The questions are on the electrophilic addition test. Take it under real conditions before reading the mark scheme.
Quick answer: Alkenes are hydrocarbons with a C=C double bond whose exposed π electrons attract electrophiles. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on electrophilic addition, from bonding in propene to why bromine adds, with the equivalent answers to accept.
The model answers below give the expected working and a marking guide. Award marks for correct reasoning, even if a final number differs slightly by rounding. Do not give marks for a mechanism whose arrow starts from the hydrogen.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.4
Question 1 Bonding in propene (2 marks)
- 8 σ bonds and 1 π bond (6 C–H, 2 C–C σ; plus one π).
- About 120°.
Marking: 1 mark for 8 σ and 1 π; 1 mark for 120°.
Question 2 Test for a double bond (2 marks)
- Shake with bromine water; the colour changes from orange-brown to colourless.
- C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).
Marking: 1 for reagent and colourless; 1 for the balanced equation.
Question 3 Mechanism of addition (3 marks)
- Electrophilic addition.
- Intermediate CH₃CH₂⁺ (with Br⁻); product CH₃CH₂Br.
Marking: 1 for the name; 1 for the carbocation; 1 for bromoethane.
Question 4 Markovnikov product (3 marks)
- 2-bromobutane.
- H adds to the terminal carbon, giving a secondary carbocation. Two alkyl groups push electron density towards it, making it more stable than the primary ion that leads to 1-bromobutane, so it forms faster.
Marking: 1 for the name; 1 for secondary versus primary; 1 for alkyl groups stabilising.
Question 5 Industrial hydration (2 marks)
- Phosphoric acid on silica; about 300 °C.
- Very high pressure is expensive and makes ethene polymerise, so 65 atm is a compromise.
Marking: 1 for catalyst and temperature; 1 for cost or polymerisation.
Question 6 Yield of 2-bromobutane (3 marks)
- n = 5.60 / 56.0 = 0.100 mol; mass = 0.100 × 136.9 = 13.7 g.
- 10.2 / 13.69 × 100 = 74.5 %.
Marking: 1 for amount; 1 for mass; 1 for percentage.
Question 7 Polymer from but-1-ene (2 marks)
- –CH₂–CH(CH₂CH₃)–.
- It contains only strong non-polar C–C and C–H bonds and no C=C, so no electrophile or nucleophile can attack it easily.
Marking: 1 for the repeat unit with the ethyl branch; 1 for the explanation.
Question 8 Why bromine adds (3 marks)
- The π electrons repel the electrons in Br₂, inducing a dipole (Brδ+ near the alkene). The positive end is attacked by the π electrons.
- 1,2-dibromoethane.
Marking: 1 for the induced dipole; 1 for the electrophilic end; 1 for the name.
