OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 6

✅ Correct Answer

• Smooth best-fit curve drawn through points on Fig. 6.1.
• Tangent drawn carefully at exactly t = 50 s .
• Calculated gradient: 0.44 ± 0.2 cm³ s⁻¹ .

🧠 Exam Technique

• Use a sharp pencil and a clear plastic ruler to draw your tangent so you can see the graph grid underneath.
• Make your gradient triangle as large as possible to minimise percentage reading errors.
• Show your working clearly on the graph and write down the coordinates used for Δy / Δx.

❌ Common Errors

• Interpolation trap: Taking a direct reading from the graph at 50 s instead of finding the gradient of the tangent. Examiners strictly penalise this!
• Misreading scale divisions on the time or volume axes.

✅ Correct Answer

• Advantage: No loss of gas (system is closed or easier to capture mass change without gas escaping unmeasured, though watch for vapor loss).
• Disadvantage: Very small loss in mass (hydrogen gas has a very low molar mass, making total mass changes extremely small and harder to measure accurately on a 2 d.p. balance).

💡 Key Knowledge

• Hydrogen gas ( H₂ ) escapes into the atmosphere during the reaction, causing the total mass of the flask and contents to decrease.
• A 2 d.p. balance lacks sensitivity for tiny mass changes when dealing with sub-gram quantities of reactants.

❌ Common Errors

• Vague statements like "easier to set up" or "more accurate" without specifying *why* (examiners ignore generic terminology).

📐 Step-by-Step Calculation

1. Find moles of strontium used:
   Molar mass of Sr = 87.6 g mol⁻¹
   Moles = 0.26 g / 87.6 g mol⁻¹ = 2.968 × 10⁻³ mol
2. Apply stoichiometric ratio:
   The equation shows a 1:1 molar ratio between Sr and Ba (when using the "same amount, in moles"):
   Moles of Ba required = 2.968 × 10⁻³ mol
3. Calculate mass of barium:
   Molar mass of Ba = 137.3 g mol⁻¹
   Mass = Moles × Molar mass = 2.968 × 10⁻³ × 137.3 = 0.4075 g
4. Round to required format:
   Round to 2 decimal places = 0.41 g

❌ Calculation Traps

• Forgetting to round to the requested 2 decimal places.
• Using incorrect molar masses from the periodic table.

✅ Correct Answers ((ii) & (iii))

• Graph sketch (ii): Steeper initial gradient (faster reaction) and levels off earlier, reaching the *same* maximum volume of gas (since moles of reactant are identical).
• Reactivity (iii): Barium is more reactive with water than strontium.

💡 Key Knowledge: Explaining Down Group 2

• Atomic Radius: Barium has a greater atomic radius / more electron shells than strontium, increasing distance from the nucleus.
• Shielding: Greater inner-shell electron shielding in barium.
• Nuclear Attraction: The increased shielding and distance outweigh the increased nuclear charge, resulting in *less* overall attraction from the nucleus on outer electrons.
• Ionisation Energy: First and second ionisation energies decrease down the group, making outer electrons easier to remove ( Ba(s) → Ba²⁺(aq) + 2e⁻ ).

❌ Common Errors

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• Failing to make a direct comparative statement ("Ba is more reactive than Sr" vs just describing Ba).
• Using imprecise terminology like "more nuclear charge" without mentioning distance/shielding balance, or writing "less effective nuclear charge" (which is disallowed).