AQA A-Level Chemistry Paper 3, 2019: Question 1

✅ Correct Answer

S₂O₃²⁻ + 2H⁺ → SO₂ + S + H₂O

🧠 Exam Technique

• Identify ionic aqueous compounds: Na₂S₂O₃(aq) splits into 2Na⁺ and S₂O₃²⁻; HCl(aq) splits into H⁺ and Cl⁻.
• Identify spectator ions: Na⁺ and Cl⁻ do not undergo oxidation or reduction and remain unbonded in solution as NaCl(aq) . Cancel them out completely.

❌ Common Errors

• Leaving spectator ions in the equation (e.g. including Na⁺ or Cl⁻ on either side) scores 0 marks.
• Writing non-simplest integer multiples (e.g. 2S₂O₃²⁻ + 4H⁺ → 2SO₂ + 2S + 2H₂O) is rejected because the question explicitly specifies the simplest ionic equation.

✅ Correct Answer

Property (1 Mark): It is acidic (or produces H⁺ / lowers water pH).

Equation (1 Mark):

SO₂ + H₂O → H₂SO₃

💡 Key Knowledge

• Non-metal oxides are generally acidic. When SO₂ dissolves in water, it forms sulfurous acid (H₂SO₃), a weak diprotic acid.
• The increase in [H⁺] lowers the pH of natural water bodies, causing severe harm to aquatic ecosystems, fish gills, and insect larvae.

❌ Common Errors

• Stating properties such as "toxic" or "soluble" without mentioning acidity — these do not explain the specific chemical pollutant action requested.
• Writing "sulfuric acid" ( H₂SO₄ ) for the equation without an oxidizing agent: SO₂ with H₂O produces sulfurous acid ( H₂SO₃ ), not sulfuric acid.
• Merely writing "causes acid rain" — the question specifies what happens when it enters rivers directly.

✅ Correct Answers & Marking Points

• M1 (Diagram): Bent / V-shaped arrangement showing 2 distinct lone pairs on the central oxygen atom.
• M2 (Bond Angle): 104.5° (allow 104° – 105°).
• M3 (Repulsion comparison): Lone pairs repel more strongly than bonding pairs.
• M4 (Angle reduction): Bond angle is reduced from the standard tetrahedral angle ( 109.5° ).

📐 Molecule Structure Diagram

🧠 Exam Technique: Structuring VSEPR Answers

Always hit the 4 essential steps for molecular shape explanations:

1. State the number of electron pairs around the central atom (2 bond pairs, 2 lone pairs).
2. State the basic electron pair geometry based on 4 pairs (tetrahedral, 109.5°).
3. State the relative repulsion order: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair.
4. Quantify the effect: each lone pair reduces the ideal tetrahedral bond angle by approximately 2.5° (109.5° − 5.0° = 104.5°).

❌ Common Errors

• Drawing a linear molecule (H–O–H) with 180° angle — completely ignores lone pair repulsion.
• Omitting lone pairs from the oxygen atom when the question explicitly states "Include any lone pairs of electrons".
• Vague phrasing such as "bonds repel" rather than specifically identifying bonding pairs of electrons.

Stage 1: Method

• Place a known volume of sodium thiosulfate solution into a conical flask placed over a paper card marked with a black cross (or use a colorimeter).
• Warm/equilibrate the reactants to a chosen temperature using a thermostatted water bath.
• Add the dilute hydrochloric acid, start the stopwatch immediately, and swirl the flask once to mix.

Stage 2: Measurements

• Look vertically down through the flask; stop the timer the exact moment the black cross is no longer visible due to colloidal sulfur precipitate (S(s)).
• Record temperature ( T ) and time taken ( t ).
• Control variables: Keep volumes and concentrations of Na₂S₂O₃ and HCl identical, use the same flask, same total depth of liquid, and same cross mark.
• Repeat at at least 5 different temperatures (e.g. 20°C, 30°C, 40°C, 50°C, 60°C).

Stage 3: Graph & Results Analysis

• Recognise that the amount of sulfur formed is constant, so Rate ∝ 1/t (or use 1000/t).
• Calculate 1/t for each temperature.
• Plot a graph of Rate (or 1/t) on the y-axis against Temperature (T in °C or K) on the x-axis.
• Sketch shows an upward curve with increasing gradient (exponential-like shape), NOT a straight line!

❌ Common Errors that Cap Responses

• Plotting raw time vs temperature: Plotting t vs T gives a downward decaying curve, but the question asks for initial rate. You must specify plotting 1/t or calculate rate.
• Drawing a straight line through origin: Rate does not increase linearly with temperature; drawing a straight line loses the graph mark.
• Inadequate temperature range: Saying "repeat at a different temperature" without specifying multiple temperatures (at least 3 to 5 values) limits Stage 2.
• Failing to state controlled variables: Neglecting to mention that concentrations and volumes must be kept constant.

💡 Examiner Secrets for 6/6 Marks

• Always state explicitly: "The amount of sulfur precipitate formed is constant at the moment the cross disappears, therefore rate is proportional to 1/time."
• Mention using a water bath to pre-heat both solutions separately before mixing, ensuring the initial reaction temperature is accurately known.