Chemistry 2020 Oct/Nov | Past Paper Questions & Answers

Q1.

Short Answer 1 mark

Represent the ions 1H+, 2H+ and 3H+. State, in terms of subatomic particles in the nucleus, what is the same about each of these ions and what is different.

Model Answer:

Same: each ion has 1 proton in the nucleus.

Explanation: All hydrogen ions are isotopes of hydrogen and therefore each nucleus contains one proton.

Model Answer:

Different: they have different numbers of neutrons.

Explanation: 1H+ has 0 neutrons, 2H+ has 1 neutron and 3H+ has 2 neutrons.

Q2a.

Short Answer 1 mark

State the trend in acid/base behaviour of the oxides of the Period 3 elements, from Na to S.

Model Answer:

The oxides change from basic to amphoteric to acidic across Period 3.

Explanation: Metal oxides are basic, aluminium oxide is amphoteric, and non-metal oxides are acidic.

Q2b.

Short Answer 2 marks

State and explain the trend, from Na to S, in the maximum oxidation number of the Period 3 elements in their oxides.

Model Answer:

The maximum oxidation number increases from +1 to +6 across the period.

Explanation: The number of outer-shell electrons increases from Na to S, so the highest oxidation state in oxides increases.

Q2c.

Short Answer 2 marks

Sodium oxide and phosphorus(V) oxide both react with water. Name the product of each reaction.

Model Answer:

Sodium hydroxide

Explanation: Na2O + H2O forms NaOH.

Model Answer:

Phosphoric(V) acid

Explanation: P4O10 / P2O5 reacts with water to form H3PO4.

Q2d.

Short Answer 3 marks

Explain why phosphorus(V) oxide has a low melting point of approximately 300 °C but magnesium oxide has a high melting point of approximately 2850 °C.

Model Answer:

Phosphorus(V) oxide is a simple molecular substance with weak intermolecular forces between molecules, so little energy is needed to melt it.

Model Answer:

Magnesium oxide has a giant ionic lattice with strong electrostatic attractions between Mg2+ and O2− ions.

Model Answer:

A large amount of energy is needed to overcome these strong ionic bonds, so MgO has a much higher melting point.

Q2e(i).

Short Answer 1 mark

Write an equation for the reaction of aluminium oxide with excess aqueous hydrochloric acid.

Model Answer:

Al2O3 + 6HCl → 2AlCl3 + 3H2O

Q2e(ii).

Short Answer 1 mark

Write an equation for the reaction of aluminium oxide with excess aqueous sodium hydroxide.

Model Answer:

Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4]

Q2f.

Short Answer 2 marks

Describe the lattice structure of silicon(IV) oxide. Your answer should include reference to the arrangement of the silicon and oxygen atoms and the bonds between them.

Model Answer:

A giant covalent lattice with each silicon atom covalently bonded to four oxygen atoms and each oxygen atom covalently bonded to two silicon atoms.

Q2g(i).

Short Answer 1 mark

Write an equation for the reaction of sodium oxide with silicon(IV) oxide to form sodium silicate(IV), Na2SiO3.

Model Answer:

Na2O + SiO2 → Na2SiO3

Q2g(ii).

Short Answer 1 mark

Write an equation for the thermal decomposition of sodium carbonate, forming sodium oxide and carbon dioxide.

Model Answer:

Na2CO3 → Na2O + CO2

Q3a(i).

Short Answer 1 mark

Write an equation for the formation of PCl5 from the reaction of phosphorus and chlorine.

Model Answer:

P4 + 10Cl2 → 4PCl5

Q3a(ii).

Short Answer 1 mark

State the type of structure and bonding shown by liquid PCl5.

Model Answer:

Simple molecular structure with covalent bonding.

Q3b(i).

Short Answer 1 mark

Give one other observation you would make when PCl5 reacts with excess water.

Model Answer:

Steamy fumes / white mist / vigorous effervescence may be observed.

Q3b(ii).

Short Answer 1 mark

Write the equation for the reaction of PCl5 with excess water.

