OCR A-Level Chemistry AS Depth in chemistry (02), June 2025: Question 2

✅ Correct Answer

Ca⁺(g) → Ca²⁺(g) + e⁻

Also allowed: Ca⁺(g) - e⁻ → Ca²⁺(g) . State symbols on the electron are not required.

❌ Common Errors

• Writing the first ionisation energy: Ca(g) → Ca⁺(g) + e⁻
• Writing the two-electron ionisation: Ca(g) → Ca²⁺(g) + 2e⁻
• Omitting state symbols or writing solid (s) instead of gaseous state (g) .

💡 3 Core Themes Needed for Level 3 (5–6 Marks)

A. Trend & Explanation:

• Calcium is more reactive than magnesium.
• Ca has a larger atomic radius / more electron shells / more shielding than Mg.
• Nuclear attraction to the outer electrons is weaker in Ca.
• Outer electrons are lost more easily (first & second ionisation energies of Ca are lower).

B. Equation & Observations:

• Ca + 2H₂O → Ca(OH)₂ + H₂
• Effervescence / fizzing / bubbles of gas.
• Calcium metal dissolves / disappears, or a white precipitate / cloudy suspension forms.

C. Redox Explanation:

• Both oxidation and reduction occur simultaneously.
• Ca is oxidised: oxidation number increases from 0 in Ca to +2 in Ca(OH)₂ (loss of 2 electrons).
• H is reduced: oxidation number decreases from +1 in H₂O to 0 in H₂ (gain of 1 electron per H atom).

🧠 Exam Technique & Structure

To secure a Level 3 (6 marks), structure your answer clearly under three distinct headings:

1. Equation & Observations: Give the balanced symbol equation first, followed by at least two distinct sensory observations.
2. Redox: Define oxidation and reduction specifically using oxidation numbers for both Ca and H. Avoid vague statements like "water is reduced".
3. Reactivity comparison: Directly compare Ca with Mg. Do not just quote a general group trend; state specifically why calcium's outer electrons feel less pull to the nucleus despite higher nuclear charge.

📐 Step-by-Step Calculation

Step 1: Calculate amount in moles of BaCO₃

n(BaCO₃) = mass / M = 1.51 / 197.3 = 0.0076533... mol (7.6533 × 10⁻³ mol)

Step 2: Use stoichiometry to find moles of CO₂

From the balanced equation, ratio is 1:1, so:

n(CO₂) = 7.6533 × 10⁻³ mol

Step 3: Calculate volume of gas in cm³

At RTP, 1 mol of gas = 24 000 cm³ (or 24.0 dm³):

Volume = 7.6533 × 10⁻³ × 24 000 = 183.68 cm³

Step 4: Round to 3 significant figures

Volume = 184 cm³

🧠 Calculation Tips & Guidance

• Full Marks Shortcut: If the final answer on the line is 184 cm³, both marks are awarded automatically.
• Unit Consistency: The question specifies cm³. If you use 24 dm³, you get 0.184 dm³ — remember to multiply by 1000!
• Error-Carried-Forward (ECF): If you miscalculate the moles of BaCO₃, you can still gain the second mark by multiplying your value by 24 000 and rounding to 3 SF.
• Ideal Gas Equation: Alternatively, using pV = nRT at 293 K and 100 kPa gives 186 cm³, which is also fully credited.

✅ Any Two Valid Reasons (1 Mark Each)

• Gas escape: Gas escapes before the stopper / bung can be inserted into the conical flask after adding the carbonate.
• Gas solubility: Carbon dioxide dissolves in the water inside the trough/measuring cylinder.
• Non-standard conditions: Laboratory temperature was lower than 20 °C (293 K) OR pressure was higher than 101 kPa (conditions not at RTP).

❌ Ignored & Rejected Answers

• "Incomplete reaction" — rejected because nitric acid was specified as being in excess.
• "Human errors" (e.g. faulty apparatus, misreading the scale, gas leaking around the delivery tube).
• "Impurities in the solid" without experimental justification.

✅ Acceptable Modifications (Choose One)

• Use a gas syringe instead of collecting the gas over water (directly solves the issue of CO₂ dissolving in water).
• Place the barium carbonate in a small ignition tube / suspended container inside the conical flask and tip it into the acid after sealing with the stopper (prevents gas escape when replacing the bung).
• Use a water bath set to room temperature (20 °C / 293 K) to control temperature conditions.

🧠 Examiner Trap

Writing simply "use a gas syringe" without specifying replacing the water collection is sometimes penalised if not distinct. Clearly state: "Collect the gas in a gas syringe instead of an upturned measuring cylinder over water".