Study of Compounds

The One Thing

The preparation, properties, reactions, identification tests and uses of hydrogen chloride, ammonia, nitric acid and sulphuric acid are determined by the ions or molecules they form and by their chemical composition and structure. Their laboratory and industrial chemistry therefore depends on acid–base behaviour, oxidation, dehydration, solubility, equilibrium and controlled preparation conditions.

Reactions, Processes and Experiments

Key Terms

• Hydrogen chloride: A covalent compound with the formula HCl; its aqueous solution is called hydrochloric acid.
• Hydrochloric acid: The aqueous solution of hydrogen chloride, which ionises in water to form hydrogen ions or hydronium ions and chloride ions.
• Ammonia: A basic compound with the formula NH3, having a pungent smell and high solubility in water.
• Ammonium ion: The positively charged ion NH4+ formed when ammonia accepts a proton.
• Nitric acid: A strong acid with the formula HNO3 and a powerful oxidising agent in concentrated form.
• Sulphuric acid: A strong dibasic acid with the formula H2SO4; concentrated sulphuric acid is a dehydrating and oxidising agent.
• Covalent chloride: A chloride formed by sharing electrons, such as hydrogen chloride, rather than by transfer of electrons.
• Ionisation: The formation of ions when a covalent substance reacts with or dissolves in water.
• Acidic nature: The ability of a substance in aqueous solution to produce hydronium ions, H3O+.
• Basic nature: The ability of a substance to accept protons or produce hydroxide ions in aqueous solution.
• Dehydrating agent: A substance that removes water or hydrogen and oxygen in the ratio of two to one from another substance.
• Deliquescence: The absorption of moisture from air by a substance until it dissolves in the absorbed water.
• Ostwald process: The industrial manufacture of nitric acid by catalytic oxidation of ammonia followed by absorption of nitrogen dioxide in water.
• Contact process: The industrial manufacture of sulphuric acid involving oxidation of sulphur dioxide to sulphur trioxide and absorption in concentrated sulphuric acid.

Easily Confused

• Hydrogen chloride and hydrochloric acid: Hydrogen chloride is the covalent compound HCl; hydrochloric acid is its aqueous solution.
• Dry and moist hydrogen chloride: Dry hydrogen chloride does not change dry blue litmus, whereas moist hydrogen chloride turns blue litmus red because water enables ionisation.
• Ammonia and ammonium ion: Ammonia is NH3 and accepts a proton; the ammonium ion is NH4+ formed after proton acceptance.
• Ammonia and hydrogen chloride: Ammonia is basic and reacts with hydrogen chloride to form ammonium chloride, producing dense white fumes.
• Hydrogen chloride and nitric acid with metals: Hydrogen chloride reacts with zinc to liberate hydrogen, whereas concentrated nitric acid generally does not liberate hydrogen and passivates iron, aluminium and chromium.
• Concentrated and dilute nitric acid with copper: Concentrated nitric acid produces nitrogen dioxide, whereas dilute nitric acid produces nitric oxide.
• Brown ring and precipitation tests: The brown ring test identifies nitrate ions by a brown ring; chloride and sulphate ions are identified by white precipitates of AgCl and BaSO4 respectively.
• Ostwald process and Contact process: The Ostwald process manufactures nitric acid from ammonia; the Contact process manufactures sulphuric acid through sulphur dioxide and sulphur trioxide.
• Dehydrating and oxidising actions of sulphuric acid: Concentrated sulphuric acid removes water from sugar, but it oxidises copper to form copper sulphate and sulphur dioxide.
• Sulphuric acid preparation and sodium chloride reaction: NaCl + H2SO4 → NaHSO4 + HCl produces hydrogen chloride and is not a preparation of sulphuric acid.
• Diluting concentrated sulphuric acid: Acid must be added slowly to water with stirring; water must never be added to concentrated acid because dilution is highly exothermic.

What Gets Asked

• Laboratory preparation equations: Questions may require the preparation of hydrogen chloride, ammonia or nitric acid. Marks are lost by omitting the specified reactants, conditions or products, such as heating sodium chloride with concentrated sulphuric acid.
• Distinguishing acid–base behaviour: Questions may ask why moist hydrogen chloride turns blue litmus red or why ammonia turns moist red litmus blue. The key distinction is that water is required for hydrogen chloride ionisation, while ammonia accepts a proton and forms NH4+.
• Industrial processes: Questions may require the equations and conditions for the Haber, Ostwald or Contact process. Marks are lost by confusing the processes or omitting conditions such as the iron catalyst in the Haber process or V2O5 at about 450°C in the Contact process.
• Reactions of oxidising acids: Questions may compare copper reactions with concentrated and dilute nitric acid or with concentrated sulphuric acid. The specific gaseous products—NO2, NO or SO2—must be identified correctly.
• Qualitative analysis: Questions may ask for the tests for chloride, nitrate or sulphate ions. The required observations are a white AgCl precipitate, a brown ring and a white BaSO4 precipitate respectively.
• Properties, uses and safety: Questions may ask about dehydration, passivation, gas collection or industrial uses. Marks are lost by stating that ammonia is collected over water, overlooking nitric-acid passivation, or reversing the safe dilution procedure for concentrated sulphuric acid.