Ionisation Constant of Acids and Bases - JEE Important Topic

Acidic substances are substances which accept the electrons and these are also known as electron acceptors. They donate protons in the aqueous solution. They are sour in taste and they release  $H^+$ions in the aqueous solution. Basic substances donate electrons and are known as electron donors. They accept protons. They release $OH^-$ ions in the aqueous solution.

Classification of Acids: On the Basis of Ionisation in Water

Acids are divided into two types on the basis of their ionisation in water.

• Strong acids

• Weak acids

Strong Acids

Acids which are completely ionised into the aqueous solution are called strong acids. For example, hydrogen chloride $HCl$, sulphuric acid $H_2SO_4$, nitric acid$HNO_3$. Hydrochloric acid ionises completely into hydrogen ions and chloride ions when dissolved in water. The following equation represents the ionisation of hydrochloric acid in aqueous solution:

$HCl\left(g\right)\rightarrow\ H^+\left(aq\right)+{Cl}^-\left(aq\right)$

Weak Acids

Acids which are not completely ionised into the aqueous solutions are called weak acids. For example, acetic acid $CH_3\ COOH$, phosphoric acid $\left(H_3P\left\{O\right\}_4\right)$  ,   carbonic acid $$. The ionisation of weak acids depends on their ionisation strength and the equation of ionisation of weak acids shows a double arrow. The ionisation of acetic in aqueous solution is represented by following equation:

$CH_3COOH(aq)\leftrightarrow CH_3COO^-(aq)+H^-(aq)$

Acid Ionisation Constant K_a

It represents the strength of acid ionised in the aqueous solution. It is also known as an equilibrium constant for the ionisation of acids. The ionisation of general weak acid HA is represented by the equation:

$HA\leftrightarrow H^+\left(aq\right)+A^-\left(aq\right)$  

Acid ionisation constant for the weak acid HA is represented as:

$K_a=\frac{\left[H^+\right]\left[A^-\right]}{\left[HA\right]}$

Acid ionisation constant value of strong weak acid is higher as compared to weaker weak acids.

Reason to Study the Ionisation Constant for Weak Acids and Bases

When the strong acids and bases ionise into the aqueous solution, then they dissociate completely into ions in the solution and we can easily find out their concentration in the solution but weak acids and bases do not dissociate completely. Hence, ionisation constant helps us to find out the extent to which weak acids and bases dissociate.

Classification of Bases: On the Basis of Ionisation in Aqueous Solution

• Strong Bases

• Weak Bases

Strong Bases 

Bases which completely ionise into the aqueous solution are called strong Bases. Mostly soluble metal hydroxides are strong bases. For example, Barium hydroxide $\left[Ba\left(OH\right)_2\right] $, sodium hydroxide. When sodium hydroxide is dissolved in aqueous solution, it ionises into hydroxide and sodium ions. The following equation represents the ionisation of sodium hydroxide in water:

$NaOH\left(aq\right)\rightarrow{Na}^+\left(aq\right)+OH^-\left(aq\right)$

Weak Bases

Bases which are partially ionised in the aqueous solution are called weak bases. For example, ammonium hydroxide, pyridine, water, copper hydroxide, methylamine. When the ammonium hydroxide dissolves in aqueous solution, then it dissociates in ammonium cation and hydroxide anions. The following equation represents the ionisation of ammonium hydroxide in aqueous solution:

$NH_4OH(aq)\leftrightarrow NH_4^+(aq)+{\rm OH}^-(aq)$

Base Ionisation Constant

It shows the strength of the base ionised in aqueous solution. The base ionisation constant of ammonia is represented as:

K_b = \[ \frac{\left[{NH}_4^+\right]\left[{OH}^-\right]}{\left[{NH}_3\right]}\]

K_b value is higher for strong weak bases as compared to weaker weak bases.

Numerical Relationship Between Degree of Ionisation and Ionisation Constant

Degree of ionisation is represented by the 𝛂 and it is derived with the help of given formula:

$K_c=\frac{C^2\alpha^2}{C\left(1-\alpha\right)}$

Here, $K_C$ = ionisation constant

       𝛂 = degree of ionisation

       C = concentration of acids /bases

When 𝛂<<1 then 

$K_c = C\alpha^2$

$\alpha = \sqrt{\frac{K_C}{C}}$

Ionisation Yield (Degree of Ionisation)

It is the capacity of acids and bases to ionise itself. It is the proportion of neutral molecules ionising itself into the charged particles. When the electrolyte is weak, then it ionises partially and gives a low ionisation degree but if the electrolyte is strong, then it ionises completely and gives a high ionisation degree.

Conclusion

Acids and bases are divided into strong acid/ base and weak acid/base on the basis of their ionisation in the aqueous solution. Strong acid/base completely ionises in water weak acid/base partially ionises in water and this ionisation is found out with the help of the ionisation constant. The ionisation constant is used mainly for weak acids and bases. Ionisation yield (degree of ionisation) decreases or increases according to the strength of acids and bases. Ionisation yield increases if the strength of acid or base increases and decreases if the strength of acids and bases decreases.