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Conjugate Acid Base Pairs

Learn and apply Conjugate Acid Base Pairs in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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Conjugate Acid–Base Pairs: Orientation

A conjugate pair differs by exactly one proton. Mark the proton donor and acceptor first; the pairings then follow without guesswork.

H1 8873 scope
  • Retain proton-transfer and conjugate-pair reasoning.

Definitions (Must Know)

  • A conjugate acid is formed when a base accepts H+.
  • A conjugate base is formed when an acid donates H+.
  • Members of a conjugate acid–base pair differ by exactly one H+.

Detailed Explanations

For NH4+ + H2O ⇌ NH3 + H3O+, NH4+/NH3 is one pair and H3O+/H2O is the other.

HCO3− is amphiprotic: it can accept H+ to form H2CO3 or donate H+ to form CO3²−.

Charge usually increases by +1 when a species gains H⁺ and decreases by 1 when it loses H⁺.

Pair species across the equation, not merely species written beside one another.

Worked Examples

Modelled example 1

Core application

Core

Problem

Identify both conjugate pairs in H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺.
Study the worked solution

  1. Follow the donor

    Method

    Pair dihydrogenphosphate with hydrogenphosphate.

    Reason

    H₂PO₄⁻ loses one H⁺.

    Working

    H₂PO₄⁻/HPO₄²⁻.

  2. Follow the acceptor

    Method

    Pair hydronium with water.

    Reason

    H₂O gains one H⁺.

    Working

    H₃O⁺/H₂O.

Guided practice 2

Finding both conjugate pairs

About 6 min

Problem

Identify both conjugate acid–base pairs in HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻ and state which reactant accepts the proton.

Try this before viewing the solution

Hints

Hint 1: compare hydrogencarbonate
It gains one hydrogen and its negative charge becomes neutral.
Hint 2: compare water
It loses one proton and becomes hydroxide.
View solution step by step

  1. Pair the proton acceptor

    Method

    Match HCO₃⁻ with H₂CO₃.

    Reason

    Hydrogencarbonate gains one H⁺ and is the base.

    Working

    H₂CO₃/HCO₃⁻.

  2. Pair the proton donor

    Method

    Match water with hydroxide.

    Reason

    Water loses one H⁺ and is the acid.

    Working

    H₂O/OH⁻.

Common misconception 3

Keep conjugate direction and charge consistent

Find and correct the mistake

Learner answer

A learner states that H₂SO₄ is the conjugate base of HSO₄⁻ because it differs by one proton. Identify and correct the error.

Choose the conjugate base

The conjugate base of HSO4− is

View solution step by step

  1. Use the word base

    Method

    Remove one proton from HSO₄⁻.

    Reason

    A conjugate base is what remains after an acid donates H⁺.

    Working

    HSO₄⁻ → SO₄²⁻ + H⁺.

  2. Audit the charge

    Method

    Make the product charge one unit more negative.

    Reason

    Removing a positive ion changes -1 to -2.

    Working

    SO₄²⁻ is the conjugate base; H₂SO₄ is the conjugate acid.

Examiner practice 4

Pair an acid with its conjugate base

4 marks

Problem

For NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, identify both conjugate pairs and state the Brønsted–Lowry acid and base on the left. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Identify the acid

    1 mark

    Method

    Name ammonium as the proton donor.

    Reason

    It loses H⁺ to form ammonia.

    Working

    NH₄⁺ is the acid.

  2. Give its pair

    1 mark

    Method

    Pair ammonium with ammonia.

    Reason

    They differ by one proton.

    Working

    NH₄⁺/NH₃.

  3. Identify the base

    1 mark

    Method

    Name water as the proton acceptor.

    Reason

    It gains H⁺ to form hydronium.

    Working

    H₂O is the base.

  4. Give its pair

    1 mark

    Method

    Pair hydronium with water.

    Reason

    They differ by one proton.

    Working

    H₃O⁺/H₂O.

Challenge 5

Demonstrate both roles of dihydrogenphosphate

Minimal support

Problem

Use two equations with water to demonstrate that H₂PO₄⁻ is amphiprotic. State its role in each equation.

Try this before viewing the solution

Hints

Hint 1: act as a base
Add one proton to form H₃PO₄ while water becomes OH⁻.
Hint 2: act as an acid
Remove one proton to form HPO₄²⁻ while water becomes H₃O⁺.
View solution step by step

  1. Show proton acceptance

    Method

    Form phosphoric acid and hydroxide.

    Reason

    H₂PO₄⁻ accepts H⁺ and therefore acts as a base.

    Working

    H₂PO₄⁻ + H₂O ⇌ H₃PO₄ + OH⁻

  2. Show proton donation

    Method

    Form hydrogenphosphate and hydronium.

    Reason

    H₂PO₄⁻ donates H⁺ and therefore acts as an acid.

    Working

    H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺

Mind Stretchers

Attempt the independent prompts before opening a hint or solution.

  • Write the conjugate base of HSO4− and the conjugate acid of NH3.
  • Use two equations to demonstrate that H2PO4− is amphiprotic.

Mind stretcher 1: Deduction from chargeExtension

Question. Species X⁻ is the conjugate base of HX. Predict the formula and charge of the conjugate acid of X⁻.

Show Hint

A conjugate acid is formed by adding one H⁺.

Show Answer

Adding H⁺ to X⁻ gives HX, which is neutral. HX is therefore the conjugate acid of X⁻.

Mind stretcher 2: Rejecting a false pairExtension

Question. A student pairs H₂CO₃ with CO₃²⁻ as conjugates. Find the error and give the intermediate species.

Show Hint

Count the protons by which the species differ.

Show Answer

They differ by two protons, so they are not a conjugate pair. HCO₃⁻ is the intermediate; H₂CO₃/HCO₃⁻ and HCO₃⁻/CO₃²⁻ are conjugate pairs.