A-Level Chemistry — Questions and Answers
Question 1: What type of bonding exists in a sodium chloride (NaCl) crystal as studied in Singapore A-Level Chemistry?
- Covalent bonding
- Ionic bonding — electrostatic attraction between Na⁺ and Cl⁻ ions (Correct answer)
- Metallic bonding
- Van der Waals forces
Correct answer: Ionic bonding — electrostatic attraction between Na⁺ and Cl⁻ ions
NaCl consists of Na⁺ and Cl⁻ ions held together by strong electrostatic forces of attraction in a giant ionic lattice structure. Sodium transfers its valence electron to chlorine.
Question 2: According to Le Chatelier's Principle, what happens when pressure is increased on the equilibrium N₂ + 3H₂ ⇌ 2NH₃?
- Equilibrium shifts to the left
- Equilibrium shifts to the right (towards fewer moles of gas) (Correct answer)
- No change occurs
- The reaction stops completely
Correct answer: Equilibrium shifts to the right (towards fewer moles of gas)
Increasing pressure favours the side with fewer moles of gas. Left side has 4 moles (1 + 3); right side has 2 moles. Equilibrium shifts right to produce more ammonia.
Question 3: What is the hybridisation of carbon in methane (CH₄)?
- sp
- sp²
- sp³ (Correct answer)
- sp³d
Correct answer: sp³
In methane, carbon forms four equivalent C-H bonds by hybridising its 2s and three 2p orbitals to form four sp³ hybrid orbitals, resulting in a tetrahedral shape with 109.5° bond angles.
Question 4: What is the oxidation state of manganese in KMnO₄?
- +4
- +5
- +6
- +7 (Correct answer)
Correct answer: +7
In KMnO₄: K is +1, each O is -2 (total -8). So +1 + Mn + (-8) = 0, giving Mn = +7. Manganese is in its highest common oxidation state.
Question 5: What is the rate-determining step in a reaction mechanism?
- The fastest step
- The slowest step that determines the overall rate of reaction (Correct answer)
- The step with the most reactants
- The final step of the mechanism
Correct answer: The slowest step that determines the overall rate of reaction
The rate-determining step is the slowest step in a multi-step reaction mechanism. It acts as a bottleneck, controlling the overall rate of the reaction.
Question 6: In the Singapore A-Level syllabus, what is an SN2 reaction?
- A first-order substitution
- A bimolecular nucleophilic substitution with a single concerted step, resulting in inversion of configuration (Correct answer)
- A free radical reaction
- An electrophilic addition reaction
Correct answer: A bimolecular nucleophilic substitution with a single concerted step, resulting in inversion of configuration
SN2 (Substitution Nucleophilic Bimolecular) involves a single concerted step where the nucleophile attacks the carbon from the opposite side to the leaving group, causing Walden inversion of stereochemistry.
A-Level Chemistry
What type of bonding exists in a sodium chloride (NaCl) crystal as studied in Singapore A-Level Chemistry?