WJEC A-Level Chemistry AS Unit 1, June 2025: Question 12

15 marks · Medium difficulty · Structured Questions

Deduce the structure of lithium hydride, determine stoichiometric ratios and an equation for the thermal decomposition of lithium aluminium hydride, calculate the volume of hydrogen gas formed from lithium reacting with water, and calculate the equilibrium constant Kc and equilibrium concentration of methane.

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Question

Question 12 consisting of parts (a) to (e). Part (a) asks for the type of structure and electrical conductivity explanation for lithium hydride. Part (b) asks for a balanced equation between LiH and AlCl3 to form LiAlH4 and LiCl. Part (c) gives mass and volume data for the thermal decomposition of LiAlH4 and asks for molar ratios and the balanced equation for the unknown product. Part (d) provides the reaction of lithium with water to calculate the volume of hydrogen gas produced at 15 °C and 1 atm. Part (e) gives the steam reforming equilibrium of methane, asking for the Kc expression, units, and a concentration calculation.

Mark scheme

Show the mark scheme Mark scheme for Question 12 showing marking points and mark allocations across AO1, AO2, AO3, Maths, and Practical. Part (a) awards 2 marks for (giant) ionic and ions free to move when molten. Part (b) awards 1 mark for 4LiH + AlCl3 -> LiAlH4 + 3LiCl. Part (c)(i) awards 3 marks for mole calculations showing a 3:2:3 ratio, and (c)(ii) awards 1 mark for 3LiAlH4 -> 2Al + 3H2 + Li3AlH6. Part (d) awards 4 marks for moles of Li, moles of H2, ideal gas calculation, and final volume of 512 cm3. Part (e)(i) awards 2 marks for Kc = [H2]^3[CO] / ([CH4][H2O]) and unit mol^2 dm^-6, and (e)(ii) awards 2 marks for rearranged calculation yielding [CH4] = 0.61 mol dm^-3.

How to answer it

WJEC AS Chemistry: Inorganic Hydrides, Gas Calculations & Chemical Equilibria

What This Question Tests

  • Bonding & Structure: Linking physical properties (electrical conductivity) to giant ionic lattices and mobile ions.
  • Stoichiometry & Balancing Equations: Constructing balanced symbol equations and determining empirical ratios from experimental decomposition data.
  • Gas Calculations: Using the ideal gas equation ( pV = nRT ) with proper SI conversions (temperature in K, pressure in Pa, volume in m³ to cm³).
  • Equilibrium Constant (Kc): Writing homogeneous equilibrium expressions, deriving correct units, and rearranging terms to find unknown equilibrium concentrations.
Part (a) • 2 Marks

Structure and Electrical Conductivity of Lithium Hydride

Explaining molten vs solid electrical conductivity

✅ Correct Answer

  • Structure: (giant) ionic [1 mark]
  • Explanation: Contains ions that are free to move when molten, but held in fixed positions in the solid lattice [1 mark] .

💡 Key Knowledge

Lithium hydride (LiH) consists of Li⁺ cations and hydride (H⁻) anions arranged in a giant 3D ionic lattice. Electrical conduction in ionic compounds requires delocalised charged particles free to move (mobile ions, not electrons).

❌ Common Errors

  • Writing "electrons are free to move" – this scores zero for the second mark. Ionic conduction is mediated purely by ions.
  • Stating "covalent" or "simple molecular" because hydrogen is non-metallic. Group 1 hydrides are ionic.

🧠 Exam Technique

Always state both states explicitly: specify that ions are mobile when molten and fixed / not free to move when solid to guarantee both points.

Part (b) • 1 Mark

Synthesis of Lithium Aluminium Hydride

Balancing the reaction between LiH and AlCl₃

✅ Correct Answer

4LiH + AlCl₃ → LiAlH₄ + 3LiCl

[1 mark] for fully correct species and balancing.

🧠 Exam Technique

Count the hydrogens first: 4 on the RHS in LiAlH₄ requires 4LiH on the LHS. That leaves 3 remaining Li atoms, which pair neatly with the 3 chlorine atoms from AlCl₃ to give 3LiCl .

Part (c)(i) • 3 Marks

Molar Ratio Determination from Decomposition Data

Calculating moles of LiAlH₄, Al, and H₂

📐 Step-by-Step Calculation

  1. Find moles of LiAlH₄:
    Mr(LiAlH₄) = 6.94 + 26.98 + (4 × 1.01) = 37.98 g mol⁻¹
    n(LiAlH₄) = 1.9 ÷ 37.98 = 0.0500 mol
  2. Find moles of Al:
    Ar(Al) = 27.0 g mol⁻¹
    n(Al) = 0.90 ÷ 27.0 = 0.0333 mol
  3. Find moles of H₂:
    At 25 °C (298 K) and 1 atm, molar volume of gas = 24.5 dm³ mol⁻¹ (WJEC data sheet).
    n(H₂) = 1.22 ÷ 24.5 = 0.0498 ≈ 0.0500 mol [1 mark for all three calculated moles]
  4. Find the simplest whole number ratio:
    Divide each by the smallest (0.0333):
    0.0500 ÷ 0.0333 = 1.50
    0.0333 ÷ 0.0333 = 1.00
    0.0500 ÷ 0.0333 = 1.50 [1 mark for working]
    Multiply by 2 → 3 : 2 : 3 [1 mark]

❌ Common Errors

  • Using 24.0 dm³ mol⁻¹ instead of the 25 °C WJEC standard value (24.5 dm³ mol⁻¹).
  • Rounding 1.5 : 1 : 1.5 prematurely to 2 : 1 : 2 instead of multiplying by 2.

