Ionization of Acids and Bases: Definition, Exaples, Problems & Class 11 Notes

 Atoms and molecules gain or lose electrons to form ions; this process is called ionization. When an atom loses an electron, it becomes positively charged (a cation), and when it gains an electron, it becomes negatively charged (an anion). This process can be achieved in different ways, such as heating a substance, passing an electric current through gas, or dissolving it in water. 

Example: Sodium (Na) loses one electron to form a sodium ion (Na⁺). 

Na → Na⁺ + e⁻ 

Important links:

NCERT Class 12 notes	NCERT Class 12 Chemistry
Class 12 Maths NCERT notes	NCERT Class 12 Physics

 

A substance that can donate hydrogen ions (H⁺) when dissolved in water is called an acid. It is sour in taste and can change the blue litmus paper red. Acids are present in foods, cleaning products and the human stomach. 

Examples: Hydrochloric acid (HCl), Citric acid, and Acetic acid (CH₃COOH). 

A substance that accepts a hydrogen ion (H⁺) or releases hydroxide ions (OH⁻) when dissolved in water is called a base. It is bitter in taste and turns the red litmus paper blue. 

Example: Ammonia (NH₃), Sodium hydroxide (NaOH), Calcium hydroxide (Ca (OH)₂). 

Arrhenius' concept of acids and bases was introduced in 1884. This concept helps to explain the behaviour of acids and bases in aqueous solutions.

Also Read: NCERT Solution | Class 11 Chemistry NCERT Solution

Arrhenius Acid: 

This is a substance that increases the concentration of hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water, and is called an Arrhenius acid. 

Example: Hydrochloric acid (HCl) ionizes in water to form (H⁺) ions. 

Arrhenius Base: 

It is a substance that increases the concentration of hydroxide ion (OH⁻) when dissolved in water.

Example: Sodium hydroxide (NaOH) ionizes in water to produce hydroxide ions (OH⁻). 

NaOH → Na⁺ + OH⁻ 

Ionization in Arrhenius Concept

1. Acids release (H⁺) ions in water during ionization process.
2. Bases dissociate to release hydroxide ions (OH⁻).

The ionization constant or dissociation constant is a measure of acid and base strength in water. It also helps to know the quantity of acid or base ionizse in a solution.  

 There are two types of ionization constants for acids and bases (Ka and Kb).  

 For Acid (Ka)

Suppose a weak acid HA partially ionizes in water.  

 Reaction: HA ⇌ H⁺ + A⁻ 

 Ionization constant of Acid (Ka):

$K{a}_{}=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[\mathrm{HA}\right]}$

 The value of  Ka determines the strength of the acid.  

•  Larger Ka, stronger acid 
•  Weaker acid smaller Ka. 

  For a Base (Kb)

 Let's consider a weak base, BOH partially ionizes in water. 

Reaction:  

BOH ⇌ B⁺ + OH⁻ 

Ionization constant of base (Kb):

$K{b}_{}=\frac{\left[B^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[\mathrm{BOH}\right]}$

How to determine Ionization Constant Experimentally: 

1. Prepare a concentration of acid or base solution.
2. Use pH meter to measure the pH solution.
3. Calculate hydrogen ions or hydroxide ions from pH or pOH.
4. Calculate the Ka or Kb using the equilibrium expression.