Topic 2 of 3
Bronsted-Lowry: follow the proton
Find donors, acceptors and conjugate pairs, including amphiprotic species.
A-Level 9476 (2026-2027)
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.