OCR A-Level Chemistry AS Depth in chemistry (02), November 2020: Question 3

18 marks · Medium difficulty · Structured Questions

Explain the bonding, structure, properties, and reactions of Period 4 elements and their compounds including calcium, bromine, barium, selenium, and sodium bromate.

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Question

Question text

3 This question is about some elements in Period 4 of the periodic table.

(a) The table shows the melting point and electrical conductivity of two elements in Period 4.

Element Melting Point / °C Electrical conductivity

Calcium 842 Good

Bromine –7 Poor

Use your knowledge of structure and bonding to explain the properties in the table.

… [5]

(b) Calcium reacts with bromine to form calcium bromide, CaBr2.

(i) Draw a ‘dot-and-cross’ diagram to show the bonding in CaBr2.

Show outer electrons only.

[2]

(ii) The reaction of barium with bromine is more vigorous than the reaction of calcium with

bromine.

Explain why.

… [3]

Selenium and oxygen both form compounds with hydrogen with the formulae H2Se and H2O

respectively.

(i) H2Se can be prepared by reacting aluminium selenide, Al2Se3, with water.

Aluminium hydroxide and hydrogen selenide are formed.

Write the equation for this reaction.

… [1]

(ii) The boiling points of H2O and H2Se are shown below.

Compound Boiling point /°C

H2O 100

H2Se –41

Explain why H2O has a higher boiling point than H2Se.

… [3]

(d) Bromine reacts with concentrated sodium hydroxide at 50 °C as in the equation below.

3Br2 + 6NaOH 5NaBr + NaBrO3 + 3H2O

(i) Write the systematic name for NaBrO3.

… [1]

(ii) This reaction is an example of disproportionation.

Use oxidation numbers to explain why. Include the meaning of the term

disproportionation.

… [3]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

3 (a) Ca: metallic bonding OR giant metallic lattice 5 1.1 ×2 ALLOW Metallic structure

DO NOT ALLOW reference to molecules or

intermolecular forces for calcium

Br2: simple molecular OR simple covalent ALLOW ‘are molecules’

Induced dipole(–dipole) forces/interactions 2.1×1 IGNORE

OR London forces • permanent dipole(–dipole) forces

• IDID and LDF

• van der Waals

Conductivity linked to mobile electrons 3.2 ×2 DO NOT ALLOW ‘free electrons’ for mobile

In Ca electrons are mobile electrons

OR electrons are delocalised

OR electrons can move

AND in Br2 charge carriers/electrons are not mobile

ALLOW comparison, e.g.

Melting point linked to bond strengths • Metallic bonds are stronger than London

Metallic bonds are strong forces

AND London forces are weak OR

OR • Metallic bonds need more energy to break

Metallic bonds need a large amount of energy to break than London forces

AND London forces need little energy to break ALLOW intermolecular forces instead of London

forces for this mark

element

(b) (i) 2 ALLOW separate Br– ions, i.e.

Ca shown with either 8 or 0 electrons

AND

Br shown with 8 electrons with 7 crosses and 1 dot 1.2 ×1

(or vice versa) 10

For first mark, if eight electrons are shown around

2.5 ×1

Correct charges on both ions Ca, the ‘extra’ electrons around Br must match the

symbol chosen for the electrons for Na.

IGNORE inner shells

Circles or brackets not required

(ii) Atomic radius 3 Comparison required throughout

Ba has a greater atomic radius than Ca ORA throughout

OR Ba has more shells 1.1 ×1

OR Ba has more shielding For more shells, ALLOW higher energy level

IGNORE more orbitals OR more sub-shells

Attraction IGNORE ‘different shell’ or ‘new shell’

Nuclear attraction is less in Ba

OR (outer) electrons in Ba are less attracted (to ALLOW Ba has less nuclear pull’

nucleus) OR ‘Ba electrons are less tightly held’

OR Increased distance / shielding in Ba outweighs

increased nuclear charge 2.3 ×2 IGNORE less effective nuclear charge’

IGNORE ‘nuclear charge’ for ‘nuclear attraction’

Ionisation energy

Ionisation energy of Ba is less ALLOW easier to oxidise Ba

OR easier to remove (outer) electrons in Ba

element

(ii) H2O has hydrogen/H-bonds (between molecules) 3 1.1 ×2

H2Se has induced dipole(-dipole) interactions ALLOW permanent dipole-dipole interactions

OR London forces

H-bonds are stronger (than other intermolecular forces) 2.1 ×1

OR more energy needed to overcome H-bonds

(d) (i) Sodium bromate(V) 1 2.5 ×1

(ii) Br is oxidised AND reduced 3 1.1 ×1 ALLOW same element is both oxidised and

OR Br oxidation number is increased and decreased reduced

Br is oxidised from 0 to +5 2.2 ×2 ALLOW 1 mark if all 3 oxidation numbers are

correct (even if oxidation/reduction incorrectly

Br is reduced from 0 to –1 assigned)

How to answer it

Period 4 Elements, Bonding, Intermolecular Forces & Redox

What this question tests

This comprehensive question assesses your understanding of bonding types (metallic vs. simple molecular), electrical conductivity mechanisms, dot-and-cross diagrams for ionic compounds, periodic trends in Group 2 reactivity down the group, writing chemical equations from descriptions, intermolecular forces (hydrogen bonding vs. London forces), systematic nomenclature, and definitions and identification of disproportionation reactions.

