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Date May 2022 Marks available 1 Reference code 22M.2.sl.TZ2.1
Level SL Paper 2 Time zone TZ2
Command term Justify and Identify Question number 1 Adapted from N/A

Question

Lithium reacts with water to form an alkaline solution.

A 0.200 g piece of lithium was placed in 500.0 cm3 of water.

Determine the coefficients that balance the equation for the reaction of lithium with water.

[1]
a.

Calculate the molar concentration of the resulting solution of lithium hydroxide.

[2]
b(i).

Calculate the volume of hydrogen gas produced, in cm3, if the temperature was 22.5 °C and the pressure was 103 kPa. Use sections 1 and 2 of the data booklet.

[2]
b(ii).

Suggest a reason why the volume of hydrogen gas collected was smaller than predicted.

[1]
b(iii).

The reaction of lithium with water is a redox reaction. Identify the oxidizing agent in the reaction giving a reason.

[1]
c.

Describe two observations that indicate the reaction of lithium with water is exothermic.

[2]
d.

Markscheme

2 Li (s) + 2 H2O (l) → 2 LiOH (aq) + H2 (g) ✔

a.

nLi«0200g6.94g=»0.0288«mol»

«nLiOH = nLi»

LiOH «=0.0288mol0.5000dm3=»0.0576 «moldm-3» ✔

 

Award [2] for correct final answer.

b(i).

«nH2=12×0.0288mol=0.0144mol»

«V=nRTP=»0.0144mol×8.31JK-1mol-1×22.5+273K103kPa «×103»

V=343 «cm3»

 

Award [2] for correct final answer.

Accept answers in the range 334 – 344 cm3.

Award [1 max] for 0.343 «cm3/dm3/m3».

Award [1 max] for 26.1 cm3 obtained by using 22.5 K.

Award [1 max] for 687 cm3 obtained by using 0.0288 mol.

b(ii).

lithium was impure/«partially» oxidized

OR

gas leaked/ignited ✔

 

Accept “gas dissolved”.

b(iii).

H2O AND hydrogen gains electrons «to form H2»

OR

H2O AND H oxidation state changed from +1 to 0 ✔

 

Accept “H2O AND H/H2O is reduced”.

c.

Any two:

temperature of the water increases ✔

lithium melts ✔

pop sound is heard ✔

 

Accept “lithium/hydrogen catches fire”.

Do not accept “smoke is observed”.

d.

Examiners report

[N/A]
a.
[N/A]
b(i).
[N/A]
b(ii).

This part-question was better answered than part (ii). 50% of the candidates drew a correct arrow between n=2 and n=3. Both absorption and emission transitions were accepted since the question did not specify which type of spectrum was required. Some teachers commented on this in their feedback. Mistakes often included transitions between higher energy levels.

b(iii).
[N/A]
c.
[N/A]
d.

Syllabus sections

Core » Topic 9: Redox processes » 9.1 Oxidation and reduction
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Core » Topic 9: Redox processes
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