A-Level H2 Chemistry 3 — Questions and Answers
Question 1: What is the mechanism for the reaction of benzene with bromine in the presence of AlBr₃?
- Nucleophilic substitution
- Electrophilic addition
- Electrophilic substitution (Correct answer)
- Free radical substitution
Correct answer: Electrophilic substitution
AlBr₃ acts as a Lewis acid catalyst, generating Br⁺ (electrophile). Benzene undergoes electrophilic substitution to maintain aromaticity, replacing H with Br.
Question 2: What is the pH of a buffer solution containing 0.1 mol/dm³ ethanoic acid (Ka = 1.8×10⁻⁵) and 0.1 mol/dm³ sodium ethanoate?
- 5
- 4.74 (Correct answer)
- 7
- 3
Correct answer: 4.74
Using Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]) = -log(1.8×10⁻⁵) + log(1) = 4.74 + 0 = 4.74.
Question 3: Which species is the oxidising agent in: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺?
- H⁺
- Fe²⁺
- Fe³⁺
- MnO₄⁻ (Correct answer)
Correct answer: MnO₄⁻
MnO₄⁻ is reduced (Mn goes from +7 to +2), so it is the oxidising agent. Fe²⁺ is oxidised to Fe³⁺ (loses electrons), making it the reducing agent.
Question 4: What is the shape of SF₆?
- Tetrahedral
- Octahedral (Correct answer)
- Trigonal bipyramidal
- Square planar
Correct answer: Octahedral
SF₆ has 6 bonding pairs and no lone pairs around sulfur. VSEPR theory predicts an octahedral shape with 90° bond angles.
Question 5: What is Le Chatelier's principle?
- A system will always produce more products when heated
- When a change is imposed on a system at equilibrium, the system shifts to oppose that change (Correct answer)
- Increasing pressure always increases the rate of reaction
- Catalysts change the position of equilibrium
Correct answer: When a change is imposed on a system at equilibrium, the system shifts to oppose that change
Le Chatelier's principle states that if a system at equilibrium is disturbed, it will shift in a direction that tends to oppose the change, restoring a new equilibrium.
Question 6: What functional group is present in an amide?
- –OH
- –NH₂
- –CO–NH– (Correct answer)
- –CHO
Correct answer: –CO–NH–
Amides contain the functional group –CONH– (or –CO–NH–), consisting of a carbonyl group (C=O) directly bonded to a nitrogen atom.
A-Level H2 Chemistry 3
What is the mechanism for the reaction of benzene with bromine in the presence of AlBr₃?