Conjugate acid

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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.[1]

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.

Acid-base reactions[edit]

In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid:

acid + base is in equilibrium with conjugate base + conjugate acid

The conjugate acid of a base is formed when the base gains a proton. Refer to the following equation:

NH
3
(g) + H
2
O
(l) is in equilibrium with NH+
4
(aq) + OH
(aq)

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
2
O
), because H
2
O
donates a hydrogen ion to form OH
, the conjugate base.

Strength of conjugates[edit]

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.

This principle is discussed in detail in the article on acid-base reaction theories. The position of the equilibrium is measured as an acid dissociation constant (Ka)

Identifying conjugate acid-base pairs[edit]

Example of a Bronsted-Lowry Reaction

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:

HNO
3
+ H
2
O
H
3
O+
+ NO
3

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
2
O
) is a base because it accepts a proton from HNO
3
and its conjugate acid is hydronium (H
3
O+
). 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
HClO
2
+ H
2
O
ClO
2
+ H
3
O+
HClO
2
H
2
O
ClO
2
H
3
O+
ClO
+ H
2
O
HClO + OH
H
2
O
ClO
OH
HClO
HCl + H
2
PO
4
Cl
+ H
3
PO
4
HCl H
2
PO
4
Cl
H
3
PO
4

Table of acids and their conjugate bases[edit]

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.

Acid Base
HeH+
Helium hydride ion
He helium
H
2
F+
Fluoronium 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

See also[edit]

References[edit]

  1. ^ Zumdahl, Stephen S., & Zumdahl, Susan A. Chemistry. Houghton Mifflin, 2007, ISBN 0618713700