Full chapter
Theories of Acids and Bases
Choose the definition that explains the reaction: aqueous ions, proton transfer or electron-pair donation.
A-Level 9476 (2026-2027)
The Arrhenius definition is about aqueous solution
Acids increase hydrogen-ion concentration; bases increase hydroxide-ion concentration.
An Arrhenius acid increases the concentration of hydrogen ions in water. An Arrhenius base increases the concentration of hydroxide ions. The H+(aq) notation is shorthand for a proton solvated by water; H3O+ is a useful explicit representation.
| Substance | What happens in water | Classification |
|---|---|---|
| HCl | HCl + H2O → H3O+ + Cl-: the water accepts a proton. | Acid |
| CH3COOH | CH3COOH + H2O ⇌ H3O+ + CH3COO-: only a fraction ionises. | Weak acid |
| NaOH | NaOH(s) → Na+(aq) + OH-(aq): dissolution releases hydroxide ions. | Base and soluble alkali |
| NH3 | NH3 + H2O ⇌ NH4+ + OH-: reaction with water generates hydroxide. | Aqueous basic behaviour, despite no OH group in its formula |
Neutralisation between a strong aqueous acid and a strong aqueous alkali has the net ionic equation H+(aq) + OH-(aq) → H2O(l). Counter-ions such as Na+ and Cl- remain in solution and do not belong in the net ionic equation.
This definition is convenient for aqueous behaviour but is not a general explanation of every acid-base reaction. A reaction between gaseous ammonia and hydrogen chloride can produce ammonium chloride without water being present. Following proton transfer explains it more broadly.
The acid donates H+; the base accepts it
A conjugate pair differs by exactly one proton and one unit of charge.
A Bronsted-Lowry acid is a proton donor; a Bronsted-Lowry base is a proton acceptor. An acid becomes its conjugate base after losing H+. A base becomes its conjugate acid after gaining H+. The role belongs to the species in the particular reaction, not simply to a familiar name.
- Find the proton transfer
A hydrogen nucleus moves from water to ammonia.
- Identify the donor and acceptor
H2O is the acid; NH3 is the base.
- Pair each species with its proton-change partner
H2O/OH- is one pair; NH4+/NH3 is the other.
- Check the reverse reaction
NH4+ can donate a proton to OH-, regenerating NH3 and H2O.
Worked example
Conjugate charge follows proton transfer
Give the conjugate acid and conjugate base of HCO3-.
- Add H+ to obtain the conjugate acid: HCO3- + H+ → H2CO3.
- Remove H+ to obtain the conjugate base: HCO3- → CO32- + H+.
- Check charges: -1 + 1 = 0 in the first equation; -2 + 1 = -1 in the second.
Conjugate acid: H2CO3. Conjugate base: CO32-. Hydrogencarbonate can act as either a proton donor or acceptor.
A species able to donate or accept a proton is amphiprotic. Water accepts a proton from HCl but donates one to NH3. Hydrogencarbonate accepts H+ from an acid and donates H+ to a sufficiently strong base. Amphiprotic concerns proton transfer; amphoteric is the wider description of reacting as an acid or a base.
For a general equilibrium HA + B ⇌ A- + BH+, conjugate pairs are HA/A- and BH+/B. A stronger acid has a weaker conjugate base. The equilibrium tends towards the side containing the weaker acid and weaker base; numerical extent can be evaluated with the equilibrium constants introduced later.
Check your understandingAre H2SO4 and SO42- a conjugate acid-base pair?Think it through, then reveal the answer
The Lewis base supplies an electron pair
A Lewis acid accepts a pair; a Lewis base donates one.
A Lewis acid accepts an electron pair to form a covalent bond. A Lewis base donates a pair. This definition includes proton-transfer reactions because H+ can accept a lone pair, but it also explains reactions with no proton transfer.
Ammonia donates to boron trifluoride
A lone pair on the nitrogen of NH3 is directed towards the electron-deficient boron of BF3. The arrow starts at the electron pair and ends at boron. No hydrogen ion is transferred, and no water is required.
In BF3, boron has only three shared pairs in its simple bonding description and can accept another pair. Nitrogen in NH3 has a lone pair. Donation forms an N-B bond in the neutral adduct H3N→BF3. Both centres then have four bonding regions. The arrow records the origin of the bonding pair; the final bond is covalent.
| Reaction feature | Acid | Base |
|---|---|---|
| H+ accepts a lone pair from water. | H+ is a Lewis acid. | H2O is a Lewis base. |
| NH3 transfers a proton from water to itself. | H2O is the Bronsted-Lowry donor. | NH3 is the proton acceptor and uses its lone pair. |
| BF3 accepts the nitrogen pair from NH3. | BF3 is a Lewis acid. | NH3 is a Lewis base. |
| A metal ion binds a water ligand through oxygen. | The metal ion accepts the pair. | The water ligand donates a lone pair. |
Check your understandingWhy does BF3 + NH3 need the Lewis description?Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Theory | Ask |
|---|---|
| Arrhenius | Does it increase H+(aq) or OH-(aq)? |
| Bronsted-Lowry | Which species donates a proton, and which accepts it? |
| Lewis | Where is the donated electron pair, and which species accepts it? |
A conjugate pair differs by one H+. Water can be acid or base depending on its partner. BF3 accepting a lone pair from NH3 shows why acid-base chemistry is broader than aqueous neutralisation.
Scope and references
Learning outcomes and sources
4. Theories of Acids and Bases. Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
4(a) Understand and apply the Arrhenius theory.
- Acids and hydrogen ions in water
- Bases and hydroxide ions in water
- Aqueous scope and neutralisation
4(b) Understand and apply proton-transfer theory.
- Bronsted-Lowry donors and acceptors
- Conjugate acids and bases
- One-proton/one-charge distinction and amphiprotic examples
4(c) Understand and apply the Lewis theory.
- Electron-pair donors and acceptors
- Non-aqueous systems
- Named BF3 and NH3 reaction
- SEAB H2 Chemistry 9476, examination 2026
Topic 4, printed pages 15-16; all three definitions and the non-aqueous BF3/NH3 example inspected.