Investigate the Brønsted-Lowry theory of acids and bases, including conjugate acid/base pairs and the behaviour of amphiprotic species

What this dot point is asking

NESA wants you to define Brønsted-Lowry acids and bases, identify conjugate acid-base pairs in a chemical equation, explain how a species can be amphiprotic, and compare Brønsted-Lowry to the earlier Arrhenius model. This is the conceptual foundation for every acid-base calculation in HSC Chemistry, including pH and pOH, titration analysis, and buffer systems.

The answer

Definitions

• Brønsted-Lowry acid: a species that donates a proton ($H^+$).
• Brønsted-Lowry base: a species that accepts a proton ($H^+$).

The definition focuses on the proton transfer itself, not on whether the reaction occurs in water.

Conjugate acid-base pairs

When an acid donates a proton, it becomes a base (because it can now accept the proton back). When a base accepts a proton, it becomes an acid. The acid and base that differ by a single $H^+$ form a conjugate acid-base pair.

For the reaction:

• HCl donates $H^+$, so $HCl$ is the acid. Its conjugate base is $Cl^-$.
• Water accepts $H^+$, so $H_2O$ is the base. Its conjugate acid is $H_3O^+$.

Two conjugate pairs: $HCl / Cl^-$ and $H_3O^+ / H_2O$.

Amphiprotic species

An amphiprotic species can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base depending on what it reacts with. The most important examples:

Water.

• Acts as a base: $H_2O + HCl \rightleftharpoons H_3O^+ + Cl^-$.
• Acts as an acid: $H_2O + NH_3 \rightleftharpoons OH^- + NH_4^+$.

Bicarbonate ion ($HCO_3^-$).

• Acts as a base: $HCO_3^- + H_3O^+ \rightleftharpoons H_2CO_3 + H_2O$.
• Acts as an acid: $HCO_3^- + OH^- \rightleftharpoons CO_3^{2-} + H_2O$.

Hydrogen sulfate ion ($HSO_4^-$). Similarly acts as both an acid and a base.

Amino acids (like glycine, $H_2NCH_2COOH$) are amphiprotic because they contain both an acidic $-COOH$ group and a basic $-NH_2$ group.

A useful term to distinguish: amphoteric is the broader concept (can react with both acids and bases), which includes species like $Al_2O_3$ that are not necessarily proton donors. Amphiprotic specifically means proton donor and acceptor.

Comparison with Arrhenius theory

Arrhenius (1887): an acid produces $H^+$ in water, a base produces $OH^-$ in water.

Brønsted-Lowry (1923) extends this in three ways:

1. Defines acid-base behaviour by proton transfer, not by what ions form in water.
2. Works in non-aqueous solvents.
3. Explains the basicity of species like $NH_3$, $CO_3^{2-}$, $HCO_3^-$ that contain no hydroxide.

Every Arrhenius acid is also a Brønsted-Lowry acid, but the reverse is not true.

Examples in context

Example 1. Sydney Water Prospect treatment plant pH adjustment. The Prospect water filtration plant adjusts incoming Warragamba water to a pH near 7.8 by dosing carbon dioxide or hydrated lime. The chemistry is a textbook Brønsted-Lowry transfer: hydrated lime deprotonates water pulled from a moderately acidic supply to give $Ca^{2+}_{(aq)} + 2OH^-_{(aq)}$, with the hydroxide accepting a proton from any free $H_3O^+$ in the source water. Operators see the conjugate-base $HCO_3^-$ formed from the dissolved $CO_2$ acting amphiprotically, buffering the network against tiny upstream pH fluctuations as water moves through the supply tunnel to Sydney homes.

Example 2. Bicarbonate as the body's amphiprotic ion. Bicarbonate is the most important amphiprotic species in human physiology. In stomach acid it accepts a proton: $HCO_3^- + H_3O^+ \rightarrow H_2CO_3 + H_2O$, neutralising acidity in antacids such as Mylanta. In the bloodstream the same ion donates a proton to keep plasma pH near 7.4: $HCO_3^- + H_2O \rightleftharpoons CO_3^{2-} + H_3O^+$. A single ion species playing both roles, depending on its partner, is exactly what Brønsted-Lowry theory predicts but the older Arrhenius framework cannot describe.

Try this

Q1. Define a Brønsted-Lowry acid and identify the conjugate base of $H_2SO_4$, $NH_4^+$ and $HCO_3^-$. [3 marks]

• Cue. Acid as proton donor; conjugate bases are $HSO_4^-$, $NH_3$ and $CO_3^{2-}$ respectively, each formed by removing one $H^+$.

Q2. Calculate the $[H_3O^+]$ produced when 0.0500 mol of HCl is dissolved in 250 mL of water, assuming full dissociation. [2 marks]

• Cue. Strong acid donates one proton per molecule, $[H_3O^+] = 0.0500 / 0.250 = 0.200 \text{ mol L}^{-1}$.

Q3. Hydrogen carbonate is described as amphiprotic. (a) Write equations showing $HCO_3^-$ acting as an acid and as a base in water. (b) Identify the conjugate pairs in each equation. (c) Explain why Arrhenius theory cannot accommodate this behaviour. [2+2+1 marks]

• Cue. (a) Acid: $HCO_3^- + H_2O \rightleftharpoons CO_3^{2-} + H_3O^+$; base: $HCO_3^- + H_2O \rightleftharpoons H_2CO_3 + OH^-$. (b) Label by one-proton difference. (c) Arrhenius limits acids to producers of $H^+$ in water and cannot describe a species that both donates and accepts protons.