Master AP Chemistry Topic 4.8 with this free practice quiz. Test your skills on bronsted lowry theory, proton donors and acceptors, and conjugate acids and bases to prepare for your AP exam.
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Two Definitions of an Acid
The older definition is Arrhenius: an acid produces H⁺ ions in water, a base produces OH⁻ ions in water. Sodium hydroxide is an Arrhenius base because it releases OH⁻ directly. But ammonia, NH₃, contains no OH⁻ to release, yet behaves as a base: the Arrhenius picture has a gap.
The Brønsted-Lowry definition, the CED’s, is built on proton transfer: an acid is a proton (H⁺) donor, a base a proton acceptor. A proton can only be donated if something accepts it, so acid and base always appear together.
In Water, the Proton Travels as H₃O⁺
A bare H⁺ never exists alone in water: it attaches to a water molecule, forming hydronium, H₃O⁺. H⁺(aq) is shorthand for H₃O⁺(aq).
Hydrogen chloride dissolving: HCl + H₂O → H₃O⁺ + Cl⁻. HCl donates the proton (the acid); water accepts it (the base).
With ammonia, water’s role reverses: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, water donating the proton and acting as the acid. The OH⁻ was made by the transfer, not released from the ammonia.
No acid stronger than H₃O⁺ survives in water: it hands its proton to water instead, making H₃O⁺ the strongest acid present in aqueous solution. In NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ the equilibrium therefore lies well to the left: only a small fraction of NH₄⁺ ions have donated at any moment.
Conjugate Pairs Differ by Exactly One Proton
When an acid donates its proton, what remains is its conjugate base. When a base accepts a proton, what forms is its conjugate acid. A conjugate pair is two species differing by exactly one H⁺, their charges differing by one too.
Conjugate base: remove one H⁺ (H₂SO₄ gives HSO₄⁻, not SO₄²⁻). Conjugate acid: add one H⁺ (NH₃ gives NH₄⁺). Every proton transfer contains two pairs: in HCl + H₂O → H₃O⁺ + Cl⁻, HCl/Cl⁻ and H₃O⁺/H₂O.
A weak acid together with its conjugate base forms a buffer: the acid neutralises added OH⁻, the conjugate base added H⁺, so the pH changes far less than in water alone.
Amphiprotic Species Play Both Roles
An amphiprotic species donates a proton in one reaction, accepts one in another; its partner decides which. Water accepts from HCl, donates to NH₃.
Hydrogencarbonate, HCO₃⁻, is amphiprotic: with water it acts as a base, HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻; with hydroxide as an acid, HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O. HSO₄⁻ and H₂PO₄⁻ behave the same way.
Strong Means Fully Ionised, Weak Means Barely
A strong acid donates its proton completely: no intact acid molecules remain. The strong acids are HCl, HBr, HI, HNO₃, HClO₄ and the first proton of H₂SO₄; single arrow. Strong bases ionise completely too: Group 1 hydroxides, such as NaOH and KOH, and Ba(OH)₂ are entirely cations and OH⁻ ions.
A weak acid ionises only slightly: CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻ is an equilibrium, and in 0.10 mol/L solution only about one molecule in a hundred has ionised at any moment. A weak base accepts protons only partially: most NH₃ in solution is still NH₃.
Strength is the fraction ionised; concentration is the amount dissolved. Independent ideas: a concentrated weak acid can hold more H₃O⁺ than a very dilute strong acid.
Neutralisation: A Proton Meets Hydroxide
An acid reacting with an OH⁻ base produces a salt and water: HCl + NaOH → NaCl + H₂O, the salt pairing the base’s cation with the acid’s anion.
For any strong acid with any strong base the net ionic equation is Topic 4.2’s: H⁺(aq) + OH⁻(aq) → H₂O(l); the spectators differ, the proton transfer never does. With hydronium: H₃O⁺(aq) + OH⁻(aq) → 2H₂O(l).
If the acid is weak it stays whole in the net ionic equation, because most of it is whole in the flask: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l). If the base is ammonia, no OH⁻ appears: NH₃(aq) + H⁺(aq) → NH₄⁺(aq); with hydrochloric acid the only product is the salt NH₄Cl. Neutralisation is the proton transfer, not the appearance of water.
Strength changes how ionised an acid is, not its total proton capacity: a mole of CH₃COOH neutralises exactly as much NaOH as a mole of HCl.
Polyprotic Acids Donate One Proton at a Time
A polyprotic acid has more than one donatable proton: H₂SO₄ has two, H₃PO₄ three. They leave one at a time, in separate steps, never together.
Sulfuric acid’s first step is complete, H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻; the second is an equilibrium, HSO₄⁻ + H₂O ⇌ H₃O⁺ + SO₄²⁻. Hence the conjugate base of H₂SO₄ is HSO₄⁻: one proton removed, one step taken.
Carbonate runs the same staircase in reverse: CO₃²⁻ + H⁺ → HCO₃⁻, then HCO₃⁻ + H⁺ → H₂O + CO₂ (through H₂CO₃, which falls apart). Together: 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g), the net ionic equation behind the Topic 4.7 result.
Worked Examples
Example 1: labelling a proton transfer. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, NH₃ accepts a proton: it is the base, and NH₄⁺ is its conjugate acid. H₂O donates the proton: it is the acid, and OH⁻ its conjugate base. Two pairs, each differing by one proton.
Example 2: three neutralisations, three net ionic equations. Nitric acid with potassium hydroxide: K⁺ and NO₃⁻ spectate: H⁺(aq) + OH⁻(aq) → H₂O(l). Acetic acid with sodium hydroxide: Na⁺ spectates, the weak acid stays whole: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l). Hydrochloric acid with ammonia: Cl⁻ spectates, the proton lands on ammonia: NH₃(aq) + H⁺(aq) → NH₄⁺(aq). One family, three net equations, because what is present in each flask differs.
Example 3: carbonate, one proton at a time. A solution contains 0.0200 mol of CO₃²⁻. Adding 0.0200 mol of H⁺ runs the first step only, CO₃²⁻ + H⁺ → HCO₃⁻: 0.0200 mol of HCO₃⁻, no gas. A second 0.0200 mol of H⁺ runs the second step, HCO₃⁻ + H⁺ → H₂O + CO₂, and the 0.0200 mol of CO₂ escapes: 0.0200 × 22.4 = 0.448 L at STP. Total acid: 0.0400 mol, two protons per carbonate.
Traps That Cost Marks in Topic 4.8
Removing both protons at once. The conjugate base of H₂SO₄ is HSO₄⁻. A conjugate pair differs by exactly one proton, so SO₄²⁻, two protons away, cannot be the answer.
Miscounting the gap in a pair. H₂CO₃ and CO₃²⁻ are not a conjugate pair: two protons apart. The real pairs are H₂CO₃/HCO₃⁻ and HCO₃⁻/CO₃²⁻. H₂O and OH⁻, by contrast, differ by one proton and are a genuine pair.
Splitting weak acids and bases in ionic equations. CH₃COOH and NH₃ are written whole. Only strong acids and strong bases appear as separated ions, exactly as in Topic 4.2.
Confusing strong with concentrated. Strong describes how completely a substance ionises; concentrated, how much is dissolved. Dilute HCl is still strong; concentrated CH₃COOH is still weak.
Expecting water, or OH⁻, in every neutralisation. NH₃ + HCl → NH₄Cl forms a salt, no water, and its net ionic equation has no OH⁻: the base took the proton itself.
Pinning a permanent label on a substance. Water is the base when HCl dissolves in it, the acid when NH₃ does; HCO₃⁻ is a base toward water, an acid toward OH⁻. Assign roles from the equation in front of you.
Forgetting the second water. H₃O⁺(aq) + OH⁻(aq) → 2H₂O(l): written with hydronium, neutralisation makes two waters, not one.
Key Relationships
- Brønsted-Lowry: an acid donates H⁺, a base accepts H⁺; every acid-base reaction is a proton transfer needing both partners. Arrhenius is the water-only version: acid gives H⁺, base gives OH⁻
- In water the proton travels as H₃O⁺ (H⁺(aq) is shorthand), the strongest acid that can exist in aqueous solution
- A conjugate pair differs by exactly one H⁺: base + H⁺ gives its conjugate acid, acid − H⁺ gives its conjugate base, the charge shifting by one each way
- Every proton transfer contains two conjugate pairs; a weak acid with its conjugate base forms a buffer resisting pH change
- Strong acids and bases ionise completely and are written as ions; weak ones reach equilibrium and stay mostly intact; strength (fraction ionised) is not concentration (amount dissolved)
- Neutralisation net ionic equations: H⁺ + OH⁻ → H₂O (strong with strong); CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O (weak acid); NH₃ + H⁺ → NH₄⁺ (ammonia)
- Polyprotic acids donate stepwise, H₂SO₄ → HSO₄⁻ → SO₄²⁻; carbonate accepts stepwise, CO₃²⁻ → HCO₃⁻ → H₂O + CO₂, so acid plus carbonate gives a salt, water and carbon dioxide