Acid-Base Chemistry

An acid produces H⁺ ions in aqueous solution. An alkali is a soluble base that produces OH^- ions in water. A base neutralises an acid; insoluble copper(II) oxide is a base but is not an alkali. Acidic and alkaline solutions both contain H⁺ and OH^-; their relative concentrations differ.

Compare colours with the supplied indicator chart. Universal Indicator estimates a pH range; a suitable calibrated pH probe gives a numerical reading. We use qualitative comparisons here, not calculations of pH from hydrogen-ion concentration.

Soil pH affects nutrient availability and plant growth. Calcium hydroxide can treat excessively acidic soil because it neutralises acid. Adding too much creates a different problem: overly alkaline soil. Determine the starting pH and add an appropriate amount rather than assuming more is better.

The acid supplies the salt anion: hydrochloric acid gives chlorides, sulfuric acid sulfates, and nitric acid nitrates. The metal or base supplies the cation. Not every metal reacts with dilute acid: copper, for example, does not release hydrogen from dilute hydrochloric acid.

For an acid reacting with an alkali, the reacting ions are H⁺(aq) + OH^-(aq) -> H₂O(l). This is neutralisation. The other dissolved ions are spectators. For an ammonium salt and alkali, NH₄⁺(aq) + OH^-(aq) -> NH₃(g) + H₂O(l); warming helps ammonia escape.

For example, CO₂(g) + 2NaOH(aq) -> Na₂CO₃(aq) + H₂O(l) with excess alkali shows acidic-oxide behaviour. CuO reacting with an acid shows basic behaviour. An insoluble oxide can still be acidic or basic: classification is not determined by whether it dissolves in water.

A strong acid ionises essentially completely in water: HCl(aq) -> H⁺(aq) + Cl^-(aq). A weak acid ionises only partly: CH₃COOH(aq) reversibly forms H⁺(aq) and CH₃COO^-(aq). Many acid molecules remain un-ionised.

Concentration describes the amount of acid per unit volume, whether or not its molecules ionise. A strong acid can be dilute, and a weak acid can be concentrated. At the same concentration, comparing suitable monoprotic acids under the same conditions, the stronger acid produces more H⁺, usually gives a lower pH and reacts faster initially with magnesium.

Precipitation occurs when dissolved ions meet and form an insoluble solid. Mixing Ba(NO₃)₂(aq) and Na₂SO₄(aq) gives BaSO₄(s). The ionic equation is Ba²⁺(aq) + SO₄^2-(aq) -> BaSO₄(s). Choose soluble starting salts so both required ions are available to mix.

A suitable metal such as magnesium can prepare a soluble salt with dilute acid, but extremely reactive metals such as sodium are unsuitable. An insoluble carbonate such as zinc carbonate is another route; effervescence helps track acid consumption. For sodium chloride, both hydrochloric acid and sodium hydroxide remain dissolved, so excess cannot be removed by filtration: titration is needed.

In the Haber process, nitrogen from air reacts with hydrogen to form ammonia: N₂(g) + 3H₂(g) reversibly forms 2NH₃(g). The syllabus connects hydrogen supply to cracking crude-oil hydrocarbons. The reverse reaction breaks ammonia down, so conversion is not necessarily complete.

Industrial plants use elevated pressure, a moderately high temperature and an iron catalyst. Ammonia is cooled and removed; unreacted nitrogen and hydrogen are recycled. A catalyst speeds attainment of the final composition without improving the equilibrium yield. This chapter uses supplied trends, not Le Chatelier's principle.

Acids supply H⁺; alkalis supply OH^-. Acid + base gives salt and water; carbonate also gives CO₂; suitable metal gives H₂. Alkali and ammonium salt release NH₃ on warming.