Beginner Topic 514 of 1102 Acids, Bases & Electrochemistry
Conjugate Acid-Base Pairs
Species differing by one proton, such as NH4+ and NH3.
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In-depth explanation of Conjugate Acid-Base Pairs
Two species that differ by exactly one proton form a conjugate acid-base pair, with the acid being the member that carries the extra proton. Each acid HA has a conjugate base A-, and adding or removing a proton converts one member of the pair into the other. Equilibria always link the two: the acid donates a proton to a base, which in turn becomes its own conjugate acid, and the reverse reaction also holds. The relative strengths within a pair are inversely related, so a strong acid has a very weak conjugate base that scarcely accepts protons. Identifying pairs is the first step in predicting the direction of any proton transfer and the composition of a buffer.
Going deeper: Conjugate Acid-Base Pairs
Key ideas: Remove one H+ from an acid to get its conjugate base; add one H+ to a base to get its conjugate acid. Pairs differ by exactly one proton and charge differs by one unit: H2PO4- pairs with HPO4 2- and with H3PO4. Ka times Kb equals Kw (1.0e-14 at 25 C) for any pair.
Common mistakes and connections: A common error is changing atoms other than H, or subtracting the wrong charge, so check charge balance. Another is thinking the conjugate base of a strong acid is basic: Cl- and NO3- are neutral spectators in water. Pairs are the basis of buffers and salt hydrolysis.
Visualization of Conjugate Acid-Base Pairs
Carbonic acid system: H2CO3 and HCO3- form one conjugate pair, crossing at pH 6.35 where acid and base are equal; HCO3- and CO3(2-) form the next pair. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In the carbonic acid system, H2CO3 and HCO3- form one pair, and HCO3- and CO3^2- form another.
Frequently asked questions about Conjugate Acid-Base Pairs
What is Conjugate Acid-Base Pairs?
Species differing by one proton, such as NH4+ and NH3.
Is Conjugate Acid-Base Pairs a beginner, intermediate or advanced topic?
Conjugate Acid-Base Pairs is a beginner topic in Acids, Bases & Electrochemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Conjugate Acid-Base Pairs?
Study these earlier topics in Acids, Bases & Electrochemistry first: Arrhenius Acids and Bases, Bronsted-Lowry Theory, Lewis Acids and Bases.
What comes after Conjugate Acid-Base Pairs?
The next topics in Acids, Bases & Electrochemistry are Amphoteric Substances and Autoionization of Water.
Can I practise Conjugate Acid-Base Pairs with quizzes?
Yes. The interactive lesson includes a self-check question on Conjugate Acid-Base Pairs, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Conjugate Acid-Base Pairs fits in Acids, Bases & Electrochemistry
Conjugate Acid-Base Pairs is topic 4 of 105 in Acids, Bases & Electrochemistry.