What is conjugated acid and base

Conjugated acid-base pair

from Latin: coniungere - to connect
Synonym: Corresponding acid-base pair
English: conjugate acids and conjugate bases

1 definition

Under a conjugated acid-base pair one understands the combination of an acid and a corresponding base that absorbs the proton of the acid.

2 background

If an acid-base reaction takes place in which protons are transferred, this is also known as protolysis. It is interesting, however, that free protons (H.+) exist. Thus, every reaction of a partner as an acid necessarily requires the presence of a second partner as a base. After all, the proton of the acid has to be transferred (definition according to Broensted). A system of this kind is also called a corresponding or conjugated acid-base pair, between which a chemical equilibrium is established after a certain period of time.

3 General

If an acid or base dissociates in water, this is a proton transfer reaction. Both the uptake and release of protons are reversible and take place very quickly. Ultimately, an equilibrium is established again (protolysis equilibrium).

Example hydrochloric acid:
HCl + H2O ⇌ H3O+ + Cl-
Acid A + Base B ⇌ Acid B + Base A

The example shows that one reaction partner (HCl) is the proton donor (acid A), the other (H.2O) inevitably the proton acceptor (base B). Looking at the right hand side of the reaction equation, the H is3O+-Ion the acid - it can give off a proton during the reverse reaction (acid B), and the Cl--Ion is accordingly the base (base A). Thus one designates the Cl- also as the conjugate base of the acid HCl, correspondingly is H3O+ the conjugate acid of the base H2O. Thus:

are conjugated (corresponding, assigned) acid-base pairs.

Dissociation equilibrium
The two involved in the reaction
conjugated acid-base pairs
HA + H2O ⇌ H3O+ + A-
HA / A- and H3O+/H2O
HA .. acid
H2O .. water
A.-.. anion

4 Multi-protonic acids

In addition to monoprotonic acids, there are also acids that can give off more than one proton when dissociating in water and are correspondingly biprotonic (e.g. sulfuric acid, H.2SO4 and carbonic acid, H2CO3) or tri-protonic (e.g. phosphoric acid, H3PO4) are. If such a multi-protonic acid is present, there are correspondingly several dissociation stages, which are formulated one after the other.

The following example shows that sulfuric acid (H.2SO4) the acid and hydrogen sulfate (HSO4) is the conjugate base. In the 2nd stage, however, the hydrogen sulfate represents the acid and sulfate the conjugate base. The hydrogen sulfate can thus function as an acid or as a base.

Example sulfuric acid:
1st stage:
H2SO4 + H2O ⇌ H3O+ + HSO4-
Acid A + Base B ⇌ Acid B + Base A
2nd stage:
HSO4- + H2O ⇌ H3O+ + SO42-
Acid A + Base B ⇌ Acid B + Base A

ad stage 1:
conjugated acid-base pairs:

H2SO4/ HSO4- and H3O+/H2O
ad stage 2:
conjugated acid-base pairs:

HSO4-/SO42- and H3OH2O

5 bases

Water then reacts as a base when acids dissociate in water. However, if this reaction takes place in the presence of the base ammonia, water reacts as an acid - according to the known dissociation equilibrium.

Example ammonia:
NH3 + H2O ⇌ OH- + NH4+ conjugated acid-base pairs:
Base B + acid A ⇌ base A + acid B NH4+/ NH3 and H2O / OH-

It is therefore clear: For the bases of the general formula B, where B must have at least one free electron pair, the following applies:

Dissociation equilibrium:
conjugated acid-base pairs:
B + H2O ⇌ bra+ + OH-
bra+/ B and H2O / OH-

It can be remembered that all acid anions, which become an acid again after the uptake of protons, are bases. This also includes the alkali and alkaline earth oxides (e.g. K2O), where O2- is the base and by the uptake of a proton from the water to OH- becomes. However, this is not a dissociation of the oxide, but a reaction of the oxide with the water, which gives off a proton:

  • K2O + H2O ⇌ 2 K+ + 2 OH-
  • CaO + H2O ⇌ approx2+ + 2 OH-

Alkali and alkaline earth hydroxides are basic and can also be isolated in solid form. In water they experience dissociation as follows. OH--Ion is the base here, which is available as a proton acceptor.

NaOH + H2O ⇌ Well+ + OH-
Ca (OH)2 + H2O ⇌ approx2+ + 2 OH-

As is well known, the Broensted definition states:

  • Acids give off protons, they are proton donors.
  • Bases take up protons, they are proton acceptors.

However, this rule does not only apply to aqueous solutions, but also to acid-base reactions in general. The prerequisite for this, however, is that the acid can donate protons to a base. To do this, the base must have at least one free electron pair to which a proton can attach.

HA + B ⇌ BH+ + A-
Acid A + Base B ⇌ Acid B + Base A

6 ampholytes

With the dissociation of acids and bases, we have seen that water is a base compared to HCl, compared to NH3 reacts as an acid. It is therefore amphoteric.

Substances that have amphoteric properties are called ampholytes. They can be acid as well as base and also react like that.

So it depends on the reaction partner how the water reacts in the individual case. So you can remember the following: If the water encounters a substance that has a greater proton donor strength than itself, it reacts as a base. In the opposite direction, water reacts with the base ammonia as an acid, since it has a greater proton donor strength than this. If one wants to predict the direction of acid-base reactions, one has to measure the proton donor strength. Here are a few more examples of ampholytes:

  • H2O
  • HSO4-
  • HS-
  • HCO3-
  • H2PO4-
  • HPO42-

7 example

Take the ampholyte H2PO4- take a closer look: As a base it can take up a proton, H is formed3PO4. At the same time, however, it is able to give off a proton, i.e. to react as an acid. The doubly negatively charged anion HPO is formed42-.

a.) H.2PO4- + H+ ⇌ H3PO4 b.) H.2PO4- ⇌ HPO42- + H+
ad a.)

Dihydrogen phosphate + proton ⇌ phosphoric acid
(reacts as a base)
ad b.)

Dihydrogen phosphate ⇌ hydrogen phosphate + proton
(reacts as an acid)

8 hermaphrodite ions

The amino acids are a special group of amphoteric compounds that contain the acidic COOH group (carboxy group) and the basic NH group in the same molecule2-Group (amino group) included. Amino acids are also not in the neutral form. In aqueous solution they form predominantly the hermaphrodite ion through proteolysis. This hermaphrodite ion is the actual ampholyte. It can proceed as follows: It can take up protons and thus becomes a cation, a biprotonic acid, or it can release protons and thus become an anion.