OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify which ionic compound requires the most energy to convert one mole into its gaseous ions based on lattice enthalpy trends.

Practise this question

Question

Multiple choice question 8 asking which compound requires the most energy to convert one mole into its gaseous ions, with options A (NaF), B (Na2O), C (MgF2), and D (MgO), along with a box for the answer and 1 mark indicated.
Question text

8 Which compound requires the most energy to convert one mole into its gaseous ions?

A NaF

B Na2O

C MgF2

D MgO

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 8 is option D, worth 1 mark.

8 D 1 AO1.1

How to answer it

Lattice Enthalpy and Ionic Compound Stability

What this question tests

This question assesses your understanding of lattice dissociation enthalpy (the energy required to convert one mole of a solid ionic compound into its gaseous ions). It tests your ability to apply electrostatic principles—specifically ionic charge and ionic radius—to compare the magnitude of lattice enthalpies across different ionic lattices.

Question 8

Identifying the Compound with the Highest Lattice Enthalpy

✅ Correct Answer: D (MgO)

Magnesium oxide has the highest magnitude of lattice enthalpy among the choices because it contains ions with the highest charges (Mg²⁺ and O²⁻) and relatively small ionic radii, resulting in the strongest electrostatic attraction.

💡 Key Knowledge: Factors Affecting Lattice Enthalpy

  • Ionic Charge: Higher charges lead to stronger electrostatic forces of attraction between ions (proportional to Product of Charges: q₁ × q₂).
  • Ionic Radius: Smaller ions can get closer together, increasing the force of attraction (inversely proportional to ionic radii sum: r⁺ + r⁻).
  • MgO features 2+ and 2- ions, giving a charge product of 4, whereas NaF features 1+ and 1- ions (charge product of 1).

🧠 Exam Technique: Systematic Elimination

  • Step 1: Translate the phrase "energy required to convert one mole into its gaseous ions" as lattice dissociation enthalpy.
  • Step 2: Look at the charges first. Compounds with divalent ions (Mg²⁺, O²⁻) like MgO and Na₂O require significantly more energy to break apart than monovalent ones ( NaF , MgF₂ ). Eliminate A and C.
  • Step 3: Compare Na₂O (Na⁺ and O²⁻) with MgO (Mg²⁺ and O²⁻). Magnesium has a smaller radius and higher charge than sodium, making the Mg²⁺-O²⁻ bond much stronger than Na⁺-O²⁻.

❌ Common Student Errors

  • Confusing formulas with stoichiometry: Students sometimes assume MgF₂ or Na₂O have higher lattice enthalpies simply because they contain more atoms per formula unit.
  • Ignoring ionic radius: Forgetting that ionic radius also plays a crucial role when comparing ions with the same charges.
Mark Scheme Allocation: 1 mark available for selecting option D (AO1.1 - Demonstrating knowledge and understanding of scientific ideas).

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.