Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 2

14 marks · Medium difficulty · Open Response

Describe and compare the bonding, structure, properties, and reactions of sodium, sodium chloride, and aluminium hydride compounds.

Practise this question

Question

Question 2 with parts (a) and (b). Part (a)(i) provides two boxes to draw labelled 2D diagrams showing the structures of sodium metal and sodium chloride, each containing nine ions. Part (a)(ii) asks to describe what holds the ions together in both substances. Part (a)(iii) asks what the difference in melting points indicates about bonding. Part (a)(iv) asks to compare and contrast their electrical conductivity and charge carriers. Part (b)(i) asks for the equation forming sodium hydride from its elements. Part (b)(ii) asks to balance the reaction between NaH and AlCl3 to form NaAlH4 and NaCl. Part (b)(iii) asks for the name and explanation of the H-to-Al dative bond in AlH4-. Part (b)(iv) is a multiple-choice question choosing the name of the shape of AlCl3 and the diagram of AlH4-.

Mark scheme

Show the mark scheme Mark scheme for Question 2 detailing: (a)(i) 3 marks for regular 3x3 array of positive ions with labelled delocalised electrons for Na, and alternating positive and negative ions for NaCl. (a)(ii) 2 marks for electrostatic attraction between cations and delocalised electrons in Na, and oppositely charged ions in NaCl. (a)(iii) 1 mark for metallic bonding in sodium being weaker than ionic bonding in NaCl. (a)(iv) 3 marks for similarity (both conduct when molten) and differences (Na conducts as solid, electrons carry charge in Na vs ions in NaCl). (b)(i) 1 mark for 2Na + H2 -> 2NaH. (b)(ii) 1 mark for 4NaH + AlCl3 -> NaAlH4 + 3NaCl. (b)(iii) 2 marks for dative covalent bond and lone pair donation from H- to Al. (b)(iv) 1 mark for option D (trigonal planar and tetrahedral wedge-and-dash diagram).

How to answer it

Structure, Bonding & Shapes of Molecules: Sodium & Aluminium Compounds

📌 WHAT THIS QUESTION TESTS

This question assesses core understanding across physical and inorganic chemistry fundamentals:

  • Lattice Diagrams: Constructing accurate 2D representations of giant metallic and giant ionic structures (9 ions alternating in regular 3×3 grids).
  • Bonding Definitions: Electrostatic attractions defining metallic and ionic bonds.
  • Physical Properties: Explaining melting points and electrical conductivity mechanisms (delocalised electrons vs. mobile ions).
  • Equations & Stoichiometry: Balancing simple synthesis and multi-reactant redox/salt precipitation equations.
  • Dative Covalent Bonding: Explaining coordinate bond formation via lone pair donation.
  • VSEPR & Molecular Shapes: Predicting and representing 3D shapes (trigonal planar vs. tetrahedral with wedge-dash convention).

Part (a)(i) — Drawing 2D Crystal Lattices

3 Marks | Metallic Lattice (Na) & Giant Ionic Lattice (NaCl)

✅ Marking Requirements

Sodium Metal (2 marks):

  • A regular array (e.g. 3×3 grid) of exactly 9 singly positively charged ions labelled + or Na⁺ . [1]
  • Delocalised electrons labelled between ions (e.g. e⁻ or dots labelled "delocalised electrons"). [1]

Sodium Chloride (1 mark):

  • A regular 3×3 array of strictly alternating cations and anions ( + and − or Na⁺ and Cl⁻ ). [1]

💡 Visualizing the Drawings

Sodium Metal Drawing:
Draw a 3×3 grid of touching/adjacent circles, each containing a + sign. In the gaps between the circles, scatter small symbols labelled e⁻ with an arrow pointing to them annotated "delocalised electrons".
Sodium Chloride Drawing:
Draw a 3×3 grid of touching circles with strictly alternating charges:
Row 1: (+) (−) (+)
Row 2: (−) (+) (−)
Row 3: (+) (−) (+)
(Or starting with −; 5 anions & 4 cations or vice versa).

❌ Common Errors

  • Drawing random numbers of circles (the question strictly specifies nine ions).
  • In NaCl, placing like charges next to each other in the row or column (violates the alternating lattice).
  • Leaving the electrons unlabelled in the sodium diagram, or omitting charges entirely.

🧠 Exam Technique

  • Count your ions carefully! Exactly 9 circles per box.
  • Always add a label to the electrons: write "delocalised electron" with a direct pointer.
Total: 3 Marks — 1 mark for 9 Na⁺ in a regular array, 1 mark for delocalised electrons between ions, 1 mark for alternating 3×3 ionic grid.

Part (a)(ii) — Bonding Forces Holding Ions Together

2 Marks | Definitions of Metallic and Ionic Bonding

✅ Model Answer

Sodium metal: Electrostatic attraction between positive metal ions ( Na⁺ ) and the sea of delocalised electrons. [1]

Sodium chloride: Electrostatic attraction between oppositely charged ions ( Na⁺ and Cl⁻ ). [1]

❌ Examiner Pitfall & Guidance

  • Critical Word: The term "electrostatic attraction" must be used! If "electrostatic" is omitted completely across both answers, a maximum of 1 mark can be awarded.
  • Do not refer to "intermolecular forces" in giant lattices—these are primary chemical bonds.
Total: 2 Marks — 1 mark per correct definition explicitly mentioning electrostatic attractions.

