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:

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


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