A-Level H2 Chemistry 4 — Questions and Answers
Question 1: Why does HF have a higher boiling point than HCl despite having a lower molar mass?
- HF has stronger dispersion forces
- HF has stronger hydrogen bonding due to the high electronegativity of fluorine (Correct answer)
- HF has ionic bonding
- HCl has stronger dipole-dipole forces
Correct answer: HF has stronger hydrogen bonding due to the high electronegativity of fluorine
Although HF has a lower molar mass than HCl, it has much stronger hydrogen bonds due to the high electronegativity and small size of fluorine, leading to a higher boiling point.
Question 2: What is the test for a primary amine?
- Add FeCl₃: purple colour
- React with nitrous acid at 0-5°C, then couple with a phenol to form an azo dye (Correct answer)
- Add NaOH: white precipitate
- Burn with orange flame
Correct answer: React with nitrous acid at 0-5°C, then couple with a phenol to form an azo dye
Primary amines react with nitrous acid (HNO₂) at 0-5°C to form a diazonium salt, which can be coupled with a phenol to produce a brightly coloured azo dye.
Question 3: Explain why aluminium chloride exists as a dimer (Al₂Cl₆) in the vapour phase.
- To achieve 18-electron configuration
- The Al in AlCl₃ is electron-deficient (only 6 electrons) and accepts a lone pair from Cl of another AlCl₃, forming a coordinate bond (Correct answer)
- Due to ionic bonding
- AlCl₃ is unstable alone
Correct answer: The Al in AlCl₃ is electron-deficient (only 6 electrons) and accepts a lone pair from Cl of another AlCl₃, forming a coordinate bond
In AlCl₃, Al has only 6 electrons (incomplete octet). It acts as a Lewis acid and accepts a lone pair from Cl of another AlCl₃ unit, forming Al₂Cl₆ with two coordinate bonds.
Question 4: What is the colour change when Cr₂O₇²⁻ is reduced?
- Yellow to green
- Orange to green (Correct answer)
- Blue to colourless
- Purple to colourless
Correct answer: Orange to green
Dichromate (Cr₂O₇²⁻) is orange. When reduced, Cr³⁺ ions are formed, which are green. This is used to test for the presence of a reducing agent.
Question 5: What is the common ion effect?
- The increase in solubility when a common ion is added
- The decrease in solubility of an ionic compound when a solution already containing one of its ions is added (Correct answer)
- When two salts share an ion, solubility increases
- The formation of a precipitate from two solutions
Correct answer: The decrease in solubility of an ionic compound when a solution already containing one of its ions is added
The common ion effect: adding an ion that already exists in the equilibrium suppresses dissociation, reducing solubility — consistent with Le Chatelier's principle.
Question 6: What is the definition of a standard electrode potential (E°)?
- The potential of any electrode under any conditions
- The potential measured under standard conditions (1 mol/dm³, 298K, 100kPa) relative to the standard hydrogen electrode (Correct answer)
- The potential needed for electrolysis
- The energy released per mole of electrons transferred
Correct answer: The potential measured under standard conditions (1 mol/dm³, 298K, 100kPa) relative to the standard hydrogen electrode
Standard electrode potential is measured at 298K, 1 mol/dm³ ion concentration, and 100 kPa pressure, with all species in their standard states, relative to the SHE (E°=0V).
A-Level H2 Chemistry 4
Why does HF have a higher boiling point than HCl despite having a lower molar mass?