OCR A-Level Chemistry AS Depth in chemistry (02), June 2024: Question 2

17 marks · Medium difficulty · Structured Questions

Analyze periodicity, electron configurations, redox reactions of calcium with hydrochloric acid, and first ionisation energy trends for Group 2 and related elements.

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Question

A structured chemistry question about periodicity, electron configurations, redox equations, moles calculations, and an ionisation energy graph from Li to Mg. It includes tables to fill in electron configurations, chemical equations to balance or explain, and a line graph plotting first ionisation energy against elements.
Question text

2 This question is about periodicity and the reaction of some Group 2 metals.

(a) Periodicity is the repeating trend in properties of elements across different periods in the periodic

table.

(i) Complete the table below with the electron configurations and blocks.

Group 2 Group 17 (7)

Period 2 Be F

1s2 … 1s2 …

Period 3 Mg Cl

1s2 … 1s2 …

Block

[3]

(ii) Use your answers to (a)(i) to explain why electron configuration is an example of a periodic trend.

… [2]

(iii) Mg forms 2+ ions but Cl usually forms 1– ions in their reactions. Explain why.

… [2]

(iv) Magnesium reacts with oxygen in the air.

Write the equation for this reaction.

… [1]

(b) The reaction between calcium and hydrochloric acid is a redox reaction.

Ca(s) + 2HCl(aq) CaCl2(aq) + H2(g) Equation 2.1

(i) Explain, in terms of electron transfer, why the reaction shown in equation 2.1 is a redox reaction.

… [2]

(ii) A student plans to add 0.0100 mol of Ca to 120 cm3 of 0.100 mol dm–3 HCl (aq).

When the student carries out this reaction, they are surprised that all the calcium reacts, despite

being in excess of the HCl(aq).

• Show by calculation that calcium is in excess of the HCl(aq).

• Suggest a reason for this unexpected result.

… [3]

(c) The graph shows the first ionisation energies for the elements Li to Be and for C to Na.

2000

1500

First ionisation

energy 1000

/ kJ mol–1

Li Be B C N O F Ne Na Mg

(i) Complete the graph by adding points for the missing values of B and Mg. [2]

(ii) Write an equation, including state symbols, to represent the second ionisation energy of B.

… [2]

Mark scheme

Show the mark scheme The official mark scheme showing required electron configurations, explanations for periodic trends and ion changes, redox explanations in terms of electron transfer, mole calculations showing reactant ratios, and ionisation energy graph plotting points with corresponding equations for successive ionisation energy.

Question Answer Marks Guidance

2 (a) (i) Be: 1s22s2 F: 1s22s22p5 ✓ 3 1 mark per correct row

Mg: 1s22s22p63s2 Cl: 1s22s22p63s23p5 ✓ ALLOW upper case letter S and P, and subscripts,

e.g. 2S22P5

Block: s p ✓ IGNORE superscripts/numbers given on block (e.g. s2 and p5) if

the letter is clear

(ii) Across period 2, the (2)s subshell fills first, followed 2 ALLOW Elements in the same group have same number of

by the (2)p ✓ electrons in their outer shells or subshell

e.g. s2 in group 2/ s2p5 in group 17(7)

ALLOW Elements in the same period have the same number of

energy levels/shells

same pattern or trend of filling (the subshells) ALLOW for both marks for indication that the pattern repeats

repeated in other periods ✓ across each period e.g

Across each period, elements repeat the pattern of electrons

filling the s-subshell then p-subshell ✓✓

(iii) Mg loses (2) electrons 2 ALLOW Mg is oxidised AND Cl is reduced

AND

Cl gains an electron ✓

To gain a full/complete shell

OR

Noble gas configuration

OR

Stable/full octet✓

(iv) 2Mg + O2 → 2MgO ✓ 1 ALLOW multiples

e.g. Mg + O2 → MgO

IGNORE state symbols even if wrong

(b) (i) Ca loses 2 electrons AND Oxidised✓ 2

H gains 1 electron (per atom) AND Reduced✓ ALLOW H gains an electron OR gains electrons OR gains 2

electrons

ALLOW 1 mark for Ca is oxidised AND H is reduced

ALLOW 1 mark for Ca loses electron(s) AND H gains

electron(s)

IGNORE oxidation numbers even if incorrect

(ii) n(HCl) = 0.012 (mol)✓ 3

n(Ca) required to react with HCl = 0.006 (mol) Second mark must show recognition of the 2:1 ratio

OR e.g. ALLOW ratio is 1:2 but here only 1:1.2 so Ca is in excess

0.0100 mol Ca would need 0.02 mol HCl to

completely react ✓

Ca reacts with water ✓

(c) (i) B is below Be but above Li (about 800 kJ mol–1) ✓ 2

Mg is above Na but below Be (about 700 kJ mol–1) ✓ DO NOT ALLOW if on the line of 900 kJ mol–1. It must be clear

that IE for Mg is less than Be as below it in group 2

+ 2+ – 13 + – 2+

(ii) B (g) → B (g) + e 2 ALLOW B (g) - e → B (g) for 2 marks

Equation correct ✓

The second mark is dependent upon the first mark except for

Correct state symbols ✓ the following close attempts:

