Learn first: Solutions and concentration
Why does a solution of table salt conduct electricity while a sugar solution does not? The answer is electrolytic dissociation: when some substances dissolve in water they break up into freely moving ions, and the ions carry the current. This theory explains reactions in solution: the properties of acids and bases, precipitation, gas evolution, the acidity of a solution (pH) and the hydrolysis of salts are all processes between ions. Writing ionic equations, telling strong from weak electrolytes and working out the pH are among the most common exam tasks and require precise knowledge.
The theory was proposed by the Swedish scientist S. Arrhenius (1887). Its main points:
Dissociation happens because polar water molecules surround the ions of the crystal lattice (hydrate them) and pull them apart. So ions in solution are really hydrated. For example, the blue colour of copper(II) sulfate solution belongs to hydrated Cu²⁺ ions, while anhydrous CuSO₄ is white.
NaCl → Na⁺ + Cl⁻; H₂SO₄ → 2H⁺ + SO₄²⁻; Ba(OH)₂ → Ba²⁺ + 2OH⁻; Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄²⁻
In this theory an acid is an electrolyte whose only cation on dissociation is H⁺; a base gives OH⁻ as its only anion; a salt splits into metal cations (or NH₄⁺) and acid-residue anions.
Polyprotic acids and polyhydroxy bases dissociate in steps, the first step being the strongest: H₃PO₄ ⇌ H⁺ + H₂PO₄⁻, H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻, HPO₄²⁻ ⇌ H⁺ + PO₄³⁻.
α = n(dissociated molecules)/n(dissolved molecules), as a fraction or a percentage
The degree of dissociation depends on the nature of the electrolyte, its concentration (α rises on dilution) and the temperature.
How many ions form when 0.2 mol of Al₂(SO₄)₃ dissociates completely?
Reactions between electrolyte solutions really take place between ions. For an ion-exchange reaction to go to completion, one product must be a precipitate, a gas or a weak electrolyte (such as water); otherwise the ions simply stay mixed in solution.
Rules for ionic equations: write strong, soluble electrolytes as ions, and precipitates, gases, water and other weak electrolytes as molecules. Cancel identical ions on both sides.
Molecular: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl
Full ionic: Ba²⁺ + 2Cl⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄↓ + 2Na⁺ + 2Cl⁻
Net ionic: Ba²⁺ + SO₄²⁻ → BaSO₄↓
The net ionic equation shows the essence of the reaction: any soluble barium salt reacts with any soluble sulfate in exactly the same way. More examples:
Water is a very weak electrolyte: H₂O ⇌ H⁺ + OH⁻. At 25 °C pure water has [H⁺] = [OH⁻] = 10⁻⁷ mol/L.
[H⁺]·[OH⁻] = 10⁻¹⁴ (ionic product of water); pH = −lg[H⁺]; pH + pOH = 14
pH < 7 acidic, pH = 7 neutral, pH > 7 alkaline
Indicators: litmus (red in acid, blue in alkali), phenolphthalein (colourless in neutral and acidic solutions, pink in alkali), methyl orange (red in acid, orange in neutral, yellow in alkali).
Find the pH of 0.001 M HCl and of 0.01 M NaOH.
100 mL of 0.2 M HCl was mixed with 100 mL of 0.1 M NaOH. Find the pH.
Hydrolysis is the reaction of a salt’s ions with water to form a weak electrolyte, changing the pH. The rule: "the stronger partner wins" — the solution takes the character of the stronger component that formed the salt.
That is why mixing AlCl₃ and Na₂CO₃ solutions gives not aluminium carbonate but a hydroxide precipitate and CO₂: 2AlCl₃ + 3Na₂CO₃ + 3H₂O → 2Al(OH)₃↓ + 6NaCl + 3CO₂↑. Hydrolysis increases on heating and dilution. In practice, washing soda (Na₂CO₃) solution is alkaline and so cleans greasy dishes; Al(OH)₃ from the hydrolysis of aluminium sulfate traps and settles suspended particles in water treatment; and soil pH also depends on the hydrolysis of the salts it contains.