Edexcel A-Level Chemistry AS Paper 2, June 2016: Question 3

✅ Correct Answer

CH₃CH₂CHBrCH₃ + OH⁻ → CH₃CH₂CH(OH)CH₃ + Br⁻

(Also accepts CH₃CH₂CHBrCH₃ + H₂O → CH₃CH₂CH(OH)CH₃ + H⁺ + Br⁻ )

💡 Key Knowledge

• An ionic equation must omit spectator ions (like K⁺).
• Structural or displayed formulae must clearly show the location of the functional groups.

❌ Common Errors

• Including potassium ions ( K⁺ ) in the equation when the question specifically asks for an ionic equation.
• Using molecular formulae (e.g., C₄H₉Br ) instead of structural formulae for the organic compounds.

✅ Correct Answer / Indicative Content

• Reagent / Solvent: Add ethanol as a common solvent to the halogenoalkanes in separate containers.
• Fair Test: Use equal volumes/amounts of each halogenoalkane and warm the tubes in a water bath to a specified temperature.
• Test Reagent: Add aqueous silver nitrate ( Ag⁺(aq) ) to each tube.
• Measurement: Time how long it takes for a precipitate to form.
• Trend: 2-iodobutane reacts fastest, 2-bromobutane is intermediate, and 2-chlorobutane is slowest.
• Justification: Carbon-halogen bond enthalpy decreases down the group (C-I is weakest, C-Cl is strongest), requiring less energy to break the C-I bond.

🧠 Exam Technique & Structuring

This is a leveled response question (6 marks total: up to 4 marks for chemistry content, and up to 2 marks for the structure and logical flow of reasoning). To secure top-level marks:

• Clearly separate your experimental method from your scientific explanation.
• Ensure you explicitly mention controlling variables (fair test) and timing precipitate formation.

❌ Common Errors

• Mentioning bond polarity or electronegativity instead of bond enthalpy as the primary reason for the rate differences.
• Forgetting to add ethanol, meaning the halogenoalkane and aqueous silver nitrate will not mix properly.

✅ Correct Answer

An additional curve drawn with:

• A lower peak than the original curve.
• A peak shifted to the right (higher energy).
• The curve crossing the original curve once on the tail and staying above it, approaching the energy axis asymptotically without touching or crossing back multiple times. Starts at the origin.

🧠 Exam Technique

When sketching temperature shifts: remember that the total area under the curve must remain constant because the total number of molecules does not change. A lower peak and a rightward shift automatically account for this.

✅ Correct Answer (3 Marking Points)

1. Higher temperature gives molecules a higher kinetic energy, leading to more frequent collisions.
2. A greater number / proportion of molecules have energy greater than or equal to the activation energy ( E ≥ Eₐ ), as shown by the area under the curve past Eₐ .
3. Therefore, a greater proportion of collisions result in a successful reaction.

❌ Common Errors

• Stating that the activation energy changes with temperature (it does not!).
• Stating that the total area under the curve increases (violates the conservation of number of particles).
• Using vague terms like "molecules move faster" without explicitly linking it to collision frequency or energy distribution.