2.3
Level HL Paper 2 Time zone TZ2
Command term Annotate, Deduce, Describe, Determine, Explain, Outline, Suggest Question number 3 Adapted from N/A
3.
[Maximum mark: 12]
23M.2.HL.TZ2.3

Electrolysis and Winkler titrations are both applications of redox reactions.

(a)

An electrolytic cell was set up using inert electrodes and a dilute aqueous solution of magnesium chloride, MgCl2 (aq).

(a.i)

Annotate the diagram to show the movement of particles that conduct electricity in this cell.

[2]

Markscheme

electron flow from anode to battery OR from battery to cathode ✓

Mg2+/H+ ions to − electrode
AND
Cl−/OH− ions to + electrode ✓

Do not award M1 if electrons are shown in electrolyte.

(a.ii)

Deduce the half-equation for the reaction at each electrode. Use section 24 of the data booklet.

 

Positive electrode:  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . 


Negative electrode:  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . 

[2]

Markscheme

Positive electrode:
H2O(l) → ½ O2(g) + 2H+(aq) + 2e−
OR
2OH−(aq) → ½O2(g) + H2O(l) + 2e−✓

Negative electrode:
2H2O(l) + 2e− → H2(g) + 2OH−(aq)
OR
2H+(aq) + 2e− → H2(g) ✓ 

Accept 2Cl−(aq) → Cl2(g) + 2e− for M1.

Award [1 max] for correct equations at wrong electrodes.

(a.iii)

Graphite rods are sometimes used as inert electrodes. Describe the structure of graphite and explain why graphite conducts electricity.

[2]

Markscheme

layers «of carbon atoms in a giant structure» ✓
delocalized electrons «flow along layers» ✓

Accept suitable diagram for M1.

Accept two-dimensional network for M1.

Accept electrons are mobile/flow for M2.

(b)

Winkler titrations can be used to determine the biochemical oxygen demand, BOD, of a water sample. One set of equations for the reactions occurring is:

2Mn2+ (aq) + O2 (aq) + 4OH-→ 2MnO(OH)2 (s)

MnO(OH)2 (s) + 2I- (aq) + 4H+ → Mn2+ (aq) + I2 (aq) + 3H2O

I- (aq) + I2 (aq) → I3- (aq)

2S2O32- (aq) + I3- (aq) → S4O62- (aq) + 3I- (aq)

150 cm3 of a water sample was tested using a Winkler titration. 36.0 cm3 of 0.00500 mol dm-3 sodium thiosulfate solution, Na2S2O3 (aq), was required to reach the end point.

(b.i)

Determine the concentration, in mol dm−3, of oxygen dissolved in the water sample.

[3]

Markscheme

«n(S2O32−) = 0.00500 mol dm−3 × 0.0360 dm3 =» 0.000180 / 1.80 × 10−4 «mol» ✓
«n(O2) =» n(S2O32-)4  / 0.0000450 / 4.50 × 10−5 «mol» ✓
«[O2] = 4.50×10-5 mol0.150 dm3  =» 0.000300 / 3.00 × 10−4 «mol dm−3» ✓

 

Award [3] for correct final answer.

(b.ii)

Outline how the BOD of the water sample could be determined.

[2]

Markscheme

titrate/measure dissolved oxygen in «another» water sample «stored under controlled conditions five days» later ✓

difference between two values «is BOD»✓

(b.iii)

Suggest what a low BOD value indicates about a water sample.

[1]

Markscheme

low levels of «organic/oxygen consuming» water pollution ✓