Question

Explain how you know the conjugated acid base pairs for a
reaction.

Answer 1

Use the Brønsted–Lowry idea: an acid is a proton (H+) donor, a base is a proton acceptor. A conjugate base is what remains after an acid donates a proton; a conjugate acid is what you get when a base accepts a proton. Conjugate acid–base pairs always differ by exactly one H+.

How to identify conjugate pairs in a reaction (step‑by‑step)

1. Find the proton transfer. Look for which species loses an H+ and which gains an H+.
2. Label the acid (the one that donates H+) and the base (the one that accepts H+).
3. Form the conjugate base by removing one H+ from the acid; form the conjugate acid by adding one H+ to the base.
4. Check charges: removing H+ reduces the charge by +1, adding H+ increases the charge by +1.
5. Conjugate pairs are on opposite sides of the equation.

Examples

- HCl + H2O → Cl− + H3O+
- HCl donates H+ → HCl is the acid; its conjugate base is Cl−.
- H2O accepts H+ → H2O is the base; its conjugate acid is H3O+.
- Pairs: (HCl / Cl−) and (H2O / H3O+).

- NH3 + H2O → NH4+ + OH−
- NH3 accepts H+ → base; its conjugate acid is NH4+.
- H2O donates H+ → acid; its conjugate base is OH−.
- Pairs: (NH3 / NH4+) and (H2O / OH−).

- H2SO4 → HSO4− → SO4^2− (stepwise deprotonation of a diprotic acid)
- Each step removes one H+, so successive conjugate pairs are (H2SO4 / HSO4−) and (HSO4− / SO4^2−).

Tips and reminders

- Conjugate pairs differ by one H+ only. If two species differ by more than one H+ (or other atoms), they are not a conjugate pair.
- Amphiprotic species (like H2O, HSO4−) can act as either acid or base depending on the reaction.
- Strength relation: the stronger the acid, the weaker its conjugate base (and vice versa).
- If a reaction is shown without H+, look for changes in formulas/charges to see which species gained or lost H+.

If you want, give me a specific reaction and I’ll identify the conjugate acid–base pairs for you.