Dissociation of base in aqueous solution

These notes are based on the chapter Acids Bases and Salts from NCERT book based on CBSE syllabus.

Let us see what happens when a base is dissolved in water. A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound. A base is not going to show basic properties in absence of water. It needs to be in aqueous solution to show the bhaviour of a base. Example:

Example 1: When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.

NaOH (aq) ⇨ Na⁺ + OH⁻

Example 2: When potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.

KOH (aq) ⇨ K⁺ + OH⁻

Thus, base shows its basic character because of dissociation of hydroxide ion.

Neutralisation Reaction

In previous section, you read about a general definition of neutralisation reaction. In thise section, you will learn about neutralisation reaction in terms of dissociation and recombination of different ions of acid and base when they react with each other.

When an acid reacts with a base, the hydrogen ion of acid combines with the hydroxide ion of base and forms water. As these ions combine together and form water instead of remaining free, both neutralize each other. Here, neutralisation means that the acid is erasing the basic property of base, and bases is erasing the acidic property of acid.

OH⁻ + H⁺ ⇨ H₂O

Example: When sodium hydroxide (a base) reacts with hydrochloric acid, sodium hydroxide breaks into sodium ion and hydroxide ion and hydrochloric acid breaks into hydrogen ion and chloride ion. Hydrogen ion and hydroxide ion combine together and form water, while sodium ion and chloride ion combine together and form sodium chloride.

NaOH + HCl ⇨ OH⁻ + Na⁺ + H⁺ + Cl⁻ ⇨ NaCl + H₂O

Dilution of Acid and Base

The concentration of hydrogen ion in an acid and hydroxide ion in a base (per unit volume) shows the concentration of acid or base.

By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted.

While diluting an acid or a base, either of them is added to water rather than water being added to them. Why?

The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.

Monoacedic or Monoprotic Base

When one molecule of a base produces 1 ion of OH^- on complete ionisation, the base is called monoacedic or monoprotic base. Examples: NaOH, KOH, etc.

Diacedic or Diprotic Base

When one molecule of a base produces 2 ions of OH^- on complete ionisation, the base is called diacedic or diprotic base. Examples: Ca(OH)₂, Mg(OH)₂, etc.

Triacedic or Triprotic Base

When one molecule of a base produces 3 ions of OH^- on complete ionisation, the base is called triacedic or triprotic base. Examples: Al₂(OH)₃, Fe₂(OH)₃, etc.