OCR A-Level Chemistry AS Depth in chemistry (02), June 2025: Question 5

✅ Correct Answer & Diagram Construction

The question strictly demands a different symbol for each element (e.g. dots • for H, crosses × for C, filled squares ▪ for Cl):

• Central C atom: 4 covalent bonding pairs (8 electrons total around C).
• Each H atom: Shares 1 pair with C (duplet complete).
• Cl atom: 1 shared pair with C + 3 non-bonding lone pairs (6 non-bonded outer electrons).

❌ Common Errors & Examiner Traps

• Forgetting Cl lone pairs: The most frequent error is drawing only the C–Cl bonding pair and leaving Cl with only 2 electrons.
• Using only 2 symbols: The rubric specifically instructs: "Use a different symbol for the electrons of each element." Using dots and crosses only scores 0.
• Drawing inner shells: Unnecessary and increases the chance of counting mistakes. Stick to outer electrons only.

✅ Correct Answers

• Name of shape: Tetrahedral [1 mark]
• Bond angle: 109.5° [1 mark] (Allow 109° to 110°)

💡 Key Knowledge

The central carbon atom has 4 single bonded pairs and 0 lone pairs of electrons. According to Electron Pair Repulsion Theory, 4 electron pairs repel each other equally into a 3D tetrahedral geometry with an angle of 109.5°.

🧠 Exam Technique

Always state the standard ideal tetrahedral angle as 109.5° . While OCR accepts a range of 109°–110°, writing 109.5° ensures you are never penalised across any UK exam board.

✅ Correct Answers & Model Response

• Bond angle: 120° [1 mark] (Allow 116° – 124°)
• Explanation:
  • CH₃Cl has 4 bonded pairs around the carbon atom, whereas H₂C=CHCl has 3 bonded regions (or 2 single bonds and 1 double bond) around the carbon atom. [1 mark]
  • Electron pairs / bonded regions repel each other as far apart as possible. [1 mark]

❌ Major Misconceptions & Examiner Traps

• Saying "atoms repel": Zero marks awarded for stating atoms or bonds push each other away. You MUST specify that electron pairs or bonded regions repel.
• Calling a double bond "one pair": Refer to it as a "bonded region", "double bond", or "group of electrons". C in chloroethene has 3 bonded regions, giving a trigonal planar arrangement around C.
• Discussing lone pairs on chlorine: The H–C–Cl angle is centred at the carbon atom. Repulsion occurs between electron regions around carbon, not the chlorine lone pairs.

✅ Stage 1: Purification Procedure (Step-by-Step)

1. Separating Funnel:
   • Pour the reaction mixture into a separating funnel.
   • Allow the two layers to separate. Because the organic layer is less dense ( 0.87 g cm⁻³ ) than the aqueous layer ( 1.00 g cm⁻³ ), the organic layer is the UPPER layer.
   • Open the tap to run off the lower aqueous layer, then collect the upper organic layer.
2. Drying:
   • Add an anhydrous inorganic salt (e.g. anhydrous MgSO₄ or CaCl₂ ) to dry the organic liquid.
   • Swirl until the liquid turns from cloudy to clear; filter off the drying agent.
3. Distillation:
   • Transfer the dried liquid to a distillation apparatus.
   • Heat gently and collect the pure fraction distilling at its boiling point ( 70 °C ) to remove unreacted starting materials and side-products.

📐 Stage 2: Mass Calculation

1. Calculate molar masses (M_r):
   • Butan-2-ol, C₄H₁₀O: (4 × 12.0) + (10 × 1.0) + 16.0 = 74.0 g mol⁻¹
   • 2-Chlorobutane, C₄H₉Cl: (4 × 12.0) + (9 × 1.0) + 35.5 = 92.5 g mol⁻¹
2. Calculate moles of reactant butan-2-ol:
   • n = 9.25 g / 74.0 g mol⁻¹ = 0.125 mol
3. Theoretical moles of product:
   • 1 : 1 stoichiometric ratio → Theoretical moles of 2-chlorobutane = 0.125 mol
4. Theoretical maximum mass (100% yield):
   • Mass = 0.125 mol × 92.5 g mol⁻¹ = 11.5625 g
5. Actual expected mass (65.0% yield):
   • Expected mass = 11.5625 g × 0.650 = 7.5156 g
   • Round to 3 significant figures: 7.52 g

🧠 Level 3 Marking Criteria (5–6 Marks)

• Full Practical Description: Must describe ALL 3 steps in logical order (separating funnel → drying agent → distillation) AND explain the purpose of each step.
• Density Justification: Explicitly state that the organic layer sits on top because its density is lower (0.87 vs 1.00 g cm⁻³).
• Distillation Target: Must mention collecting specifically at/around 70 °C.
• Calculation: Completely correct numerical answer of 7.52 g with clear layout.

❌ Common Errors in Level of Response

• Failing to identify the top layer: Simply saying "separate the layers" without explaining which layer is organic based on density misses out on top-band credit.
• Vague drying agent: Saying "add salt" or "add a drying agent" without specifying anhydrous or naming a suitable salt ( MgSO₄ / CaCl₂ ).
• Premature rounding: Rounding intermediate values too early leads to rounding errors (e.g. 7.50 g instead of 7.52 g). Always keep unrounded values in your calculator.

✅ Correct Structures & Reagents

1. Alkene X: Structure of 4-methylpent-1-ene

Skeletal or structural formula accepted: CH₂=CHCH₂CH(CH₃)₂ [1 mark]

• Stage 1 Reagent: Br₂ (or bromine) [1 mark]
• Stage 2 Reagent: NaOH or KOH (or OH⁻(aq) / aqueous hydroxide) [1 mark]

💡 Reaction Pathway Breakdown

• Stage 1 (Electrophilic Addition): Bromine ( Br₂ ) adds across the C=C double bond in Alkene X to form the vicinal dibromoalkane, Compound Y (1,2-dibromo-4-methylpentane).
• Stage 2 (Nucleophilic Substitution): Aqueous hydroxide ions ( OH⁻ ) replace both bromine atoms via nucleophilic substitution, producing the diol, Compound Z (4-methylpentane-1,2-diol).

✅ Correct Answer

Mechanism: Nucleophilic substitution [1 mark]

❌ Critical Examiner Trap: "Hydrolysis" vs "Mechanism"

DO NOT write "Hydrolysis"!

The mark scheme explicitly states: "DO NOT ALLOW hydrolysis". Hydrolysis describes the type of reaction (breaking a bond using water/hydroxide), whereas the question specifically asks for the name of the mechanism (the step-by-step electronic pathway: Nucleophilic Substitution).