Exam (elaborations)
AS & A Level CHEMISTRY9701/42 Paper 4 A Level Structured Questions May/June 2023 Authentic Marking Scheme
AS & A Level CHEMISTRY9701/42 Paper 4 A Level Structured Questions May/June 2023 Authentic Marking Scheme
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Cambridge International AS & A Level* 8 9 3 6 5 2 3 2 3 2 *
CHEMISTRY 9701/42
Paper 4 A Level Structured Questions May/June 2023
2 hours
You must answer on the question paper.
No additional materials are needed.
INSTRUCTIONS
● Answer all questions.
● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs.
● Write your name, centre number and candidate number in the boxes at the top of the page.
● Write your answer to each question in the space provided.
● Do not use an erasable pen or correction fluid.
● Do not write on any bar codes.
● You may use a calculator.
● You should show all your working and use appropriate units.
INFORMATION
● The total mark for this paper is 100.
● The number of marks for each question or part question is shown in brackets [ ].
● The Periodic Table is printed in the question paper.
● Important values, constants and standards are printed in the question paper.
This document has 24 pages.
DC (CE/SG) 311841/5
© UCLES 2023 [Turn over
, 2
1 (a) Group 2 carbonates decompose when heated to form the metal oxide and carbon dioxide.
(i) Suggest a mechanism for the decomposition of the carbonate ion by adding two curly
arrows in Fig. 1.1.
O
C CO2 + O2–
–O O–
Fig. 1.1
[1]
(ii) Describe the variation in the thermal stability of Group 2 carbonates. Explain your
answer.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [3]
(b) (i) Define lattice energy.
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [2]
o
(ii) The lattice energy of the Group 2 carbonates, ΔH latt (MCO3), becomes less exothermic
down the group.
o
The lattice energy of the Group 2 oxides, ΔH latt (MO), also becomes less exothermic
down the group.
o o
ΔH latt (MCO3) and ΔH latt (MO) change by different amounts going down the group.
Suggest how the standard enthalpy change of the decomposition reaction for Group 2
carbonates changes down the group.
Explain your reasoning in terms of the relative sizes of the anions and the relative
changes in lattice energy down the group.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [2]
© UCLES 2023 9701/42/M/J/23
, 3
(c) Potassium sulfite, K2SO3, is used as a food additive.
The concentration of sulfite ions, SO32–, can be determined by titration using aqueous
acidified manganate(VII) ions, MnO4–.
• A 250 cm3 solution contains 3.40 g of impure K2SO3.
• 25.0 cm3 of this solution requires 22.40 cm3 of 0.0250 mol dm–3 acidified MnO4– to
reach the end-point. All the SO32– ions are oxidised. None of the other species in
the impure K2SO3 are oxidised.
The reaction occurs as shown by the two half-equations.
H2O + SO32– SO42– + 2H+ + 2e–
MnO4– + 8H+ + 5e– Mn2+ + 4H2O
(i) Give the ionic equation for the reaction between SO32– and acidified MnO4–.
..................................................................................................................................... [1]
(ii) Calculate the percentage purity of the sample of K2SO3.
Show your working.
percentage purity of K2SO3 = .............................. [3]
(d) Potassium disulfite, K2S2O5, is another food additive. The disulfite ion, S2O52–, has the
displayed formula shown in Fig. 1.2.
O
O–
α
S S O
O
O–
Fig. 1.2
Deduce the geometry (shape) around the S(α) atom in S2O52–.
geometry around S(α) ................................................. [1]
[Total: 13]
© UCLES 2023 9701/42/M/J/23 [Turn over
, 4
2 (a) State two typical chemical properties of a transition element.
1 ................................................................................................................................................
2 ................................................................................................................................................
[1]
(b) Aqueous solutions of cobalt(II) salts contain the complex ion [Co(H2O)6]2+.
(i) Define complex ion.
...........................................................................................................................................
..................................................................................................................................... [1]
(ii) Samples of [Co(H2O)6]2+ are reacted separately with an excess of aqueous ammonia,
with an excess of concentrated HCl and with an excess of aqueous sodium hydroxide,
as shown in Fig. 2.1.
excess excess
NH3(aq) NaOH(aq)
A [Co(H2O)6]2+(aq) C
excess
concentrated HCl
B
Fig. 2.1
Complete Table 2.1 about the reactions shown by [Co(H2O)6]2+.
Table 2.1
reagent added to formula of cobalt colour and state of
type of reaction
[Co(H2O)6]2+(aq) species formed cobalt species formed
an excess of NH3(aq) A=
an excess of
B=
concentrated HCl
an excess of NaOH(aq) C=
[4]
© UCLES 2023 9701/42/M/J/23
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