AQA A-Level Chemistry Paper 3, June 2018: Question 3

📐 Step-by-Step Calculation

1. Mass of cyclohexanol:
   Mass = volume × density = 10 cm³ × 0.96 g cm⁻³ = 9.60 g
2. Molar masses (M_r):
   Cyclohexanol (C₆H₁₁OH): (6 × 12.0) + (12 × 1.0) + 16.0 = 100.0 g mol⁻¹
   Cyclohexene (C₆H₁₀): (6 × 12.0) + (10 × 1.0) = 82.0 g mol⁻¹
3. Moles of cyclohexanol (reactant):
   Moles = 9.60 g / 100.0 g mol⁻¹ = 0.0960 mol [Mark 1]
4. Theoretical mass of cyclohexene (1:1 ratio):
   Theoretical moles = 0.0960 mol
   Theoretical mass = 0.0960 mol × 82.0 g mol⁻¹ = 7.872 g [Mark 2]
5. Percentage Yield:
   % yield = (actual mass / theoretical mass) × 100
   % yield = (5.97 g / 7.872 g) × 100 = 75.8% to 76% [Mark 3]

❌ Common Errors & Traps

• Forgetting density: Assuming 10 cm³ = 10 g gives a yield of 62.2%. This is a frequent error and only scores a maximum of 1 mark out of 3.
• Incorrect M_r: Counting 12 hydrogens for cyclohexanol without the OH gives C₆H₁₂O (correct: C₆H₁₂O, M_r = 100.0). For cyclohexene, remember the double bond removes two hydrogens (C₆H₁₀, M_r = 82.0).

✅ Correct Answer

• Reagent: Add bromine water (or bromine). [1 mark]
• Observation: Turns from orange/brown to colourless OR decolourises. [1 mark]

🧠 Exam Technique

• Always state both the initial colour and the final colour ("orange to colourless"), or explicitly use the term "decolourises".
• Do NOT write "turns clear" — "clear" means transparent, not colourless! Orange bromine water is already clear (not cloudy).
• Do not write "discolours" (spelling and meaning error).

💡 03.3: Washing with Na₂CO₃ Solution

Answer: To neutralise / react with / remove residual acid (conc. H₃PO₄ / H⁺).

💡 03.4: Inverting and Opening the Tap

Answer: To release pressure / release built-up CO₂ gas / prevent the stopper blowing out.

✅ Correct Answer

It does not dissolve in and does not react with cyclohexene (it is inert towards the organic product).

❌ Common Errors

• Saying simply "it is insoluble" without specifying what it is insoluble in (it must be insoluble in the organic layer / cyclohexene).
• Stating "it does not react with cyclohexanol" — the primary compound you are drying is the product, cyclohexene.
• Writing "it can be filtered off easily" — while true, the question asks for a property of the chemical agent in this reaction system.

✅ Required Apparatus Description

• Funnel: Büchner funnel (or flat-bottomed funnel) containing filter paper. [Mark 1]
• Flask & Connection: Büchner flask (side-arm flask) sealed with a rubber bung/gasket, connected via tubing to a vacuum pump / water aspirator. [Mark 2]

🧠 Diagram / Description Details

• If you draw a diagram, ensure the connection between funnel and flask shows an airtight seal (rubber bung).
• Do not draw a standard conical gravity funnel (Y-shaped filter funnel) — standard funnels fail under vacuum and will lose both marks.
• Remember to clearly show or label the filter paper lying flat covering the holes in the Büchner plate.

💡 Why Cyclohexene Elutes Faster (Shorter Retention Time)

1. Relative Polarity: Cyclohexene is less polar / non-polar, whereas cyclohexanol is more polar (capable of hydrogen bonding via –OH group). [Mark 1]
2. Relative Affinities: Cyclohexene has a greater affinity/attraction for the mobile phase (hexane) OR cyclohexanol has a greater affinity for the stationary phase (silica) and is held longer. [Mark 2]

🧠 Linking Polarity to Phases

Always state the "like dissolves like" principle explicitly:

• Silica = polar stationary phase → holds strongly to polar molecules via dipole-dipole / H-bonds.
• Hexane = non-polar mobile phase → dissolves and carries non-polar molecules quickly down the column.

✅ Correct Answer (Either option accepted)

• There would be no peak/absorption at 3230 – 3550 cm⁻¹ corresponding to the O–H (alcohol) bond. [1 mark]
• Alternative: The spectrum matches a pure reference sample of cyclohexene exactly, with no extra peaks in the fingerprint region (< 1500 cm⁻¹). [1 mark]

❌ Common Errors

• Quoting an incorrect wavenumber range (must be 3230 – 3550 cm⁻¹ from the AQA Data Sheet for an alcohol O–H, not 2500 – 3000 cm⁻¹ which is for carboxylic acids).
• Naming only the bond or only the wavenumber: both the wavenumber range and the O–H bond must be stated to secure the mark.