OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 3

1 mark · Easy difficulty · Multiple Choice

Explain why the second electron affinity of oxygen is an endothermic (positive) value.

Practise this question

Question

A Born-Haber cycle diagram for sodium oxide, Na2O. The cycle shows energy levels starting from 2Na(s) + 1/2 O2(g), going through atomisation and ionisation to 2Na+(g) + O(g) + 2e-, then first electron affinity down to 2Na+(g) + O-(g) + e-, then second electron affinity upwards to 2Na+(g) + O2-(g), and finally lattice enthalpy downwards to Na2O(s). Below the diagram, multiple choice question 3 asks: 'Why is the value for the second electron affinity of oxygen positive?' with four options A to D.

Mark scheme

Show the mark scheme Mark scheme table row showing question number 3, correct answer B, and mark 1.

How to answer it

Born-Haber Cycles: Second Electron Affinity of Oxygen

📌 What this question tests

This question assesses your understanding of enthalpy changes of electron affinity within a Born-Haber cycle. Specifically, it tests why the first electron affinity is exothermic (negative value), whereas the second electron affinity is always endothermic (positive value), based on electrostatic principles.

Question 3 (Multiple Choice)

Identifying the reason for an endothermic second electron affinity

✅ Correct Answer

B: An electron is added to a negative ion.

Award [1 mark] for selecting B.

💡 Key Knowledge

  • 1st Electron Affinity: O(g) + e⁻ → O⁻(g)
    Exothermic (ΔH is negative). Energy is released because the incoming electron is attracted to the positively charged oxygen nucleus.
  • 2nd Electron Affinity: O⁻(g) + e⁻ → O²⁻(g)
    Endothermic (ΔH is positive). Energy must be put in to overcome the strong electrostatic repulsion between the negative incoming electron and the negatively charged O⁻ ion.
  • In the Born-Haber cycle diagram, notice the arrow going from 2Na⁺(g) + O⁻(g) + e⁻ upwards to 2Na⁺(g) + O²⁻(g) , showing that energy is absorbed.

🧠 Exam Technique: Elimination Strategy

  • Eliminate C and D immediately: Electron affinity is the gain/addition of electrons, not removal. Removal of electrons defines ionisation energy.
  • Evaluate A vs B: The second electron added to oxygen enters the 2p subshell (not a high-energy or new shell; oxygen simply completes its octet: 2s² 2p⁴ → 2s² 2p⁵ → 2s² 2p⁶). Thus, statement A is false.
  • Confirm B: The electron is being forced onto an already negative species ( O⁻ ), which requires energy to overcome like-charge repulsion.

❌ Common Errors

  • Confusing EA with IE: Mixing up definitions and choosing options C or D because "positive enthalpy usually means ionisation".
  • Assuming forming a full shell is always exothermic: Students often mistakenly think that achieving a noble gas electron configuration (O²⁻ having 8 valence electrons) must automatically release energy. Overcoming electron-electron repulsion outweighs this.
  • Misreading cycle arrows: Failing to link the direction of arrows in the cycle (downwards = exothermic, upwards = endothermic).

Topics

Module 5: Physical chemistry and transition elements · 5.2 Energy

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