Edexcel A-Level Chemistry AS Paper 2, June 2016: Question 5

✅ Correct Answer

A diagram of a separating funnel showing two distinct layers with correct labels:

• Top layer labelled: 2-chloro-2-methylpropane (organic layer)
• Bottom layer labelled: aqueous layer (or hydrochloric acid)

💡 Key Knowledge

Layers separate based on density. Using the data table: 2-chloro-2-methylpropane has a density of 0.84 g cm⁻³ , whereas the aqueous hydrochloric acid solution has a higher density ( ~1.0 g cm⁻³ ). Therefore, the organic product floats on top.

✅ Correct Answer

• Step 4 reason: To react with and neutralise any unreacted/excess hydrochloric acid.
• Tap opening reason: To release the carbon dioxide gas produced and relieve the build-up of pressure.

❌ Common Errors

Students often lose marks by stating the tap is opened "because pressure builds up" without explaining why (due to carbon dioxide gas generation from the reaction between sodium hydrogencarbonate and excess acid). Never state that hydrogen gas is released.

✅ Correct Answer

D (sodium sulfate)

🧠 Exam Technique

An effective drying agent must be an anhydrous inorganic salt that chemically removes traces of water from the organic layer without reacting with the organic product itself. Sodium hydroxide is a strong base and would react; sodium chloride is not a drying agent; sodium nitrate is incorrect.

✅ Correct Answer

Three essential modifications for safe/efficient distillation:

• The bulb of the thermometer must be positioned directly opposite the side-arm opening to the condenser.
• The water flow in the condenser must be reversed (water in at the bottom, out at the top) to ensure the jacket fills completely.
• The apparatus must be left open (put a vent after the condenser / leave a gap before the receiver / use an open conical flask) to prevent a closed system and an explosion hazard.

❌ Common Errors

Sealing the collection vessel entirely is a major safety hazard. Students also frequently reverse the water jacket flow direction incorrectly or place the thermometer bulb too high or too low in the still head.

✅ Correct Answer

50 - 52°C (Accept any range between 49°C and 53°C provided it includes the boiling temperature of 51°C).

❌ Common Errors

Do not give a single fixed temperature value like 51°C ; distillation is carried out over a narrow temperature window to collect the pure fraction effectively.

📐 Step-by-Step Calculation

1. Calculate mass and moles of alcohol used:
   Mass = Volume × Density = 15.0 cm³ × 0.79 g cm⁻³ = 11.85 g
   Moles of 2-methylpropan-2-ol = 11.85 g / 74.0 g mol⁻¹ = 0.16014 mol
2. Determine theoretical yield:
   Since the stoichiometric ratio is 1:1, theoretical moles of 2-chloro-2-methylpropane = 0.16014 mol
   Theoretical mass = 0.16014 mol × 92.5 g mol⁻¹ = 14.8125 g
   *(Alternatively, theoretical volume = 14.8125 g / 0.84 g cm⁻³ = 17.634 cm³ )*
3. Calculate percentage yield:
   Actual volume produced = 6.9 cm³
   Percentage Yield = (6.9 / 17.634) × 100 = 39.1%
   *(Using moles/mass yields the same result: 39.1% )*

❌ Calculation Traps

• Forgetting to convert volume to mass using density ( mass = volume × density ) before finding moles.
• Mixing up molar masses for reactant and product.

✅ Correct Answer

• Stage 2: Draw a curly arrow starting from the C–O single bond and pointing towards the oxygen atom (breaking the C–O bond to form a carbocation and water).
• Stage 3: Draw a curly arrow starting from the lone pair of electrons on the chloride ion ( :Cl⁻ ) and pointing to the positively charged carbon atom ( C⁺ ).

❌ Common Errors

• Drawing curly arrows starting in the middle of space instead of explicitly originating from a bond or a lone pair of electrons.
• Using incorrect single-headed arrows instead of double-headed curly arrows representing movement of electron pairs.