Bronsted-Lowry acids and basesAQA A-Level Chemistry: Revision notes
Section 1
Acids and bases as proton donors and acceptors
In the Brønsted–Lowry model:
- An acid is a proton donor.
- A base is a proton acceptor.
A proton is a hydrogen ion, H⁺. In water a free H⁺ does not exist alone. It is bonded to a water molecule as the oxonium ion, H₃O⁺, though chemists often write H⁺(aq) for short.
This definition is wider than 'an acid contains H' or 'a base contains OH⁻'. Ammonia, NH₃, is a base even though it has no hydroxide ions, because it accepts protons.
A base does not have to contain OH⁻. Define it as a proton acceptor.
Section 2
Acid–base reactions are proton transfers
An acid can only act as an acid if a base is present to accept its proton.
- HCl + H₂O → H₃O⁺ + Cl⁻: HCl is the acid, water is the base.
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻: water is the acid, ammonia is the base.
- HCl(g) + NH₃(g) → NH₄Cl(s): proton transfer from HCl to NH₃ with no water present.
A base accepts a proton by using a lone pair of electrons to form a dative covalent bond to H⁺. In NH₃ the lone pair on nitrogen forms the fourth N–H bond in NH₄⁺.
To identify the acid in an equation, look for the reactant that loses H⁺, and the base as the reactant that gains it.
Section 3
Acid–base equilibria and conjugate pairs
Many acid–base reactions are reversible, so proton transfer occurs in both directions. Each equilibrium contains two conjugate acid–base pairs, which differ by one proton.
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
- Pair 1: CH₃COOH (acid) and CH₃COO⁻ (conjugate base)
- Pair 2: H₃O⁺ (conjugate acid) and H₂O (base)
In the reverse reaction H₃O⁺ acts as the acid and CH₃COO⁻ as the base. At equilibrium the forward and reverse proton transfers continue at equal rates.
A conjugate pair differs by exactly one H⁺. Do not pair CH₃COOH with H₃O⁺ just because both are acids.
Section 4
Water and other species that can be acids or bases
Whether a species acts as an acid or a base depends on what it reacts with.
- Water accepts a proton from HCl to give H₃O⁺, so it is a base there.
- Water donates a proton to NH₃ to give OH⁻, so it is an acid there.
- HCO₃⁻ can donate a proton (to give CO₃²⁻) or accept one (to give H₂CO₃).
The nitrating mixture shows the same idea. In HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻, sulfuric acid is the acid and nitric acid is the base.
Must Know
- Acid = proton donor; base = proton acceptor.
- Acid–base reactions are proton transfers; no water or OH⁻ is needed.
- A base uses a lone pair to bond to the proton.
- Conjugate pairs differ by one H⁺; equilibria have two pairs.
- Water and some ions can act as acid or base.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Bronsted-Lowry acids and bases
- Ammonia gas dissolves in water to form a solution that turns red litmus blue. The equilibrium in the solution can be written as NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.Explain, in terms of proton transfer, why ammonia acts as a base in water.2 marks
- Sodium hydrogencarbonate is added to water. The hydrogencarbonate ion, HCO₃⁻, reacts with acids and with alkalis according to the equations: (1) HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O and (2) HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O.Use the two equations to explain why the hydrogencarbonate ion can act as both an acid and a base.2 marks
- Two acid–base reactions are studied. Reaction 1: hydrogen chloride gas and ammonia gas meet in a dry tube and form a white smoke of solid ammonium chloride, HCl(g) + NH₃(g) → NH₄Cl(s). Reaction 2: in the nitrating mixture used to make nitrobenzene, concentrated nitric acid is mixed with concentrated sulfuric acid, HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻.Explain, in terms of proton transfer, why Reaction 1 is an acid–base reaction, identifying the acid and the base.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).