AQA A-Level Chemistry Paper 3, June 2018: Question 2

✅ Correct Answer

Equation:

4Na + O₂ → 2Na₂O

(Also accepted: 2Na + O₂ → Na₂O₂ )

Observations (both required for M2):

• Flame: Yellow flame or orange flame
• Product: White solid or white smoke / white fumes / white ash

🧠 Exam Technique

• Two observations = two sensory details: Combustion reactions typically show a characteristic flame colour and a solid/fume residue. Always state both.
• State symbols are not required unless explicitly requested, but writing correct empirical formulas is essential.
• Do not confuse observations with identity: say "white smoke/solid", never just "sodium oxide forms".

❌ Common Errors

• Writing "orange-yellow precipitate" — precipitates form only in liquid solutions, not gas-phase burning! Examiners apply the list principle and penalise the use of "precipitate".
• Listing conflicting flame colours (e.g. "lilac" or "white"). Sodium is uniquely associated with a yellow-orange flame.

✅ Correct Answer

Equation:

4P + 5O₂ → P₄O₁₀

(Also accepted: P₄ + 5O₂ → P₄O₁₀ or formation of P₄O₆: 4P + 3O₂ → P₄O₆ )

Observation (one required):

• White flame / brilliant white light
• OR White smoke / white fumes / white solid

💡 Key Knowledge

• Phosphorus burns vigorously with oxygen to form phosphorus(V) oxide.
• Molecular formula of phosphorus(V) oxide is P₄O₁₀ (a dimer of P₂O₅).
• AQA mark schemes strictly penalise equations giving P₂O₅ as a product because P₄O₁₀ is the actual molecular unit.

❌ Common Errors

• Writing P₂O₅: The mark scheme explicitly notes: "Do not allow equations with P₂O₅".
• Writing "white precipitate" instead of "white fumes" or "white solid".

✅ Correct Answer

• M1: The magnesium ion has a higher charge (Mg²⁺ vs Na⁺) / higher charge density / Mg²⁺ is smaller than Na⁺.
• M2: Stronger electrostatic attraction between oppositely charged ions (between Mg²⁺ and O²⁻) / stronger ionic bonding.

🧠 Exam Technique: Preventing Contradiction Errors (CE=0)

This is an ionic lattice comparison. If your answer mentions:

• "Molecules" or "intermolecular forces"
• "Covalent bonds" breaking
• "Metallic bonds"

The examiner will award a Chemical Error (CE = 0), scoring 0/2 immediately. Always specify ionic attraction between ions.

❌ Common Errors

• Referring to the atom rather than the ion: writing "Mg has more protons" or "Mg atom is smaller" fails to score M1. You must explicitly state magnesium ion or Mg²⁺.
• Failing to mention the word ions or ionic when discussing attractions.

✅ Correct Answer

• M1 (Structure of SiO₂): Silicon dioxide is a giant covalent / macromolecular structure.
• M2 (Structure of P₄O₁₀): Phosphorus oxide is a simple molecular (molecular) structure.
• M3 (Comparison of forces): Covalent bonds (throughout the lattice of SiO₂) are much stronger than the weak intermolecular forces (van der Waals' forces) between P₄O₁₀ molecules.

💡 The Golden Rule of Melting Point Comparisons

Always address three checkpoints:

1. Name the structure type of both substances.
2. Identify the exact particles and forces broken during melting (covalent bonds vs intermolecular forces).
3. Explicitly compare the energy required (covalent bonds require much more energy to break than intermolecular forces).

❌ Common Errors

• Describing P₄O₁₀ as "simple covalent" — the mark scheme accepts simple molecular, not "simple covalent".
• Describing the elements instead of the oxides (e.g. discussing Si and P rather than SiO₂ and P₄O₁₀).
• Claiming that covalent bonds break when P₄O₁₀ melts. Only intermolecular forces break between P₄O₁₀ molecules!
• Note: M3 is strictly dependent on achieving both M1 and M2.

✅ Correct Answer

• M1 (Apparatus): Place the solid sample into a capillary tube and insert it into an electrically heated melting point apparatus (or Thiele tube / oil bath).
• M2 (Procedure): Heat slowly / gently / gradually as the temperature nears the melting point to accurately identify the melting range.
• M3 (Purity evaluation): A pure sample melts sharply at the known literature value (573 K). An impure sample melts over a broad range and at a lower temperature.

🧠 Practical Trap: Choosing the Heating Medium

Table 2 states the melting point of phosphorus oxide is 573 K (300 °C).

Fatal Error: Suggesting a water bath will immediately lose M1! Water boils at 100 °C (373 K), making it impossible to melt a compound at 300 °C. You must use an oil bath or a dedicated electrical melting point apparatus.

❌ Common Errors

• Saying "heat strongly" instead of "heat slowly/gradually" — rapid heating causes thermal lag, making temperature readings inaccurate.
• Vague purity statements: stating only "see if it melts at 573 K". You must mention sharp melting point or that impurities cause a lower melting point / broader range.