DP Chemistry (first assessment 2025)

Question 22M.2.SL.TZ2.1

Date	May 2022	Marks available	[Maximum mark: 9]	Reference code	22M.2.SL.TZ2.1
Level	SL	Paper	2	Time zone	TZ2
Command term	Calculate, Describe, Determine, Identify, Justify, Suggest	Question number	1	Adapted from	N/A

[Maximum mark: 9]

22M.2.SL.TZ2.1

Lithium reacts with water to form an alkaline solution.

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

(a)

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

[1]

Markscheme

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

(b(i))

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

[2]

Markscheme

$n_{L i} « \frac{0200 g}{6.94 g} = » 0.0288 « m o l »$ ✔

«n_LiOH = n_Li»

$[LiOH] « = \frac{0.0288 m o l}{0.5000 d m^{3}} = » 0.0576 « m o l d m^{- 3} »$ ✔

Award [2] for correct final answer.

(b(ii))

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]

Markscheme

$« n_{H_{2}} = \frac{1}{2} \times 0.0288 m o l = 0.0144 m o l »$

$« V = \frac{n R T}{P} = » (\frac{0.0144 m o l \times 8.31 J K^{- 1} m o l^{- 1} \times (22.5 + 273) K}{103 k P a}) « \times 10^{3} »$ ✔

$V = 343 « {cm}^{3} »$ ✔

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(iii))

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

[1]

Markscheme

lithium was impure/«partially» oxidized

gas leaked/ignited ✔

Accept “gas dissolved”.

Examiners report

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.

(c)

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

[1]

Markscheme

H₂O AND hydrogen gains electrons «to form H₂»

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

Accept “H₂O AND H/H₂O is reduced”.

(d)

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

[2]

Markscheme

Any two: