Worked example 1
Identify the conjugate acid-base pairs in NH₃ + H₂O → NH₄⁺ + OH⁻.
Try it first: Track the hydrogen atoms, one of them changes owner.
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Chemical ReactionsAcid-Base ReactionsContent level: Core 22 min
Acid-Base Neutralization and Proton Transfer
What you'll be able to do: Describe neutralisation as proton transfer, identify conjugate pairs and predict the products of an acid-base reaction.
Introduction
Acid-base chemistry is easier once you stop thinking about litmus paper and start thinking about one particle: the proton. Every reaction in this lesson is a hydrogen ion moving from one species to another.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Define Bronsted-Lowry acids and bases in terms of proton transfer
- Identify conjugate acid-base pairs in an equation
- Distinguish strong from weak acids and bases
- Predict the products of a neutralisation reaction
Lesson
Proton transfer, not just pH
The Bronsted-Lowry definition is deliberately simple: an acid donates H⁺, a base accepts it. Because a hydrogen ion is just a bare proton, the whole of acid-base chemistry becomes bookkeeping about where that proton goes. Nothing else needs to change for a reaction to count as acid-base.
HA + B → A⁻ + HB⁺
Conjugate pairs
When an acid gives up its proton, what remains is its conjugate base. When a base accepts a proton, the result is its conjugate acid. Members of a pair always differ by exactly one H and one unit of charge. HCl and Cl⁻ are a pair; H₂O and OH⁻ are a pair; H₂O and H₃O⁺ are also a pair, which is why water appears on both sides of so many equations.
To find a conjugate, add or remove one H⁺ and adjust the charge by one. Never change anything else.
Strong versus weak
Strong acids ionise essentially completely in water, and the list is short: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄ and HClO₃. Strong bases are the group 1 hydroxides plus Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂. Everything else is weak, which means it ionises only partially and stays mostly intact in solution.
Strong and weak describe how completely a substance ionises, not how concentrated or how dangerous it is.
Products of neutralisation
Acid plus base gives salt plus water. The salt is the cation of the base combined with the anion of the acid, and its formula comes from charge balance as always. Watch for diprotic acids such as H₂SO₄, which need two hydroxides per molecule to be fully neutralised.
H₂SO₄(aq) + 2 NaOH(aq) → Na₂SO₄(aq) + 2 H₂O(l)
Gas-forming relatives
Some acid reactions produce a gas rather than only water. Carbonates and bicarbonates react with acids to give carbon dioxide, water and a salt, and the fizzing is the visible clue. The mechanism is still proton transfer, the unstable carbonic acid formed simply falls apart.
CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
One species can play either acid or base
An amphiprotic ion can donate a proton to a stronger base or accept one from a stronger acid. Its role is determined by the reaction partner, not its name alone. Bicarbonate HCO₃⁻ sits between H₂CO₃ and CO₃²⁻ in a conjugate series, so it can react in either direction.
Identify which species gains H⁺ in the specific equation.
Key ideas
Definition
Bronsted-Lowry acid
A proton donor, a species that gives away H⁺.
Definition
Bronsted-Lowry base
A proton acceptor, a species that takes on H⁺.
Definition
Conjugate pair
Two species differing by exactly one proton, such as NH₄⁺ and NH₃.
Rule
Strong acid list
HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄ and HClO₃ ionise completely in water.
Key concept
Amphoteric water
Water can donate or accept a proton, acting as an acid with bases and as a base with acids.
Equation
Strong acid-strong base net ionic
H⁺(aq) + OH⁻(aq) → H₂O(l)
- H⁺ = proton supplied by the strong acid
- OH⁻ = hydroxide supplied by the strong base
Worked examples
Worked example 2
Write the balanced molecular and net ionic equations for H₂SO₄(aq) reacting with KOH(aq).
Try it first: Count how many protons the acid can donate.
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Worked example 3
What is the conjugate base of HCO₃⁻, and what is its conjugate acid?
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Worked example 4
Show bicarbonate acting as a base with H₃O⁺ and as an acid with OH⁻.
Try it first: Write the conjugate products for gaining or losing one proton.
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Common mistakes
Treating strong and concentrated as the same word.
Why it's wrong: Strength describes the fraction that ionises, concentration describes how much solute is present.
Check instead: Ask whether the substance is on the strong list, then read the molarity separately.
Saying the conjugate base of H₂SO₄ is SO₄²⁻.
Why it's wrong: A conjugate differs by only one proton, not two.
Check instead: Remove a single H⁺ to get HSO₄⁻.
Assuming neutralisation always gives a neutral pH 7 solution.
Why it's wrong: The salt formed can itself be acidic or basic when a weak acid or weak base is involved.
Check instead: Only a strong acid with a strong base gives a neutral salt solution.
Splitting a weak acid into ions in a net ionic equation.
Why it's wrong: Weak acids remain mostly as molecules in solution.
Check instead: Write weak acids and weak bases as intact formulas.
After the lesson · Question bank
Independent practice
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What you should now know
A Bronsted-Lowry acid donates a proton and a base accepts one, producing a conjugate base and conjugate acid that differ from their partners by exactly one H⁺. Strong acids and bases ionise completely, so their neutralisation reduces to H⁺ + OH⁻ → H₂O, while weak acids stay largely intact and appear whole in the net ionic equation. Recognising the small list of strong acids and bases is what makes these predictions fast.
- Acids donate protons and bases accept them
- Conjugate pairs differ by exactly one H⁺
- Strong acids and bases ionise completely, weak ones only partially
- Strong acid with strong base always reduces to H⁺ + OH⁻ → H₂O
- Acids with carbonates give a salt, water and carbon dioxide gas
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.