5.3.1Physical Chemistry and Transition Elements
Acids and Bases
5 min read 5 parts Lesson 14 of 40 Virtual lab
Part 1 of 5Brønsted–Lowry Theory
Acid–base behaviour is described as the transfer of protons, H⁺ ions.
Brønsted–Lowry acid
- A proton donor, for example HCl(aq) → H⁺(aq) + Cl⁻(aq), where HCl gives away a proton.
Brønsted–Lowry base
- A proton acceptor, for example OH⁻(aq) + H⁺(aq) → H₂O(l), where hydroxide takes a proton.
Proton transfer
- An acid can only behave as an acid if a base is present to accept the proton, so water often acts as that base.
Hydroxonium ion
- In water, H⁺ is really H₃O⁺, formed when a water molecule accepts a proton using a lone pair.
Amphoteric water
- Water can donate or accept a proton, so it acts as an acid with strong bases and as a base with acids.
Part 2 of 5Conjugate Acid–Base Pairs
Every acid–base reaction involves two pairs of related species.
1
Write the equation for the reaction, for example CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺.
2
Identify the species that loses a proton; CH₃COOH is the acid and CH₃COO⁻ is its conjugate base.
3
Identify the species that gains a proton; H₂O is the base and H₃O⁺ is its conjugate acid.
4
The members of each pair differ by exactly one proton, so label them as acid 1 / base 1 and acid 2 / base 2.
5
The stronger the acid, the weaker its conjugate base, because a strong acid releases its proton very readily.
Part 3 of 5Strength and Basicity
Strength and Basicity
Strong acid
- Fully ionised in aqueous solution, for example HCl, H₂SO₄ and HNO₃, so [H⁺] equals the acid concentration for monobasic acids.
Weak acid
- Only partially ionised, so an equilibrium exists;
- ethanoic acid is an example with the mixture containing mostly undissociated molecules.
Monobasic and dibasic
- HCl is monobasic and releases one H⁺, H₂SO₄ is dibasic and releases two, and H₃PO₄ is tribasic.
Strength versus concentration
- Strength describes the extent of ionisation,
- while concentration describes how much acid is dissolved; they are independent.
| Reaction type | Ionic equation |
|---|---|
| Acid + metal | 2H⁺(aq) + Mg(s) → Mg²⁺(aq) + H₂(g) |
| Acid + carbonate | 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g) |
| Acid + alkali | H⁺(aq) + OH⁻(aq) → H₂O(l) |
| Acid + metal oxide | 2H⁺(aq) + CuO(s) → Cu²⁺(aq) + H₂O(l) |
Part 4 of 5Worked Example
Question In the equilibrium NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), identify the two conjugate acid–base pairs.
- NH₃ gains a proton to become NH₄⁺, so NH₃ is a base and NH₄⁺ is its conjugate acid.
- H₂O loses a proton to become OH⁻, so H₂O is an acid and OH⁻ is its conjugate base.
Answer
- Pair 1: NH₄⁺ (acid) / NH₃ (base).
- Pair 2: H₂O (acid) / OH⁻ (base).
Part 5 of 5Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Brønsted–Lowry acid A species that donates a proton.
- Brønsted–Lowry base A species that accepts a proton.
- Conjugate pair Two species that differ from each other by one H⁺.
- Weak acid An acid that is only partially ionised in aqueous solution.
- Monobasic acid An acid that can release one proton per molecule.
Common Mistakes
- Confusing strong with concentrated; a dilute solution of a strong acid is still strong.
- Writing a conjugate pair with the wrong charge, for example NH₃ and NH₄ with no positive charge.
- Assuming that a weak acid has no ions at all, when it is partially ionised.
- Forgetting that H₂SO₄ can supply two protons, which affects neutralisation calculations.