Edexcel A-Level Chemistry Paper 2, November 2020: Question 2

Edexpert Study Guide: Alkenes & Reactions

Total Marks: 8

What this question tests

This question assesses your core knowledge of alkene chemistry, IUPAC nomenclature rules, addition reactions of alkenes (electrophilic addition and hydration), addition polymerisation drawing conventions, and fundamental moles-to-particles calculations using the Avogadro constant.

Part (a) — Identifying Molecular Formulae from Structures

Determine which structure corresponds to C₆H₁₀ (1 mark)

✅ Correct Answer: C

Structure C (cyclohexene) has a ring structure with one double bond, giving it the general formula CₙH₂ₙ₂ , which equates to C₆H₁₀ .

❌ Common Errors & Distractors

A: Hex-1-ene is an open-chain alkene with formula C₆H₁₂ .
B: Contains multiple conjugated double bonds (hexa-1,3,5-triene) with formula C₆H₈ .
D: Cyclohexa-1,4-diene has two double bonds, giving the formula C₆H₈ .

Mark: 1 mark for selecting C.

Part (b) — Systematic IUPAC Nomenclature

Name the branched alkene structure (1 mark)

✅ Correct Answer: C (2,3-dimethylbut-1-ene)

The longest continuous carbon chain containing the double bond has 4 carbons (but-ene). Numbering from the end closest to the double bond gives carbon 1 to the =CH₂ group. Methyl branches are located at carbons 2 and 3.

❌ Why other options fail

A & B: Incorrectly assume a 5-carbon chain (pentene) by misidentifying the longest continuous carbon pathway.
D: Fails to give the double bond the lowest possible number (numbering from the wrong end).

Mark: 1 mark for selecting C.

Part (c) — Reactions of Ethene

Complete the reaction and reagent table (3 marks)

💡 Key Knowledge & Reagents

Reaction 1 Product: Bromoethane ( CH₃CH₂Br or displayed formula showing H-C-C-Br connectivity).
Reaction 2 Reagents: Steam ( H₂O(g) ) and an acid catalyst (e.g., concentrated H₃PO₄ or H₂SO₄ ). Note: "water and heat" is also accepted.

🧠 Exam Technique

Make sure you explicitly state both the chemical reagent AND the catalyst for hydration reactions. Writing just "water" without specifying "steam" or an acid catalyst will often lose marks in stricter mark schemes.

Marks: 1 mark for Reaction 1 product, 1 mark for Reaction 2 reagent(s), 1 mark for Reaction 2 condition(s).

Part (d)(i) — Addition Polymerisation

Draw two repeat units of poly(but-1-ene) (1 mark)

✅ Correct Answer Structure

A backbone chain of 4 carbon atoms with single bonds, showing two complete repeat units joined together, with open extension bonds passing through square brackets or simply terminating outside the ends.

Side chains: Ethyl groups ( -CH₂CH₃ or -C₂H₅ ) hanging off alternating carbon atoms in the backbone.

❌ Common Errors

Leaving the double bond in the backbone of the polymer chain.
Drawing only one repeat unit when the question explicitly requests two.
Failing to include clear extension bonds through the brackets/ends.

Mark: 1 mark for showing 2 repeat units with correct backbone and extension bonds.

Part (d)(ii) — Moles and Avogadro Calculations

Calculate the number of molecules in 70.0 g of but-1-ene (2 marks)

📐 Step-by-Step Calculation

Find the molar mass of but-1-ene (C₄H₈):
M = (4 × 12.0) + (8 × 1.0) = 56.0 g mol⁻¹
Calculate moles of but-1-ene:
Moles = Mass / Molar Mass = 70.0 / 56.0 = 1.25 mol
Calculate the number of molecules:
Molecules = Moles × Avogadro constant
= 1.25 × 6.02 × 10²³ = 7.525 × 10²³ (or 7.53 × 10²³)

❌ Calculation Traps

Incorrect Molar Mass: Forgetting the hydrogen count or miscalculating atomic masses.
No Working Out: A correct final answer with zero working scores full marks, but if your answer is slightly off due to rounding, intermediate working steps secure your method mark (M1)!

Marks: M1 for calculating moles (1.25), M2 for scaling up by the Avogadro constant.