3.1.3—Amphiprotic species

An amphiprotic species can donate H+ in one reaction and accept H+ in another.

Write one equation in which the species becomes its conjugate base and another in which it becomes its conjugate acid.

For HCO₃⁻, donation gives CO₃²⁻ whereas acceptance gives H₂CO₃. Showing both reactions is the evidence for amphiprotic behaviour; one acid–base equation alone is insufficient.

Representative question

[Maximum number: 2]

Formulate two equations to show the amphiprotic nature of $\mathrm{H}_{2} \mathrm{PO}_{4}^{-}$ .

$\mathrm{H_2PO_4^-}(aq)+\mathrm{H^+}(aq) \rightarrow \mathrm{H_3PO_4}(aq)$ $\mathrm{H_2PO_4^-}(aq)+\mathrm{OH^-}(aq) \rightarrow \mathrm{HPO_4^{2-}}(aq)+\mathrm{H_2O}(l)$

Accept reactions of $\mathrm{H_2PO_4^-}$ with any acidic, basic or amphiprotic species, such as $\mathrm{H_3O^+}$ , $\mathrm{NH_3}$ or $\mathrm{H_2O}$ .
Accept:
$\mathrm{H_2PO_4^-}(aq)\rightarrow\mathrm{HPO_4^{2-}}(aq)+\mathrm{H^+}(aq)$
for M2.

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.

3.1.3—Amphiprotic species

Amphiprotic Species

Showing Amphiprotic Behaviour

Assessment in practice

Proton Transfer Reactions Summary

Question 1