AQA A-Level Chemistry AS Paper 1, June 2025: Question 4

✅ Mark Scheme Model Answer

• M1: Sr (cations / atoms) have a larger radius (or Sr has more shells / more shielding / lower charge density).
• M2: Therefore, Sr has weaker attraction between the cations (or nucleus) and the delocalised electrons (or weaker metallic bonding).

Note: The converse for Ca is fully acceptable.

💡 Key Knowledge

Metallic bond strength depends on two key factors:

• Ionic radius / size: Down Group 2, ionic radius increases.
• Attraction: Larger ionic radius means the positive nucleus is further from the sea of delocalised electrons, reducing electrostatic attraction.

🧠 Exam Technique

• Always follow the 2-step structure for melting point questions:
  1. Compare size / shielding / charge density.
  2. Explicitly state the effect on electrostatic attraction to delocalised electrons.
• Do not forget the word "delocalised" when naming the electrons in metallic bonding.

❌ Common Errors

• Referring to intermolecular forces or covalent bonds being broken (metals have metallic lattices, not molecules).
• Failing to mention delocalised electrons when describing the attraction.
• Vaguely stating "bonds are easier to break" without attributing it to ionic size or distance.

✅ Mark Scheme Model Answer

More electron shells (or more energy levels).

💡 Key Knowledge

Each successive element down a group has valence electrons entering an entirely new principal quantum energy level (shell) further from the nucleus.

🧠 Exam Technique

Keep it simple and precise: more shells or more energy levels is all that is required for this 1-mark question.

❌ Common Errors

• Saying "more sub-shells" or "more orbitals" instead of shells / energy levels.
• Explaining shielding without explicitly stating there are more shells.

✅ Mark Scheme Model Answer

Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)

Allow multiples. Allow ionic equation: Ba(s) + 2H₂O(l) → Ba²⁺(aq) + 2OH⁻(aq) + H₂(g)

💡 Key Knowledge

Metal + Water → Metal Hydroxide + Hydrogen

Group 2 hydroxides become more soluble down the group: Mg(OH)₂ is sparingly soluble, but Ba(OH)₂ is soluble, hence its state is (aq).

🧠 Exam Technique

• The question specifically states "including state symbols". Omitting them will forfeit the entire mark.
• Double-check water is a liquid (l) , not aqueous (aq) .

❌ Common Errors

• Writing barium oxide BaO instead of barium hydroxide Ba(OH)₂ (oxides form with steam at high temperatures, not liquid water).
• Incorrect formula for barium hydroxide, e.g., BaOH instead of Ba(OH)₂ .
• Writing H₂O(aq) or Ba(OH)₂(s) .

✅ Mark Scheme Model Answer

Ca⁺(g) → Ca²⁺(g) + e⁻

Also accepted: Ca⁺(g) - e⁻ → Ca²⁺(g) or Ca⁺(g) + e⁻ → Ca²⁺(g) + 2e⁻

💡 Key Knowledge

The n-th ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous ions with a charge of (n-1)+ .

For the 2nd IE, you start with the +1 gaseous ion and form the +2 gaseous ion plus an electron.

🧠 Exam Technique

• Ionisation energy definitions always require gaseous state symbols (g) for all species except the free electron.
• Check the starting charge! 2nd IE starts with Ca⁺(g) , NOT neutral Ca(g) .

❌ Common Errors

• Writing the equation for two ionisations at once: Ca(g) → Ca²⁺(g) + 2e⁻ (this is the sum of 1st + 2nd IE).
• Writing 1st IE: Ca(g) → Ca⁺(g) + e⁻ .
• Omitting the (g) state symbols.

✅ Mark Scheme Model Answer

Barium (or Ba )

Allow: Radium / Ra

💡 Key Knowledge

Ionisation energies (1st, 2nd, etc.) decrease down Group 2 because:

• Atomic / ionic radius increases.
• Shielding from inner shells increases.
• Outermost electron is further from the nucleus and less strongly held, requiring less energy to remove.

🧠 Exam Technique

The prompt specifies "metals Mg to Ba" at the start of Question 04, so Ba is the expected answer (though Ra is credited if given).

❌ Common Errors

• Naming Magnesium (confusing lowest ionisation energy with highest).
• Giving an element not in Group 2.