|Acids and bases|
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a species formed by the reception of a proton (H+), by a base - in other words, the base with a hydrogen ion added to it - while a conjugate base is formed by the removal of a proton from an acid: the conjugate base of an acid is that acid with a hydrogen ion removed.
Visually, this can be represented as:
- A conjugate acid of the base → Base + H+
- A conjugate base of the acid → Acid − H+
The Brønsted-Lowry model is based on the idea that acids are proton donors and bases are proton acceptors; the conjugate base or conjugate acid is merely what is left after an acid has lost a proton or a base has gained a proton, respectively.
In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid:
- acid + base conjugate base + conjugate acid
The conjugate acid of a base is formed when the base gains a proton. Refer to the following equation:
3(g) + H
4(aq) + OH−
We say that ammonium (NH+
4) is the conjugate acid to the base ammonia (NH
3), because NH
3 gained a hydrogen ion to form NH+
4, the conjugate acid. The conjugate base of an acid is formed when the acid donates a proton. In the equation, we say that hydroxide (OH−
) is the conjugate base to the acid water (H
2O), because H
2O donates a hydrogen ion to form OH−
, the conjugate base.
Strength of conjugates
The stronger the acid or base, the weaker the conjugate. The weaker the acid or base, the stronger the conjugate. However, a weak acid or base will not necessarily have a strong conjugate base or acid; there are a number of pairs of weak conjugates. For example, acetic acid (CH3COOH) and the acetate ion (CH3COO-) are both weak.
Identifying conjugate acid-base pairs
The acid and conjugate base as well as the base and conjugate acid are conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants. The reactants are the acids and bases, and the acid corresponds to the conjugate base on the product side of the chemical equation. This goes for the base too; the base corresponds to the conjugate acid on the product side of the equation.
To identify the conjugate acid, look for the pair of compounds that are related. The acid-base reaction can be viewed in a before and after sense. The before is the reactant side of the equation, the after is the product side of the equation. The conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base. The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation the compound that has one more hydrogen ion of the conjugate base is the acid.
Consider the following acid-base reaction:
3 + H
2O → H
Nitric acid (HNO
3) is an acid because it donates a proton to water and its conjugate base is nitrate (NO−
3). An easy way to identify the conjugate base is that it differs from the acid by one proton. Water (H
2O) is a base because it accepts a proton from HNO
3 and its conjugate acid is hydronium (H
). Again to identify the conjugate acid (or any conjugate pair) is that it differs from the base by one proton.
|Equation||Acid||Base||Conjugate Base||Conjugate Acid|
2 + H
2O → ClO−
2 + H
2O → HClO + OH−
|HCl + H
4 → Cl−
Table of Acids and their Conjugate Bases
Tabulated below are several examples of acids and their conjugate bases; notice how they differ by just one proton (H+ ion). Acid strength decreases and base strength increases down the table.
Helium hydride ion
|HF Hydrogen fluoride|
|HCl Hydrochloric acid||Cl− Chloride ion|
|H2SO4 Sulfuric acid||HSO4− Hydrogen sulfate ion|
|HNO3 Nitric acid||NO3− Nitrate ion|
|H3O+ Hydronium ion||H2O Water|
|HSO4− Hydrogen sulfate ion||SO42− Sulfate ion|
|H3PO4 Phosphoric acid||H2PO4− Dihydrogen phosphate ion|
|CH3COOH Acetic acid||CH3COO− Acetate ion|
|H2CO3 Carbonic acid||HCO3− Hydrogen carbonate ion|
|H2S Hydrosulfuric acid||HS− Hydrogen sulfide ion|
|H2PO4− Dihydrogen phosphate ion||HPO42− Hydrogen phosphate ion|
|NH4+ Ammonium ion||NH3 Ammonia|
|HCO3− Hydrogencarbonate (bicarbonate) ion||CO32− Carbonate ion|
|HPO42− Hydrogen phosphate ion||PO43− Phosphate ion|
|H2O Water (neutral, pH 7)||OH− Hydroxide ion|
- Zumdahl, Stephen S., & Zumdahl, Susan A. Chemistry. Houghton Mifflin, 2007, ISBN 0618713700