Ionic Equilibrium

IONIC EQUILIBRIUM

Peter Joseph William Debye

Peter Joseph William Debye was Dutch-American physicist greatly contributed to the theory of electrolyte solutions. He also studied the dipole moments of molecules, Debye won the Nobel Prize in Chemistry (1936) for his contributions to the determination of molecular structure through his investigations on dipole moments and X-rays diffraction.

Learning Objectives

After studying this unit, the students will be able to

• classify the substances into acids and bases based on Arrhenius, Lowry – Bronsted and Lewis concepts.

• define pH scale and establish relationship between pH and pOH

• describe the equilibrium involved in the ionisation of water.

• explain Ostwald’s dilution Law and derive a relationship between the dissociation constant and degree of dissociation of a weak electrolyte.

• recognise the concept of common ion effect and explain buffer action.

• apply Henderson equation for the preparation of buffer solution

• calculate solubility product and understand the relation between solubility and solubility product.

• solve numerical problems involving ionic equilibria.

INTRODUCTION

We have already learnt the chemical equilibrium in XI standard. In this unit, we discuss the ionic equilibria, specifically acid – base equilibria. Some of the important processes in our body involve aqueous equilibria. For example, the carbonic acid – bicarbonate buffer in the blood.

H₃O⁺ (aq) + HCO₃^- (aq) ↔ H₂ CO₃ (aq) + H₂O(l)

We have come across many chemical compounds in our daily life among them acids and bases are the most common. For example, milk contains lactic acid, vinegar acetic acid, tea tannic acid and antacid tablet aluminium hydroxide / magnesium hydroxide. Acids and bases have many important industrial applications. For example, sulphuric acid is used in fertilizer industry and sodium hydroxide in soap industry etc… Hence, it is important to understand the properties of acids and bases.

In this unit we shall learn the definitions of acids and bases and study, their ionisation in aqueous solution. We learn the pH scale and also apply the principles of chemical equilibrium to determine the concentration of the species furnished in aqueous solution by acids and bases.