DP Chemistry (first assessment 2025)

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Question 23M.2.SL.TZ2.3

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Date May 2023 Marks available [Maximum mark: 10] Reference code 23M.2.SL.TZ2.3
Level SL Paper 2 Time zone TZ2
Command term Annotate, Describe, Determine, Explain, Identify, Outline, Suggest Question number 3 Adapted from N/A
3.
[Maximum mark: 10]
23M.2.SL.TZ2.3

Electrolysis and Winkler titrations are both applications of redox reactions.

(a)

An electrolytic cell was set up using inert electrodes and molten magnesium chloride, MgCl2 (l).

 

(a.i)

Identify the product formed at the cathode.

[1]

Markscheme

magnesium/Mg «metal» ✓

 

Do not accept magnesium ions/Mg2+.

(a.ii)

Annotate the diagram to show the movement of electrons.

[1]

Markscheme

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

 

Do not award mark if electrons are shown in electrolyte.

(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 ✓