Part (a) — Explaining Melting Points and Electrical Conductivity

5 Marks

✅ Correct Answer Structure

Calcium: Metallic bonding / giant metallic lattice. Contains mobile delocalised electrons that carry charge. Strong metallic bonds require a large amount of energy to break (high melting point).
Bromine: Simple molecular / simple covalent. Contains induced dipole-dipole forces (London forces) between molecules. Weak intermolecular forces require little energy to overcome (low melting point, poor electrical conductivity as there are no mobile charge carriers).

❌ Common Student Errors

Referring to "free electrons" instead of the required "mobile" or "delocalised" electrons.
Confusing intramolecular bonds (covalent bonds within Br₂) with intermolecular forces (London forces between Br₂ molecules).
Stating that calcium contains molecules.

Mark breakdown: 1 mark for Ca structure, 1 mark for Br₂ structure, 1 mark for conductivity linked to mobile electrons in Ca and lack of charge carriers in Br₂, 2 marks for melting point linked to bond/force strength.

Part (b)(i) — Dot-and-Cross Diagram for Calcium Bromide

2 Marks

✅ Correct Answer Requirements

Ca shown with a 2+ charge and either 8 or 0 outer shell electrons.
Br shown with a 1- charge, surrounded by 8 electrons (7 crosses and 1 dot, or vice versa). Two separate Br⁻ ions or a coefficient of 2 is acceptable.

🧠 Exam Technique

When drawing ionic lattices or units, always include the correct integer charges outside square brackets. Ensure your electron symbols (dots and crosses) clearly show electron transfer from the metal to the non-metal.

Mark breakdown: 1 mark for correct electron distribution on both ions, 1 mark for correct charges on both ions.

Part (b)(ii) — Reactivity of Barium vs. Calcium with Bromine

3 Marks

💡 Key Knowledge (Group 2 Trends)

Atomic Radius: Barium has a greater atomic radius and more electron shells/shielding than calcium.
Nuclear Attraction: The outer electrons in barium experience greater shielding and increased distance, resulting in weaker nuclear attraction.
Ionisation Energy: The first and second ionisation energies of barium are lower, making its outer electrons easier to remove (easier to oxidise).

❌ Common Errors

Failing to make a direct comparison (using comparative terms like "greater", "lower", "more shielding").
Writing "nuclear charge" instead of "nuclear attraction" or "effective nuclear charge".

Mark breakdown: 1 mark for atomic radius/shells/shielding comparison, 2 marks for nuclear attraction and ionisation energy/ease of electron loss.

Part (c)(i) & (ii) — Equations and Intermolecular Forces

1 Mark (i) + 3 Marks (ii)

✅ Correct Answers

(i) Equation: Al₂Se₃ + 6H₂O → 2Al(OH)₃ + 3H₂Se
(ii) Boiling Point Explanation: H₂O has hydrogen bonding between molecules, whereas H₂Se only has weaker induced dipole-dipole interactions (London forces). Hydrogen bonds are significantly stronger and require more thermal energy to overcome.

🧠 Exam Technique

For boiling point or melting point comparisons involving hydrides of Group 16 (or 15/17), always explicitly state the type of intermolecular force present in both substances before comparing their relative strengths.

Mark breakdown: (i) 1 mark for balanced equation including correct states/formulae. (ii) 1 mark for H-bonding in water, 1 mark for London forces in H₂Se, 1 mark for relative strength/energy comparison.

Part (d)(i) & (ii) — Nomenclature and Disproportionation

1 Mark (i) + 3 Marks (ii)

💡 Key Knowledge & Definitions

(i) Systematic Name: Sodium bromate(V) (Roman numerals indicate the oxidation state of bromine).
(ii) Disproportionation Definition: A reaction in which the same element is simultaneously oxidised and reduced.
Oxidation States: Bromine starts at 0 (in Br₂), increases to +5 in NaBrO₃ (oxidation), and decreases to -1 in NaBr (reduction).

❌ Common Errors

Omitting Roman numerals in systematic names where elements exhibit multiple oxidation states.
Forgetting to explicitly state both oxidation and reduction processes when defining disproportionation.

Mark breakdown: (i) 1 mark for correct systematic name. (ii) 1 mark for defining disproportionation (element simultaneously oxidised and reduced), 1 mark for identifying Br oxidation changes, 1 mark for correct oxidation numbers (0 to +5 and 0 to -1).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.