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Inquiry Question 5: How are acids and bases defined and how do they behave in aqueous solution?

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

A focused answer to the HSC Chemistry Module 5 dot point on Brønsted-Lowry acid-base theory. Definitions, conjugate acid-base pairs, amphiprotic species (water and bicarbonate), how the theory extends Arrhenius, and the worked HSC past exam questions.

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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+H^+).
  • Brønsted-Lowry base: a species that accepts a proton (H+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+H^+ form a conjugate acid-base pair.

For the reaction:

HCl(aq)+H2O(l)⇌H3O(aq)++Cl(aq)−HCl_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + Cl^-_{(aq)}

  • HCl donates H+H^+, so HClHCl is the acid. Its conjugate base is Cl−Cl^-.
  • Water accepts H+H^+, so H2OH_2O is the base. Its conjugate acid is H3O+H_3O^+.

Two conjugate pairs: HCl/Cl−HCl / Cl^- and H3O+/H2OH_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: H2O+HCl⇌H3O++Cl−H_2O + HCl \rightleftharpoons H_3O^+ + Cl^-.
  • Acts as an acid: H2O+NH3⇌OH−+NH4+H_2O + NH_3 \rightleftharpoons OH^- + NH_4^+.

Bicarbonate ion (HCO3−HCO_3^-).

  • Acts as a base: HCO3−+H3O+⇌H2CO3+H2OHCO_3^- + H_3O^+ \rightleftharpoons H_2CO_3 + H_2O.
  • Acts as an acid: HCO3−+OH−⇌CO32−+H2OHCO_3^- + OH^- \rightleftharpoons CO_3^{2-} + H_2O.

Hydrogen sulfate ion (HSO4−HSO_4^-). Similarly acts as both an acid and a base.

Amino acids (like glycine, H2NCH2COOHH_2NCH_2COOH) are amphiprotic because they contain both an acidic −COOH-COOH group and a basic −NH2-NH_2 group.

A useful term to distinguish: amphoteric is the broader concept (can react with both acids and bases), which includes species like Al2O3Al_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+H^+ in water, a base produces OH−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 NH3NH_3, CO32−CO_3^{2-}, HCO3−HCO_3^- that contain no hydroxide.

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

Worked example

Identify the Brønsted-Lowry acids and bases and the conjugate pairs in:

NH_{3(aq)} + H_2O_{(l)} \rightleftharpoons NH_4^+_{(aq)} + OH^-_{(aq)}

Forward direction.

  • Water loses a proton (becomes OH−OH^-). So H2OH_2O is the acid.
  • Ammonia gains a proton (becomes NH4+NH_4^+). So NH3NH_3 is the base.

Reverse direction.

  • The NH4+NH_4^+ ion donates a proton, so it is the conjugate acid of NH3NH_3.
  • The OH−OH^- ion accepts a proton, so it is the conjugate base of H2OH_2O.

Conjugate pairs:

  • First pair: NH4+/NH3NH_4^+ / NH_3 (acid / base).
  • Second pair: H2O/OH−H_2O / OH^- (acid / base).

Note that water acts as an acid here but as a base in H2O+HCl⇌H3O++Cl−H_2O + HCl \rightleftharpoons H_3O^+ + Cl^-. Water is amphiprotic.

Common traps

Mixing up the conjugate. The conjugate base is the acid minus H+H^+. The conjugate acid is the base plus H+H^+. Differ by one proton, never more.

Confusing amphiprotic with amphoteric. Amphiprotic is the proton-specific subset of amphoteric. For HSC, use amphiprotic when discussing H2OH_2O, HCO3−HCO_3^-, and amino acids.

Forgetting state symbols. Acid-base reactions in HSC answers are expected to have (aq)(aq) or (l)(l) on every species.

Calling H2OH_2O the conjugate base of H3O+H_3O^+ but then forgetting it can also act as an acid. Always check both roles when water appears.

In one sentence

A Brønsted-Lowry acid is a proton donor and a base is a proton acceptor, every acid has a conjugate base differing by one H+H^+ (and every base has a conjugate acid), and a species like water or bicarbonate that can act as either is called amphiprotic.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2021 HSC4 marksUsing the equation HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻, identify each species as a Brønsted-Lowry acid or base, and explain the term amphiprotic with reference to HCO₃⁻.
Show worked answer →

A 4 mark answer needs the acid/base assignment, the conjugate pairs, and a clear demonstration that HCO3−HCO_3^- is amphiprotic.

In the forward direction:

  • Water donates a proton to HCO3−HCO_3^-, so **H2OH_2O is the Brønsted-Lowry acid**.
  • The HCO3−HCO_3^- ion accepts a proton, so **HCO3−HCO_3^- is the Brønsted-Lowry base**.

In the reverse direction:

  • The H2CO3H_2CO_3 molecule donates a proton, so it is the conjugate acid of HCO3−HCO_3^-.
  • The OH−OH^- ion accepts a proton, so it is the conjugate base of H2OH_2O.

Conjugate pairs: HCO3−/H2CO3HCO_3^- / H_2CO_3 and H2O/OH−H_2O / OH^-.

Amphiprotic means a species can act either as a Brønsted-Lowry acid (donating H+H^+) or as a Brønsted-Lowry base (accepting H+H^+). In the equation above, HCO3−HCO_3^- acts as a base. But HCO3−HCO_3^- can also donate a proton, for example HCO3−+H2O⇌CO32−+H3O+HCO_3^- + H_2O \rightleftharpoons CO_3^{2-} + H_3O^+, where it acts as an acid. Because it can do both, HCO3−HCO_3^- is amphiprotic.

Markers reward (1) correct assignment in the forward and reverse directions, (2) explicit naming of the two conjugate pairs, (3) the definition of amphiprotic with two equations showing HCO3−HCO_3^- in both roles.

2017 HSC2 marksExplain why the Brønsted-Lowry theory of acids is considered an improvement on the Arrhenius theory.
Show worked answer →

Arrhenius defined an acid as a substance that produces H+H^+ in aqueous solution and a base as a substance that produces OH−OH^-. This definition is limited to aqueous solutions and cannot explain basic behaviour without hydroxide ions.

Brønsted-Lowry defines an acid as a proton donor and a base as a proton acceptor. This extends the theory in two ways:

  1. It works in non-aqueous solvents (for example, NH3NH_3 acting as a base toward HClHCl in liquid ammonia).
  2. It explains the basic behaviour of species like NH3NH_3, CO32−CO_3^{2-} and HCO3−HCO_3^- that contain no OH−OH^- but still accept protons in water.

Markers reward (1) clearly stating both definitions, (2) at least one specific extension Brønsted-Lowry accounts for that Arrhenius cannot.

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