OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 9

1 mark · Easy difficulty · Multiple Choice

Identify the equation that represents the second ionisation energy of magnesium.

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Question

Question 9 asks: 'Which equation describes the second ionisation energy of magnesium?' followed by four multiple-choice options: A: Mg(g) → Mg2+(g) + 2e−, B: Mg(s) → Mg2+(g) + 2e−, C: Mg+(g) → Mg2+(g) + e−, and D: Mg2+(g) + e− → Mg+(g), with an answer box at the bottom worth 1 mark.
Question text

9 Which equation describes the second ionisation energy of magnesium?

A Mg(g) Mg2+(g) + 2e–

B Mg(s) Mg2+(g) + 2e–

C Mg+(g) Mg2+(g) + e–

D Mg2+(g) + e– Mg+(g)

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme snippet showing question number 9 with the correct answer C, awarded 1 mark.

9 C 1

How to answer it

Second Ionisation Energy of Magnesium

📋 What this question tests

This question assesses your precise understanding of successive ionisation energies and how to represent them using chemical equations, specifically:

  • Defining successive (second) ionisation energy.
  • Correct reactant species, product species, and stoichiometry of electrons lost.
  • Correct state symbols (all species must be in the gaseous phase).
Question 9 (Multiple Choice)

Identifying the Correct Equation

1 Mark • OCR AS Chemistry

✅ Correct Answer

C: Mg⁺(g) → Mg²⁺(g) + e⁻

Mark Scheme Breakdown:
• C is the only credited answer [1 mark].

The second ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

💡 Key Knowledge

  • Ionisation always removes electrons one by one:
    • 1st IE: Mg(g) → Mg⁺(g) + e⁻
    • 2nd IE: Mg⁺(g) → Mg²⁺(g) + e⁻
    • 3rd IE: Mg²⁺(g) → Mg³⁺(g) + e⁻
  • State Symbols: Must always be gaseous, (g) , because ionisation energies are defined for gaseous species isolated from intermolecular/lattice interactions.
  • Number of electrons: Each successive ionisation step involves the removal of exactly 1 electron ( e⁻ ) per atom/ion.

🧠 Exam Technique: Elimination Strategy

  • Check starting state: Eliminate B immediately because magnesium starts as a solid Mg(s) .
  • Check electron count: Ionisation energies are defined per single electron removed. Eliminate A and B because they remove 2e⁻ simultaneously (representing the sum of 1st and 2nd IE).
  • Check direction of reaction: Ionisation is electron removal (oxidation). Option D shows an electron being added (reduction), which is the reverse process!
  • Result: Only C remains.

❌ Common Errors & Misconceptions

  • Confusing 2nd IE with forming a 2+ ion from an atom: Many students select A ( Mg(g) → Mg²⁺(g) + 2e⁻ ), forgetting that this is the sum of the 1st and 2nd ionisation energies, not the second ionisation energy itself.
  • Forgetting state symbols: Overlooking the state symbol in B ( s instead of g ).
  • Reversing the equation: Choosing D, which represents electron capture / reverse ionisation.

Topics

Module 3: Periodic table and energy · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.