Part (a)(iii) — Comparing Melting Points and Bond Strengths

1 Mark | Deducing Relative Bond Strength

✅ Model Answer

The ionic bonding in sodium chloride is stronger than the metallic bonding in sodium (or the metallic bonding in sodium is weaker than the ionic bonding in NaCl; or more energy is required to overcome the ionic bonds in NaCl than the metallic bonds in Na). [1]

🧠 Exam Technique

Melting temperature directly reflects the strength of the electrostatic forces holding the lattice together. Since NaCl has a higher melting point, its ionic bonds are simply stronger and require more thermal energy to break.

Total: 1 Mark — Comparative statement linking higher melting point to stronger ionic bonding.

Part (a)(iv) — Compare and Contrast Electrical Conductivity

3 Marks | State-dependent Conductivity & Charge Carriers

✅ Marking Breakdown (3 Points)

  • Similarity: Both conduct electricity when molten / liquid. [1]
  • Difference: Only sodium metal conducts electricity in the solid state (sodium chloride does not conduct as a solid / only conducts when molten or aqueous). [1]
  • Charge Carriers: In sodium, delocalised electrons carry the charge, whereas in sodium chloride, mobile ions carry the charge. [1]

❌ Common Misconceptions

  • Saying "electrons move in molten NaCl" — Zero marks for charge carrier. In molten NaCl, current is carried exclusively by mobile Na⁺ and Cl⁻ ions.
  • Forgetting to state the similarity: "Compare and contrast" commands require at least one similarity and one difference!
  • Note: The mark scheme explicitly states to ignore discussions of aqueous solutions of NaCl, as the comparison is primarily about the solids/liquids themselves.
Total: 3 Marks — 1 mark similarity (both molten), 1 mark difference in solid state, 1 mark identifying both charge carriers correctly.

Part (b)(i) & (b)(ii) — Chemical Equations

2 Marks | Synthesis of NaH and Formation of NaAlH₄

✅ Balanced Equations

(b)(i) Reaction of sodium metal with hydrogen:

2Na + H₂ → 2NaH

(Multiples allowed, e.g. Na + ½H₂ → NaH. State symbols not required.) [1]

(b)(ii) Reaction of NaH with AlCl₃:

4NaH + AlCl₃ → NaAlH₄ + 3NaCl

(Fill in balancing numbers: 4, [1], [1], 3) [1]

🧠 Equation Balancing Tips

  • Hydrogen gas is diatomic ( H₂ ), whereas the hydride ion is H⁻ . Never write Na + H → NaH .
  • For (b)(ii), count the chlorine atoms first: 3 chlorines in AlCl₃ demand 3NaCl on the right. Then balance sodium: 1 in NaAlH₄ + 3 in 3NaCl = 4 Na, which gives 4NaH . This instantly balances the 4 hydrogens!
Total: 2 Marks — 1 mark each for correct balanced equations.

Part (b)(iii) — Dative Covalent Bonding in AlH₄⁻

2 Marks | Identification and Formation Mechanism

✅ Model Answer

  • Name of bond: Dative covalent bond (or coordinate bond). [1]
  • How it forms: A lone pair of electrons is donated from the hydride ion ( H⁻ ) to an empty orbital on the aluminium atom ( Al ). [1]

❌ Lost Mark Pitfall

  • Forgetting the word "pair": writing "an electron is donated" scores 0 for the second mark. It must be a lone pair or pair of electrons.
  • Specifying the wrong donor/acceptor: the arrow H → Al shows the hydride ion donates into aluminium, not the reverse.
Total: 2 Marks — 1 mark for naming dative/coordinate bond, 1 mark for lone pair donation from H⁻ to Al.

Part (b)(iv) — Shapes of AlCl₃ and AlH₄⁻

1 Mark | Multiple Choice (VSEPR Theory)

✅ Correct Answer: D

  • Shape of AlCl₃: trigonal planar
  • Diagram of AlH₄⁻: 3D tetrahedral structure with 2 solid bonds in-plane, 1 wedge coming forward, 1 dashed wedge going back into the page:
    Tetrahedral representation:
    Top Al—H (in-plane), bottom-left Al—H (in-plane), bottom-centre Al◄H (wedge), bottom-right Al┄H (dash), enclosed in square brackets with an overall negative charge [ ... ]⁻ .

🧠 Why Other Options Are Incorrect

  • Options A & B are incorrect: Al in AlCl₃ has 3 bonding pairs and 0 lone pairs, so it adopts a trigonal planar geometry with 120° bond angles (not a pyramidal shape, which requires a lone pair like NH₃).
  • Option C is incorrect: The diagram shows a flat square planar cross shape, which fails to show 3D tetrahedral geometry ( AlH₄⁻ has 4 bonding pairs, 0 lone pairs → tetrahedral, 109.5°). Option D correctly displays the wedge-and-dash convention.
Total: 1 Mark — Correct option identified is D.

Topics

Physical Chemistry · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.