Edexcel A-Level Chemistry Paper 2, June 2017: Question 3

✅ Correct Answer

• Reactivity increases down Group 7: chloroalkane < bromoalkane < iodoalkane .
• Reason: The carbon-halogen (C-X) bond enthalpy decreases / C-X bond gets weaker down the group.

💡 Key Knowledge

As you go down Group 7 (from Cl to Br to I), the halogen atoms increase in atomic radius, leading to poorer orbital overlap with carbon. This results in weaker covalent bonds that require less energy to break during nucleophilic attack.

❌ Common Errors

• Discussing halogen reactivity scores instead of the halogenoalkanes.
• Referencing halide ions or electronegativity / C-X dipoles instead of bond enthalpy.
• Incorrectly talking about atom size, shielding, or intermolecular forces.

✅ Correct Answer (SN2 Mechanism)

• Dipole & Arrow 1: Partial positive charge ( δ+ ) on C and partial negative ( δ- ) on Br. Curly arrow starting from the C-Br bond going to the Br atom (or just beyond).
• Arrow 2: Curly arrow starting from the lone pair on the oxygen of the hydroxide ion ( OH⁻ ) directed to the carbon bonded to Br.
• Transition State: Shows partial bonds ( -- ) to both OH and Br , enclosed in square brackets with an overall negative charge ( [ ]⁻ ).
• Products: Ethanol ( CH₃CH₂OH ) and a bromide ion ( Br⁻ ).

🧠 Exam Technique

Ensure your curly arrows originate precisely where required: a lone pair of electrons (start with the dot/pairs on the oxygen) and a covalent bond (start right in the middle of the bond line). The transition state must display dashed partial bonds and the negative charge clearly.

❌ Common Errors

• Drawing a curly arrow originating from a hydrogen atom instead of the oxygen lone pair.
• Forgetting to include the negative charge on the transition state or omitting the partial dipoles.
• Drawing carbocation intermediates (which apply to SN1, whereas 1-bromoethane is primary and undergoes SN2).

✅ Correct Answer

• Reagents: Nitric acid ( HNO₃ ) followed by silver nitrate solution ( AgNO₃ ).
• Result: Cream (or off-white) precipitate formed.

💡 Key Knowledge

Nitric acid is added first to remove any interfering impurity ions (such as carbonates or sulfites) that might also form precipitates with silver nitrate. Hydrochloric acid must never be used because it introduces chloride ions, giving a false positive.

❌ Common Errors

• Using hydrochloric acid ( HCl ) or sulfuric acid ( H₂SO₄ ) instead of nitric acid.
• Stating "white" precipitate (characteristic of chloride) or "yellow" precipitate (characteristic of iodide) instead of cream.

✅ Correct Answer (Skeletal Formulae)

• Alkene 1: But-1-ene ( CH₂=CHCH₂CH₃ drawn in skeletal form).
• Alkene 2: (E)-but-2-ene.
• Alkene 3: (Z)-but-2-ene.

🧠 Exam Technique

Read the question carefully: it asks for skeletal formulae. Ensure you draw zigzag lines accurately representing all three possible alkene isomers resulting from the elimination of HBr from 2-bromobutane (accounting for both positional and E/Z stereoisomerism).

❌ Common Errors

• Failing to include both E- and Z- stereoisomers of but-2-ene.
• Including 2-methylpropene, which is an isomer of butene but cannot be formed from a straight-chain 4-carbon reactant like 2-bromobutane.
• Drawing structural/displayed formulae when skeletal formulae were explicitly requested.

✅ Correct Answer

• Ethanol has hydrogen bonding (as well as London forces and permanent dipole-dipole forces).
• Ethene only has weaker London (instantaneous dipole-induced dipole) forces.
• Therefore, more energy is required to overcome the stronger intermolecular forces (hydrogen bonds) in ethanol compared to ethene.

💡 Key Knowledge

Boiling point comparisons rely strictly on identifying the types of intermolecular forces present in each substance. Hydrogen bonding is the strongest intermolecular force, requiring significantly more thermal energy to break than simple London dispersion forces found in non-polar molecules like ethene.

❌ Common Errors

• Stating or implying that covalent bonds are being broken during boiling (a common and fatal misconception).
• Failing to make a direct comparison (e.g. omitting words like "stronger" or "more energy").
• Attributing hydrogen bonding to ethene or missing the mention of London forces for ethene.