Conjugate Acid & Base Pair

Related Examples and Practice Problems

A page from an organic chemistry practice problem set with a focus on Bronsted-Lowry acid-base chemistry. It includes several chemical structures and formulas for practice, such as HF, CH3CH3, CH3CH2OH, CH3CN, HC≡CH, H2, RNH3+, and CH3OH2+.
A worksheet titled 'Conjugate Pairs Practice Questions' with multiple chemistry questions about acids, bases, conjugate acids, and conjugate bases, including chemical equations and practice problems.
A worksheet titled 'Conjugate Acid-Base Pairs Worksheet' with questions about classifying acids and bases, writing conjugate formulas, and identifying conjugate pairs.

A chemistry worksheet titled 'Conjugate Acid Base Pairs' featuring explanations of acids and bases, an example reaction, and exercises for rewriting chemical equations involving acids and bases.

Topic Summary & Highlights
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Core Concept

A conjugate acid-base pair consists of two species that differ by one proton ($H^+$).

  • The acid donates a proton ($H^+$) to become its conjugate base.

  • The base accepts a proton ($H^+$) to become its conjugate acid.

General Representation:

$\text{Acid} \rightleftharpoons \text{Conjugate Base} + H^+$

$\text{Base} + H^+ \rightleftharpoons \text{Conjugate Acid}$

Practice Tips

  • Focus on H+: When determining pairs, only look for the presence or absence of one hydrogen atom and the resulting change in charge.

  • Strong → Weak: Whenever you see a strong acid or strong base, know that the reaction will be highly favored in the direction that forms its very weak conjugate partner.

  • Charge Trick: A species with a positive charge (e.g., NH4+​) is generally a better acid than a neutral species (e.g., NH3​), which in turn is generally a better acid than an anion (e.g., NH2−​).

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Examples of Conjugate Pairs

Acid Conjugate Base Base Conjugate Acid
HCl Cl⁻ NH₃ NH₄⁺
H₂O OH⁻ OH⁻ H₂O
H₃PO₄ H₂PO₄⁻ CO₃²⁻ HCO₃⁻
HNO₃ NO₃⁻ CH₃COO⁻ CH₃COOH

Identify the Pairs: For the reaction below, identify the acid, base, conjugate acid, and conjugate base.
CH3​COOH(aq)+OH−(aq)⇌CH3​COO−(aq)+H2​O(l)

Naming Conjugates: Provide the chemical formula for the species requested: a) The conjugate base of H3​O+. b) The conjugate acid of CO32−​. c) The conjugate base of NH4+​.

How to Identify Conjugate Pairs

  1. Start with a Reaction: For example:

    $HCl + H_2O \rightleftharpoons H_3O^+ + Cl^-$

  2. Identify the Acid and Base:

    • HCl: Donates $H^+$, so it’s the acid.

    • $H_2O$: Accepts $H^+$, so it’s the base.

  3. Determine the Conjugates:

    • $Cl^−$: Formed when HCl loses $H^+$, so it’s the conjugate base.

    • $H_3O^+$: Formed when $H_2O$ gains $H^+$, so it’s the conjugate acid.

Relationship Between Acidity and Basicity

  • Strength of Acids and Bases:

    • A strong acid has a weak conjugate base (e.g., HCl and Cl^−).

    • A strong base has a weak conjugate acid (e.g., $OH^−$ and $H_2O$).

    • Weak acids and weak bases form conjugates that are relatively stronger.

  • Amphiprotic Substances:

    • Some species, like $H_2O$, can act as both an acid and a base:

      • As an acid: $H_2O \rightarrow OH^-$

      • As a base: $H_2O + H^+ \rightarrow H_3O^+$

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