ALLOW one mark for the following for state symbols

B(g) → B2+(g) + 2e–

B+(g) + e– → B2+(g) + 2e–

B(g) → B+(g) + e–

ALLOW e for electron (i.e. charge omitted)

IGNORE states on the electron

How to answer it

Periodicity and Group 2 Chemistry Study Guide

What this question tests

This question assesses your understanding of periodicity, electron configurations (subshells and blocks), ionic bonding explanations, redox definitions in terms of electron transfer, quantitative moles calculations with limiting reagents, and first/second ionisation energy trends and equations.

Question 2(a): Electron Configurations and Periodicity

Part (i): Completing Electron Configurations and Blocks

✅ Correct Answers

  • Be: 1s² 2s²
  • F: 1s² 2s² 2p⁵
  • Mg: 1s² 2s² 2p⁶ 3s²
  • Cl: 1s² 2s² 2p⁶ 3s² 3p⁵
  • Be & Mg Block: s
  • F & Cl Block: p

❌ Common Errors

  • Forgetting that Period 3 elements fill the 2p subshell completely before moving to the 3s/3p subshells.
  • Writing upper-case letters for subshells (e.g. writing 2S instead of 2s ).
Total: 3 marks (1 mark per correct row)

Part (ii): Explaining Periodic Trends

💡 Key Knowledge

Across Period 2, the 2s subshell fills first, followed by the 2p subshell. This exact same pattern or trend of subshell filling repeats in subsequent periods (e.g., filling 3s then 3p).

🧠 Exam Technique

To secure both marks, you must mention two things: how subshells fill sequentially within a period, and that this same pattern repeats across different periods.

Total: 2 marks

Part (iii): Explaining Ion Charges

✅ Correct Answers

Mg loses 2 electrons AND Cl gains 1 electron in order to achieve a full/complete outer shell (or stable noble gas configuration / stable octet).

❌ Common Errors

Only stating that "Magnesium is in Group 2" without linking it to electron loss or shell stability. You must explicitly describe the movement of electrons and the energetic driving force (noble gas configuration).

Total: 2 marks

Part (iv): Equation for Magnesium in Air

✅ Correct Answers

2Mg + O₂ → 2MgO

(Accepts multiples such as Mg + ½O₂ → MgO )

Total: 1 mark

Question 2(b): Redox Reactions and Quantitative Calculations

Part (i): Redox in Terms of Electron Transfer

✅ Correct Answers

Ca loses 2 electrons and is oxidised. H gains 1 electron (per atom) and is reduced.

🧠 Exam Technique

Always structure your redox explanation using OILRIG: explicitly state which species loses/gains electrons, how many, and whether it is oxidation or reduction.

Total: 2 marks

Part (ii): Limiting Reagents and Unexpected Observations

📐 Calculation Steps

  1. Calculate moles of HCl:
    n(HCl) = (120 / 1000) × 0.100 = 0.0120 mol
  2. Use stoichiometry:
    Equation: Ca + 2HCl → CaCl₂ + H₂
    Moles of Ca required = 0.0120 / 2 = 0.0060 mol.
  3. Compare:
    The student added 0.0100 mol of Ca, which is greater than 0.0060 mol. Therefore, Ca is in excess.

💡 Suggesting the Unexpected Result

Reason: Calcium reacts with water (present in the aqueous acid) as well as the hydrochloric acid: Ca + 2H₂O → Ca(OH)₂ + H₂ . This consumes extra calcium, which explains why all the calcium disappeared despite calculations suggesting it was in excess relative to the HCl.

Total: 3 marks

Question 2(c): Ionisation Energies

Part (i): Interpreting Ionisation Energy Trends

✅ Correct Answers

  • Boron (B): Plotted below Be, but above Li (approx. 800 kJ mol⁻¹), due to shielding by the 2s subshell as the electron enters the 2p subshell.
  • Magnesium (Mg): Plotted above Na, but below Be (approx. 700 kJ mol⁻¹), showing regular group trends down Group 2.

❌ Common Errors

Plotting Mg higher than Be, or placing the point directly on a grid line without accounting for the general decrease down a group.

Total: 2 marks

Part (ii): Second Ionisation Energy Equation for Boron

✅ Correct Answers

B⁺(g) → B²⁺(g) + e⁻

🧠 Exam Technique Checklist

  • ✔️ Start with a 1+ gaseous ion ( B⁺(g) ).
  • ✔️ Remove one electron to form a 2+ gaseous ion ( B²⁺(g) + e⁻ ).
  • ✔️ State symbols are mandatory for ionisation energies (gas phase (g) required for species).
Total: 2 marks

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), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.