Home / AS & A Level Chemistry 19.1 Primary amines Exam Style Practice Questions Paper 1

AS & A Level Chemistry 19.1 Primary amines Exam Style Practice Questions Paper 1- New Syllabus

A Level Chemistry Paper 1 - All Chapters

Question

Which reagent, when mixed with ammonium sulfate and then heated, liberates ammonia?

(A) aqueous bromine
(B) dilute hydrochloric acid
(C) aqueous calcium hydroxide
(D) potassium dichromate(VI) in acidic solution
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{C}} \)

Ammonium salts react with alkalis on warming to release ammonia gas.

\(\mathrm{2NH_4^++Ca(OH)_2\rightarrow2NH_3+2H_2O+Ca^{2+}}\)

Aqueous calcium hydroxide provides the hydroxide ions needed to liberate ammonia.

Therefore, the correct answer is (C).

Question 

Which statement about an ammonium ion is correct?

(A) All of the \( \mathrm{H-N-H} \) bond angles in the ion are \(90^\circ\).
(B) All of the \( \mathrm{H-N-H} \) bond angles in the ion are \(107^\circ\).
(C) The ion contains an \( \mathrm{N-H} \) dative covalent bond which is weaker than the other three \( \mathrm{N-H} \) covalent bonds.
(D) The ion will react with a base as it is a weak acid.
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{D}} \)

The ammonium ion, \( \mathrm{NH_4^+} \), has a tetrahedral shape with bond angles of approximately \(109.5^\circ\).

Therefore:

  • A is incorrect because the bond angles are not \(90^\circ\).
  • B is incorrect because the bond angles are \(109.5^\circ\), not \(107^\circ\).
  • C is incorrect because although one \( \mathrm{N-H} \) bond is formed by a dative (coordinate) bond, after formation all four \( \mathrm{N-H} \) bonds are identical and have the same strength and length.
  • D is correct because \( \mathrm{NH_4^+} \) can donate a proton to a base, so it behaves as a weak Brønsted–Lowry acid.

For example:

\( \mathrm{NH_4^+ + OH^- \rightarrow NH_3 + H_2O} \)

Therefore, the correct answer is (D).

Question 

An amine is produced in the following reaction.

\( \mathrm{C_2H_5I + 2NH_3 \rightarrow C_2H_5NH_2 + NH_4I} \)

What is the mechanism?

(A) Electrophilic addition
(B) Free-radical substitution
(C) Nucleophilic addition
(D) Nucleophilic substitution
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{D}} \)

Ammonia acts as a nucleophile and attacks the carbon atom bonded to iodine in the haloalkane.

The iodide ion leaves as the leaving group, so one group is substituted by another.

This is an \( \mathrm{S_N2} \) nucleophilic substitution reaction.

Therefore, the correct answer is (D).

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