Maharashtra Board Class 12 Chemistry Chapter 3 Ionic Equilibria PDF Download

3. Ionic Equilibria

What is chemical equilibrium?

What are electrolytes?

3.1 Introduction

The equilibrium between ions and unionized molecules in solution is called ionic equilibrium. The principles of chemical equilibrium we studied in standard XI will be applied to ionic equilibria. In this chapter with the help of these principles we determine equilibrium constants and concentrations of ions and unionized species. In particular examine the following ionic equilibria:

H and OH ions and unionized water molecules.

Ionization of weak acids and weak bases.

Reactions between ions of salt and ions of water.

Solid salt and its ions in water.

3.2 Types Of Electrolyte

The substances which give rise to ions when dissolved in water are electrolytes. The non electrolytes are those which do not ionize and exist as molecules in aqueous solutions.

The electrolytes are classified into strong and weak electrolytes. This classification is based on their extent of ionisation in dilute aqueous solutions.

3.2.1 Strong Electrolyte

The electrolytes ionizing completely or almost completely are strong electrolytes. For example: strong acids, strong bases and salts.

3.2.2 Weak Electrolyte

The electrolytes which dissociate to a smaller extent in aqueous solution are weak electrolytes. Weak acids and weak bases belong to this class.

The weak electrolytes dissociate only partially in dilute aqueous solutions. An equilibrium thus can be established between the ions and nonionized molecules. The ionization reaction therein is represented as double arrow between the ions and nonionized molecule.

Use Your Brain Power

Which of the following is a strong electrolyte?

HF, AgCl, CuSO4, CH3COONH4, H3PO4.

3.2.3 Degree Of Dissociation

The degree of dissociation of an electrolyte is defined as a fraction of total number of moles of the electrolyte that dissociates into its ions when the equilibrium is attained. It is denoted by symbol and given by

\(\alpha = \frac{\text{number of moles dissociated}}{\text{total number of moles}}\) (3.1)

Percent dissociation = \(\alpha \times 100\) (3.2)

If 'c' is the molar concentration of an electrolyte the equilibrium concentration of cation or anion is \((\alpha \times c)\) mol dm-3.

3.3 Acids And Bases

Acids and bases are familiar chemical compounds. Acetic acid is found in vinegar, citric acid in lemons, magnesium hydroxide in antacids, ammonia in household cleaning products. The tartaric acid is present in tamarind paste. These are some acids and bases we come across in everyday life.

Teacher's Note

Acids and bases are all around us in our daily life. For example, lemon juice has citric acid, and baking soda is a base we use in cooking.

Exam Trick

Remember: Strong acids like HCl completely break into ions. Weak acids like acetic acid only partly break into ions. Think of it like strong and weak students—strong students always give their best effort (complete), weak students sometimes don't (partial).

Points To Remember

Electrolytes are substances that give ions when dissolved in water.
Strong electrolytes break completely into ions. Weak electrolytes break only partially into ions.
Degree of dissociation is the fraction of molecules that break into ions.
Acids give H ions. Bases give OH ions.

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