Edexcel A-Level Chemistry Paper 1, November 2021: Question 6

✅ Correct Answer

Two reasons required for the 2 marks:

• There is an increase in the number of moles of particles (from 3 moles of solid reactants to 13 moles of products).
• There is a change of state producing gases, liquids, and aqueous solutions from solid reactants.

💡 Key Knowledge

Entropy measures the dispersal of thermal energy within a system. Systems that lead to more ways of arranging particles and energy quanta (more disorder) experience a positive entropy change. Gaseous states and larger numbers of moles significantly increase entropy.

🧠 Exam Technique

Always count moles carefully from the balanced equation: 1 Ba(OH)₂·8H₂O(s) + 2 NH₄SCN(s) gives 3 moles of solid on the left, turning into 1 + 2 + 10 = 13 moles of products on the right. Explicitly state both mole increase and state changes to secure both independent marks.

❌ Common Errors

Students lose marks by vague statements like "there are more types of products". Avoid referencing enthalpy change (ΔH) when explaining entropy changes. Note that examiners accept particle counts ranging from 3 to 13, but do not award credit if you write absolute statements like "11 to 13 moles".

✅ Correct Answer

The explanation must state:

• The reaction is endothermic (absorbs thermal energy from its surroundings).
• This causes the water drops on the wooden block to freeze, gluing the block to the flask.

💡 Key Knowledge

Endothermic reactions absorb heat from the immediate surroundings, causing a dramatic temperature drop. When water reaches 0 °C under a cold reaction vessel, phase change occurs from liquid water to solid ice.

🧠 Exam Technique

Link thermodynamic concepts directly to macroscopic observations. Use clear cause-and-effect language: "Because the reaction absorbs heat, the surroundings cool down, which freezes the water..."

❌ Common Errors

Confusing endothermic and exothermic directions. Simply stating "the temperature drops" without explicitly mentioning that the reaction is endothermic or missing the link to the water freezing will cause you to drop marks.

✅ Correct Answer

D (-216.6 J K⁻¹ mol⁻¹)

📐 Calculation Steps

1. Formula: ΔS_system = ΣS(products) - ΣS(reactants)
2. Products: 2 moles of MgO(s) = 2 × 26.9 = 53.8 J K⁻¹ mol⁻¹
3. Reactants: (2 × S(Mg)) + S(O₂) = (2 × 32.7) + 205.0 = 65.4 + 205.0 = 270.4 J K⁻¹ mol⁻¹
4. Subtract: 53.8 - 270.4 = -216.6 J K⁻¹ mol⁻¹

🧠 Exam Technique

Always write down the full equation including stoichiometric coefficients before plugging in values. Double-check that you multiply individual standard molar entropies by the large numbers in front of the chemical species in the balanced equation.

❌ Common Errors

Option A (-210.8) occurs if you forget to multiply by 2 for Mg and MgO and subtract backwards. Option B (-210.8) happens if you omit the stoichiometric multipliers for magnesium terms. Option C (+216.6) is the classic trap of doing reactants minus products instead of products minus reactants.