International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is the chemical equation for the reaction between hydrogen chloride and ammonia?

1. HCl + O₂ -> Cl₂ + H₂O

2. HCl + NH₃ -> NH₄Cl

3. H₂ + Cl₂ -> 2HCl

4. NH₃ + H₂O -> NH₄OH

The correct answer is: HCl + NH₃ -> NH₄Cl

The reaction between hydrogen chloride (HCl) and ammonia (NH₃) produces ammonium chloride (NH₄Cl). This process is a classic acid-base reaction where hydrogen chloride, an acid, reacts with ammonia, a base. In this reaction, HCl donates a proton (H⁺ ion) to the unshared electron pair on the nitrogen atom in ammonia. This leads to the formation of the ammonium ion (NH₄⁺) and chloride ion (Cl⁻). When these ions combine, they form the ionic compound ammonium chloride, which can be represented by the equation HCl + NH₃ → NH₄Cl. This reaction is also significant in that it is a demonstration of how gases can react to form solids, as ammonium chloride is a solid when formed from the gaseous reactants. The other answers do not represent the reaction between hydrogen chloride and ammonia accurately; instead, they depict different reactions entirely. Understanding the context and chemistry behind the formation of NH₄Cl is crucial to grasping acid-base reactions in chemistry.