A-Level H2 Chemistry 3

Question 1

What is the mechanism for the reaction of benzene with bromine in the presence of AlBr₃?

Accepted Answer

Electrophilic substitution

Answer

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?

Accepted Answer

4.74

Answer

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⁺?

Accepted Answer

MnO₄⁻

Answer

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?

Accepted Answer

Octahedral

Answer

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?

Accepted Answer

When a change is imposed on a system at equilibrium, the system shifts to oppose that change

Answer

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.

GCE A-Level Practice Test

GCE A-Level Practice Test

A-Level H2 Chemistry 3