Model Answer:

PCl5 + 4H2O → H3PO4 + 5HCl

Q3b(iii).

Short Answer 1 mark

Estimate the pH of the resulting solution.

Model Answer:

About 1

Q3c(i).

Short Answer 1 mark

Identify a reagent that could be used in reaction 1.

Model Answer:

PCl3

Q3c(ii).

Short Answer 2 marks

Explain what is meant by the term optical isomer and circle any atom(s) in T that give rise to optical isomerism.

Model Answer:

Optical isomers are non-superimposable mirror images.

Model Answer:

They arise from carbon atoms bonded to four different groups.

Q3c(iii).

Short Answer 1 mark

T is a minor product in the reaction of compound S with excess HCl. Draw the structure of the major product of the reaction of S with excess HCl.

Model Answer:

The major product is the fully substituted chloroalkane formed by replacing all hydroxyl groups in S with chlorine atoms.

Q3d(i).

Short Answer 2 marks

Predict the shape of the NCl3 molecule and the Cl-N-Cl bond angle.

Model Answer:

Trigonal pyramidal

Model Answer:

About 107°

Q3d(ii).

Short Answer 2 marks

Explain fully what is meant by the term weak Brønsted-Lowry acid.

Model Answer:

A Brønsted-Lowry acid is a proton donor.

Model Answer:

A weak acid only partially dissociates / ionises in water.

Q3d(iii).

Short Answer 4 marks

Calculate the final total pressure inside the sealed container at 20 °C after the NCl3(l) has fully decomposed.

Model Answer:

Final total pressure = 1.24 × 10^5 Pa

Q4a(i).

Short Answer 1 mark

State the type of reaction that occurs in reaction 1.

Model Answer:

Reduction

Q4a(ii).

Short Answer 2 marks

Suggest the reagent(s) and conditions required for reaction 1.

Model Answer:

NaBH4 in aqueous solution, at room temperature.

Q4a(iii).

Short Answer 1 mark

Draw the structure of the organic product, H, from reaction 2.

Model Answer:

The product is the carboxylic acid formed by oxidation of the aldehyde group to COOH.

Q4a(iv).

Short Answer 1 mark

State what you would observe in reaction 3.

Model Answer:

A silver mirror forms / a grey silver precipitate is produced.

Q4a(v).

Short Answer 1 mark

Give the type of reaction shown by reaction 4.

Model Answer:

Esterification

Q4b(i).

Short Answer 1 mark

Name the type of structural isomerism shown by G and J.

Model Answer:

Functional group isomerism

Q4b(ii).

Short Answer 2 marks

Suggest one chemical test that can distinguish G from J. Give the result of the test with each compound.

Model Answer:

Use aqueous bromine.

Model Answer:

G decolourises bromine water; J does not react / no change.

Q4b(iii).

Short Answer 1 mark

State the role of NaBH4 in the reactions with G and J.

Model Answer:

Reducing agent

Q4b(iv).

Short Answer 1 mark

Identify the organic product K.

Model Answer:

The alcohol formed by reduction of the carbonyl group in G and J.

Q4c.

Short Answer 3 marks

Complete the table with the expected observations for the reactions of P and Q with the named reagents.

Model Answer:

Br2(aq): P decolourises bromine water; Q no change.

Model Answer:

2,4-dinitrophenylhydrazine: P orange precipitate; Q orange precipitate.

Model Answer:

Aqueous sodium carbonate: P effervescence / CO2 produced; Q no reaction.

Q4d(i).

Short Answer 1 mark

Identify the bonds responsible for the absorptions marked X and Z.

Model Answer:

X: C=O

Model Answer:

Z: O–H

Q4d(ii).

Short Answer 1 mark

Name M.

Model Answer:

Hexanedioic acid

Q4d(iii).

Short Answer 1 mark

Use the information in (d) to deduce the molecular formula of L.

Model Answer:

C10H18O4

CAIE Chemistry (9701)

Questions & Answers

CAIE Chemistry `(9701)`