Edexcel A-Level Chemistry Paper 3, November 2020: Question 4

✅ Correct Answer

Mᵣ(CuSO₄·5H₂O) = 63.5 + 32.1 + (16.0 × 4) + 5 × (1.0 × 2 + 16.0) = 249.6 g mol⁻¹

Mass = moles × Mᵣ = 0.0250 × 249.6 = 6.24 g (Accept 2 or 3 significant figures)

📐 Calculation Steps

1. Find the molar mass (Mᵣ) of the hydrated salt by carefully including the 5 moles of water of crystallization.
2. Multiply the number of moles (0.0250) by the calculated Mᵣ.
3. Ensure correct units ( g ) are understood.

❌ Common Errors

• Omitting the mass of the 5 water molecules ( 5H₂O ) in the Mᵣ calculation.
• Rounding intermediate values too early, leading to slight inaccuracies in final mass.

✅ Correct Answer

Q = ΔH × n = 18.2 kJ mol⁻¹ × 0.0250 mol = 0.455 kJ = 455 J

ΔT = Q / (m × c) = 455 / (45.00 × 4.18) = 2.4189... °C

Final Answer: 2.4 °C (or 2 °C), and state that the temperature decreases.

💡 Key Knowledge

• ΔH₁ = +18.2 kJ mol⁻¹ is endothermic, meaning heat is absorbed from the solution, causing the temperature to drop.
• Watch out for unit conversions: kJ must be converted to J by multiplying by 1000 before using Q = mcΔT .

🧠 Exam Technique

• Significant figures: The data in the prompt uses 3 s.f. ( 18.2 , 45.00 , 4.18 ), so give your answer to 1 or 2 significant figures ( 2.4 °C or 2 °C ) as requested by the mark scheme.
• Don't forget to explicitly state "decreases" to secure the final marking point.

✅ Correct Answer Requirements

• y-axis: Labelled clearly as "Enthalpy" or "H" with units ( kJ mol⁻¹ ) and an appropriate linear scale.
• Entities & Levels: Place CuSO₄(s) + 5H₂O(l) at the top, CuSO₄(aq) in the middle, and CuSO₄·5H₂O(s) at the bottom.
• Arrows & Values: Draw directed arrows showing ΔH₂ = -84.5 kJ mol⁻¹ downwards from solid anhydrous to aqueous, and ΔH₁ = +18.2 kJ mol⁻¹ upwards from hydrated solid to aqueous.

🧠 Exam Technique

• Always use a ruler for energy levels and arrow lines.
• Make sure arrowheads point strictly in the correct vertical direction (down for exothermic, up for endothermic).
• Include state symbols ( s , l , aq ) for all chemical species.

❌ Common Errors

• Omitting units on the enthalpy axis.
• Placing the hydrated level above the anhydrous level (failing to reflect the relative exothermic/endothermic magnitudes).
• Using double-headed arrows instead of single-headed reaction pathway arrows.

✅ Correct Answer

ΔᵣH = ΔH₂ - ΔH₁ = -84.5 - (+18.2) = -102.7 kJ mol⁻¹ (Also accept -103 kJ mol⁻¹ )

💡 Hess's Law Route Analysis

According to Hess's Law, the total enthalpy change of a reaction is independent of the route taken. Going from CuSO₄(s) + 5H₂O(l) to CuSO₄(aq) releases 84.5 kJ mol⁻¹ ( ΔH₂ ), and reversing the hydration step removes 18.2 kJ mol⁻¹ ( ΔH₁ ).

✅ Correct Answer

It is impossible to react exactly 5 moles of water with 1 mole of anhydrous copper(II) sulfate and measure the temperature change of a solid reaction mixture accurately.

❌ Common Errors & Rejections

• Simply stating "heat loss to surroundings" (this applies to all calorimetry, but isn't the primary reason the direct hydration of a solid is impossible).
• Stating "heat is needed to start the reaction" (incorrect, the reaction is spontaneous).
• Vague statements about incomplete mixing without mentioning the specific stoichiometric water constraint (5:1 ratio).