A-Level H2 Chemistry 4

Question 1

Why does HF have a higher boiling point than HCl despite having a lower molar mass?

Accepted Answer

HF has stronger hydrogen bonding due to the high electronegativity of fluorine

Answer

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?

Accepted Answer

React with nitrous acid at 0-5°C, then couple with a phenol to form an azo dye

Answer

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.

Accepted 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

Answer

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?

Accepted Answer

Orange to green

Answer

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?

Accepted Answer

The decrease in solubility of an ionic compound when a solution already containing one of its ions is added

Answer

The common ion effect: adding an ion that already exists in the equilibrium suppresses dissociation, reducing solubility — consistent with Le Chatelier's principle.

GCE A-Level Practice Test

GCE A-Level Practice Test

A-Level H2 Chemistry 4