7.2 Brønsted–Lowry theory of acids and bases

An acid is a proton donor and a base is a proton acceptor in the Brønsted–Lowry model. In water, acids form H₃O⁺ while bases remove protons from water or another donor.

The model focuses on proton transfer, so the same species can act as an acid in one reaction and a base in another.

HCl + H₂O → H₃O⁺ + Cl⁻: HCl donates H⁺ and water accepts it. NH₃ accepts H⁺ from water, so NH₃ is a base.

A base is not defined only as a substance containing OH⁻. Ammonia is a Brønsted base even though its formula has no hydroxide ion.

Common acids include hydrochloric acid HCl, nitric acid HNO₃ and sulfuric acid H₂SO₄. Common alkalis are soluble bases such as sodium hydroxide NaOH, potassium hydroxide KOH and aqueous ammonia NH₃(aq).

In water, acids increase hydronium concentration; soluble alkalis provide or generate hydroxide ions. Formula and state determine what particles are present.

HNO₃(aq) dissociates to H⁺/H₃O⁺ and NO₃⁻; NaOH(aq) provides Na⁺ and OH⁻. Their neutralisation produces water and a salt.

“Strong” and “concentrated” are different. Strength describes ionisation; concentration describes amount per volume.

A conjugate acid is formed when a base accepts a proton; a conjugate base is formed when an acid donates one. The members of a conjugate pair differ by exactly H⁺.

Label both proton transfers in the equation, not just the reactant called “acid” in everyday language.

In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, NH₃/NH₄⁺ is a conjugate base/acid pair and H₂O/OH⁻ is a conjugate acid/base pair.

Conjugate does not mean equal strength or identical charge. It means the formulas differ by one proton under the reaction conditions.

A strong acid dissociates almost completely in water; a strong base reacts or dissociates to produce hydroxide essentially completely. Weak acids and bases establish an equilibrium with substantial undissociated species.

Strength is an equilibrium property, not a measure of how much solution is present. A dilute strong acid can have lower concentration than a concentrated weak acid while still being more completely ionised.

HCl is strong, so [H₃O⁺] is close to its analytical concentration. Ethanoic acid is weak, so only part of CH₃COOH ionises and CH₃COOH ⇌ H⁺ + CH₃COO⁻ must be considered.

Do not use pH alone to label strength without knowing concentration. Concentration and degree of ionisation are separate ideas.

A weak acid or base ionises only partially in water, so the species and its ions coexist at equilibrium. The equilibrium position depends on the acid/base strength and concentration.

Write a reversible equation and use the conjugate pair to track proton transfer. A weak acid does not mean the solution contains no hydronium; it means most solute remains un-ionised.

Ethanoic acid follows CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻. Adding water changes concentrations and pH, but the acid remains a weak equilibrium system.

Weak is not the same as dilute. Strength describes degree of ionisation; dilution describes how much solute is present per volume.

A strong acid or base ionises essentially completely in water; a weak one establishes an equilibrium with a significant fraction un-ionised.

To compare strength, compare the ionisation equilibrium, not just the measured pH. A concentrated weak acid can have a lower pH than a very dilute strong acid while still being weaker.

0.001 mol dm⁻³ HCl is strong and nearly fully ionised, whereas 0.10 mol dm⁻³ ethanoic acid is weak and only partly ionised. Their pH values cannot alone define strength.

Strong does not mean concentrated, and weak does not mean harmless. Keep degree of ionisation and amount per volume separate.

Neutralisation is a reaction in which an acid and a base react so that proton transfer produces water. In the simplest ionic form, H₃O⁺ + OH⁻ → 2H₂O.

The other ions remain in solution as the salt’s ions. Write the molecular equation, then identify the net ionic proton-transfer step when useful.

HCl(aq)+NaOH(aq)→NaCl(aq)+H₂O(l). The net reaction is H⁺(aq)+OH⁻(aq)→H₂O(l), while Na⁺ and Cl⁻ are spectators.

Neutralisation does not always produce pH 7: a weak acid/strong base mixture or excess reagent can leave the solution acidic or basic.

A salt forms when the acidic proton is replaced by a metal ion or ammonium ion. The salt remains in solution as ions, and some ions can react with water by hydrolysis.

The parent acid and base determine whether the salt solution is approximately neutral, acidic or basic. Do not infer pH from the word “salt” alone.

NaCl from strong acid/strong base is approximately neutral. CH₃COONa contains acetate, the conjugate base of a weak acid, so acetate can accept protons from water and make the solution basic.

Salt formation is not the same as complete removal of every proton from every acid. Polyprotic acids and weak conjugate ions need separate analysis.

A titration curve plots pH against the volume of acid or alkali added. Its shape reflects the relative strengths of the reactants and the equivalence point where stoichiometric neutralisation is complete.

Strong/strong curves have a steep jump centred near pH 7; weak/strong combinations shift the equivalence pH because the conjugate ion hydrolyses. Half-equivalence can identify a weak acid’s pKa.

Adding NaOH to ethanoic acid gives a buffer region before the steep rise and an equivalence point above pH 7, unlike HCl/NaOH.

Equivalence point is not always pH 7 and is not the same as the point where an indicator happens to change colour.

An indicator is suitable when its transition range lies within the sharp pH change near the equivalence point. The endpoint colour change should therefore occur close to the stoichiometric endpoint.

Choose from the expected curve, not from the acid name alone. Strong acid/strong base allows a broad choice; weak acid/strong base needs an indicator changing in the basic region.

Phenolphthalein is suitable for a weak acid–strong base titration because its transition is above pH 7; methyl orange is better suited to a strong acid–weak base curve.

An indicator does not measure exact equivalence by magic. If its transition range falls outside the steep section, the endpoint error can be significant.