Learn first: Electrolytic dissociation
Inorganic substances fall into four main classes: oxides, bases, acids and salts. The classification is not arbitrary: substances of one class have similar structures and undergo similar reactions, and the classes are "genetically" linked — a metal gives an oxide, the oxide a base, the base a salt. If you know the rules for "what reacts with what" in each class, you can solve most conversion chains by reasoning rather than memorising. This topic is the grammar of inorganic chemistry: its ideas are used in every later topic (metals, non-metals, electrolytic dissociation).
Oxides are compounds of two elements, one of which is oxygen in the −2 oxidation state. By chemical behaviour:
Only the oxides of alkali and alkaline-earth metals (forming alkalis) and most acidic oxides (forming acids) react with water; SiO₂, CuO and Fe₂O₃ do not. For elements with variable valency the valency is shown in brackets: FeO is iron(II) oxide, Fe₂O₃ iron(III) oxide.
Test tubes contain KOH, P₂O₅, CuO and SiO₂; water and litmus are added to each. In which tube does litmus turn red?
Bases consist of a metal atom (or NH₄⁺) and one or more hydroxide groups OH; on dissociation their only anion is OH⁻.
Properties of alkalis: they change indicator colours (phenolphthalein turns pink); they are neutralised by acids; with acidic oxides they form salts; they react with salts if a precipitate or gas forms (CuSO₄ + 2NaOH → Cu(OH)₂↓ + Na₂SO₄); with amphoteric metals they release hydrogen.
Acids consist of hydrogen atoms and an acid residue; the hydrogen atoms can be replaced by a metal. By composition there are oxygen-free acids (HCl, HBr, H₂S, HF) and oxoacids (HNO₃, H₂SO₄, H₃PO₄, H₂CO₃); by basicity (number of replaceable hydrogens) they are monobasic, dibasic or tribasic.
Properties: they change indicator colours (litmus turns red); with metals before hydrogen they release H₂ (except nitric and conc. sulfuric acids); they react with basic and amphoteric oxides and with bases; and with salts if a weaker or volatile acid is displaced or a precipitate forms.
Strength series: HClO₄ > H₂SO₄ ≈ HCl > HNO₃ > H₃PO₄ > HF > CH₃COOH > H₂CO₃ > H₂S > H₂SiO₃
For example, Na₂SiO₃ + 2HCl → 2NaCl + H₂SiO₃↓ — the stronger acid displaces the weaker one from its salt. Carbonates and sulfites give off gas with acids because H₂CO₃ and H₂SO₃ are unstable.
Salts consist of a metal cation (or NH₄⁺) and an acid residue; they are the products of neutralising an acid with a base.
The types interconvert: an acid salt with an alkali gives a normal salt: NaHCO₃ + NaOH → Na₂CO₃ + H₂O; a normal salt with excess acid gives an acid salt: Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃; a basic salt with acid gives a normal salt: CuOHCl + HCl → CuCl₂ + H₂O.
Equivalent mass of a salt: E = M/(number of metal atoms × valency)
Salt names depend on the acid residue: HCl — chlorides, H₂SO₄ — sulfates, H₂SO₃ — sulfites, H₂S — sulfides, HNO₃ — nitrates, H₃PO₄ — phosphates, H₂CO₃ — carbonates, H₂SiO₃ — silicates. Acid salts are named with "hydrogen", basic salts with "hydroxide": NaHCO₃ is sodium hydrogencarbonate, CuOHCl copper hydroxide chloride. General properties of salts: exchange reactions with acids, alkalis and other salts (if a precipitate, gas or weak electrolyte forms), displacement reactions with metals, and thermal decomposition of some salts (CaCO₃ → CaO + CO₂).
0.2 mol of CO₂ was passed into a solution containing 0.3 mol of NaOH. Which salts form, and in what amounts?
Metal → basic oxide → base → salt: Ca → CaO → Ca(OH)₂ → CaCO₃
Non-metal → acidic oxide → acid → salt: S → SO₂ → SO₃ → H₂SO₄ → BaSO₄
The two branches of these genetic series meet at the salt: basic and acidic compounds react with each other to form a salt. For insoluble bases the series is different: because the oxide does not react with water, the base is made from a salt by adding an alkali.
Carry out the chain Cu → CuO → CuCl₂ → Cu(OH)₂ → CuO.