# Equilibrium

## Acids, Bases and Salts

### Arrhenius Concept

Substance which dissociates in water to give hydrogen ions H+1(aq) is called acid, while a substance which dissociates in water to give hydroxyl ions OH-(aq) is called base. Following equation shows the ionization of an acid HX (aq):

HX (aq) → H+ (aq) + X-1 (aq)

This can also be written as follows:

HX (aq) + H2 (l) → H3O+ (aq) + X- (aq)

H+ is very reactive because it is a bare proton. So, it cannot exist freely in aqueous solution. Hence, it bonds to the oxygen atom of a solvent water molecule to give trigonal pyramidal hydronium ion, H3O+.

Ionization of a base molecule MOH can be given by following equation:

MOH (aq) → M+ (aq) + OH- (aq)

### Limitations of Arrhenius concept:

• It can be applicable only to aqueous solution.
• It does not account for the basicity of substances which do not have a hydroxyl group, e.g. NH3.

### Bronsted-Lowry Acids and Bases

Acid is capable of donating a hydrogen ion (or a proton) and base is capable of accepting a hydrogen ion (or proton). Following example illustrates the basic character of ammonia.

NH3 (aq) + H2O (l) ⇋ NH4+ (aq) + OH- (aq)

In this reaction, water molecule acts as proton donor and ammonia molecule acts as proton acceptor. Hence, they are respectively called Lowry-Bronsted acid and base.

Conjugate Acid Base Pair: In the reverse reaction, NH4+ acts as Bronsted acid because it is a proton donor, while OH- acts as Bronsted base. Such an acid-base pair which differs only by one proton is called a conjugate-acid-base pair.

If Bronsted acid is a strong acid then its conjugate base is a weak base and vice-versa is also true. Let us take another example to understand this.

HCl (aq) + H2O ⇋ H3O+ (aq) + Cl- (aq)

Here, HCl is a strong acid, so its conjugate base Cl- is a weak base.

It is also interesting to note that water plays the dual behavior of acid and base as per the need.

### Lewis Acids and Bases

Acid accepts electron pair and base donates an electron pair. In case of base, there is not much difference between Bronsted-Lowry and Lewis concepts because a base provides a lone pair in both the cases. But many acids do not have proton and hence they cannot be explained by Bronsted-Lowry concept. Consider following example:

BF3 + :NH3 → BF3:NH3

BF3 does not have a proton but it still acts like an acid. It reacts with NH3 by accepting its lone pair of electrons. Similarly, AlCl3, Co3+, Mg2+, etc. can act like Lewis acids while H2O, NH3, OH- can act as Lewis bases.

## Ionization of Acids and Bases

Let us consider two acids HA and H3O+ in the following equation:

HA (aq) + H2O ⇋ H3O+ (aq) + A- (aq)

Here, HA is an acid and H3O+ is a conjugate acid. If HA is a stronger acid than H3O+ then HA will donate proton and not H3O+. As a result, the solution will mainly contain A- and H3O+ ions. The equilibrium moves in the direction of formation of weaker acid and weaker base because the stronger acid donates a proton to the stronger base.

Some water soluble organic compounds behave as weak acids and exhibit different colors in their acid (HIn) and conjugate base (In-) forms.

HIn (aq) + H2O ⇋ H3O+ + In- (aq)

Such compounds are used as indicators in acid-base titrations, e.g. phenolphthalein and bromothymol blue.

#### Ionization Constant of Water

We have seen that water is unique because it behaves both like acid and base. In pure water, one H2O molecule donates a proton (and acts as an acid) and another water molecule accepts a proton (and acts as a base). Following equation shows this:

H2O (l) + H2O (l) ⇋ H3O+ (aq) + OH- (aq)

Dissociation constant is given by following:

K=([H_3O^+][OH^-])/([H_2O])

As water is a pure liquid, its concentration remains constant. So, concentration of water is omitted from the denominator in above equation. Now, the equation can be written as follows:

Kw = [H3O+][OH-]

Here, Kw is called the ionic product of water.

Concentration of water at 298 K is 1.0 × 10-7 M

As dissociation of water molecule produces equal number of H+ and OH- ions so,

[OH-] = [H3O+] = 1.0 × 10-7 M

So, vale of ionic product of water can be calculated as follows:

Kw = [H3O+][OH-]

= (1 × 10-7)2 = 1 × 10-14 M2

 [H3O+] > [OH-] Acidic [H3O+] = [OH-] Neutral [H3O+] < [OH-] Basic