OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 16

✅ Correct Answer

A labelled diagram showing a regular 2D lattice of positive ions (specifically Mg²⁺ ) surrounded by delocalized electrons (shown as e⁻ or e ). Must explicitly mention that delocalized electrons are mobile and move to conduct electricity.

💡 Key Knowledge

• Metallic bonding is the electrostatic attraction between positive metal ions and delocalized electrons.
• Conduction occurs because the delocalized electrons are mobile and can flow through the structure when a potential difference is applied.

❌ Common Errors

• Omitting the charge on the metal ions ( Mg²⁺ must be clearly stated or drawn).
• Failing to explicitly label the diagram or state that electrons are delocalized and mobile.

✅ Correct Answer

Mg³⁺(g) → Mg⁴⁺(g) + e⁻

🧠 Exam Technique

Remember that ionisation energy always refers to the removal of 1 mole of electrons from 1 mole of gaseous ions. State symbols (g) are mandatory for all species involved.

❌ Common Errors

Forgetting state symbols or writing incorrect ionic charges (e.g., starting with Mg²⁺ instead of Mg³⁺ for the fourth ionisation energy).

✅ Correct Answer

There is a large jump / exceptionally large difference in ionisation energy between the 2nd and 3rd ionisation energies.

💡 Key Knowledge

The first two electrons are removed from the outer shell relatively easily. The 3rd electron is removed from an inner, principal quantum shell closer to the nucleus, experiencing significantly less shielding and much stronger electrostatic attraction.

✅ Correct Answer

Ticks should be placed under ionisation numbers: 1, 2, 3, 4, 9, 11, 12

💡 Key Knowledge

Magnesium has the electron configuration 1s² 2s² 2p⁶ 3s² . Removing electrons: 1st & 2nd come from 3s ; 3rd, 4th, 5th come from 2p ; 9th & 10th come from 2s ; 11th & 12th come from 1s .

✅ Correct Answer

The enthalpy change when 1 mole of a compound/substance/solute dissolves (in water to form an infinitely dilute solution).

❌ Common Errors

Saying "energy released" (solution can be endothermic or exothermic) or failing to specify "1 mole of substance".

✅ Correct Answer

Top-right dotted line: Mg²⁺(aq) + 2F⁻(aq)
Bottom-middle dotted line: MgF₂(aq)

🧠 Exam Technique

Trace the pathways carefully. Aqueous ions combine or dissociate to form the aqueous solution state, keeping stoichiometry balanced ( 2F⁻ ).

📐 Step-by-Step Calculation

1. Identify the cycle relationship:
   Δsol H = Δlattice H + Δhyd H (Mg²⁺) + 2 × Δhyd H (F⁻)
2. Substitute the values from the table:
   Δsol H = (-2926) + (-1920) + (2 × -506)
3. Calculate:
   Δsol H = -2926 - 1920 - 1012 = -5858... wait, following the specific signs in the mark scheme diagram:
   -2926 + (-1920) + 2(-506) = -2926 - 1920 - 1012 = -5858 ? Let's check energy cycle direction: Lattice enthalpy is exothermic (-2926) or defined as dissociation. Using the given energy cycle arrows: Δsol H = -(-2926) + (-1920) + 2(-506) = 2926 - 1920 - 1012 = -6 kJ mol⁻¹
4. Final Answer with Unit: -6 kJ mol⁻¹

💡 Key Knowledge & Marking Points

• Ionic Radius: Halide ion gets larger down the group.
• Lattice Enthalpy: Lattice enthalpy becomes less exothermic down the group (ions are larger, weaker attraction to Mg²⁺).
• Hydration Enthalpy: Hydration enthalpy becomes less exothermic down the group (larger halide ions have weaker attraction to water molecules / H₂O).
• Conclusion: It is difficult to predict because both lattice enthalpy and hydration enthalpy decrease (become less exothermic) in magnitude, and it depends on which factor has the bigger effect on the overall enthalpy of solution.

🧠 Top-Level Response Guidance

To access all 4 marks, you must systematically discuss both contributing thermodynamic terms (lattice enthalpy and hydration enthalpy) in terms of ionic radius/charge density, and explicitly conclude why their opposing or parallel trends make a definitive prediction impossible without calculation.