🧠 Exam Technique

Always state which substance corresponds to each number in the ratio clearly: LiAlH₄ : Al : H₂ = 3 : 2 : 3 .

Part (c)(ii) • 1 Mark

Deducing the Unknown Decomposition Product

Balancing the decomposition equation

✅ Correct Answer

3LiAlH₄ → 2Al + 3H₂ + Li₃AlH₆

The unknown product formula is Li₃AlH₆ (lithium hexahydridoaluminate) [1 mark] .

🧠 Exam Technique

Use the stoichiometric ratio calculated in (c)(i) as coefficients: 3LiAlH₄ → 2Al + 3H₂ + [Product] . Count remaining atoms: 3 Li, 1 Al, and (12 − 6) = 6 H → gives precisely Li₃AlH₆.

Part (d) • 4 Marks

Gas Calculation Using the Ideal Gas Equation

Reaction: 2Li + 2H₂O → 2LiOH + H₂

📐 Step-by-Step Calculation

  1. Calculate moles of Li:
    n(Li) = 0.300 ÷ 6.94 = 0.04323 mol [1 mark]
  2. Use molar ratio to find moles of H₂:
    From equation, 2 mol Li produces 1 mol H₂ (ratio 2:1).
    n(H₂) = 0.04323 ÷ 2 = 0.02161 mol [1 mark]
  3. Convert values into SI units for pV = nRT:
    T = 15 °C + 273 = 288 K
    p = 1 atm = 1.01 × 10⁵ Pa (or 1.013 × 10⁵ Pa)
    R = 8.31 J mol⁻¹ K⁻¹
  4. Calculate Volume (V) in m³:
    V = (nRT) ÷ p = (0.02161 × 8.31 × 288) ÷ (1.01 × 10⁵) = 5.12 × 10⁻⁴ m³ [1 mark]
  5. Convert volume to cm³:
    V = 5.12 × 10⁻⁴ × 10⁶ = 512 cm³ (accepts 510 – 515 cm³) [1 mark]

❌ Common Errors

  • Molar ratio missed: Forgetting that 2 mol of Li gives only 1 mol of H₂, leading to an answer of 1024 cm³.
  • Unit conversion errors: Converting m³ to cm³ by multiplying by 10³ instead of 10⁶ (1 m³ = 100 × 100 × 100 cm³ = 10⁶ cm³).
  • Celsius trap: Using 15 °C directly in the ideal gas equation rather than 288 K.

🧠 Exam Technique

Always write down your intermediate SI units clearly. If you make an arithmetic slip, error carried forward (ecf) can still award 3 out of 4 marks provided your method is clear.

Part (e) • 4 Marks

Equilibrium Constant Expression & Concentration Calculation

Reaction: CH₄(g) + H₂O(g) ⇌ 3H₂(g) + CO(g)

✅ Part (e)(i) Answer • 2 Marks

Kc Expression:

Kc = ([H₂]³ × [CO]) ÷ ([CH₄] × [H₂O])

[1 mark]

Units derivation:

(mol dm⁻³)³(mol dm⁻³) ÷ (mol dm⁻³)(mol dm⁻³) = (mol dm⁻³)² = mol² dm⁻⁶

[1 mark]

📐 Part (e)(ii) Calculation • 2 Marks

  1. Rearrange for [CH₄]:
    [CH₄] = ([H₂]³ × [CO]) ÷ (Kc × [H₂O]) [1 mark]
  2. Substitute equilibrium concentrations:
    [H₂] = 0.55 mol dm⁻³
    [CO] = 0.26 mol dm⁻³
    [H₂O] = 0.77 mol dm⁻³
    Kc = 0.092
  3. Calculate numerator & denominator:
    Numerator = (0.55)³ × 0.26 = 0.166375 × 0.26 = 0.0432575
    Denominator = 0.092 × 0.77 = 0.07084
  4. Evaluate [CH₄]:
    [CH₄] = 0.0432575 ÷ 0.07084 = 0.61 mol dm⁻³ (to 2 s.f.) [1 mark]

❌ Common Errors in (e)

  • Forgetting to cube [H₂] in the numerator: calculating [H₂] instead of [H₂]³.
  • Inverting the Kc expression (putting reactants over products).
  • Writing round brackets ( ) instead of square brackets [ ] when stating the expression for Kc.

🧠 Exam Technique

Match the significant figures of your final answer to the data provided. The values in the stem (0.092, 0.77, 0.55, 0.26) are given to 2 significant figures, making 0.61 mol dm⁻³ the ideal presentation.

Topics

Physical Chemistry · Inorganic Chemistry · 1.1 Formulae and equations · 1.3 Chemical calculations · 1.4 Bonding · 1.5 Solid structures · 1.7 Simple equilibria and acid-base reactions

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