PHYSICAL CHEMISTRY OF MILK

Lesson 16. IONIC EQUILIBIRIA DISSOCIATION OF IONIC PRODUCT OF WATER

Module 7. Electrical conductivity

Lesson 16
IONIC EQUILIBRIUM DISSOCIATION OF IONIC PRODUCT OF WATER

16.1 Introduction

Chemical reactions mostly take place in solutions. Solution chemistry plays a very significant role in chemistry. All chemical substances are made up of either polar units (called ions) or non-polar units. The activity of these entities is more evident and pronounced in solution. The behavior of these substances depends upon their nature and conditions of the medium in which they are added. It is therefore necessary to understand the principles that govern their behavior in solution.

16.2 Ionic Equilibria

This type of equilibrium is observed in substances that undergo ionization easily, or in polar substances in which ionization can be induced. Ionic and polar substances are more easily soluble in polar solvents because of the ease of ionization taking place in the solvent medium. With the dissolution of ionic and polar substances in the solvent, these solutions become rich in mobile charge carriers (ions) and thus can conduct electricity. Substances, which are capable of conducting electricity, are called as electrolytes while those substances which are non-conducting are called as non-electrolytes.

In a water molecule the electrons are shared unevenly between the oxygen & hydrogen making it a polar molecule. Because of this water molecules have a partial negative charge on oxygen, and a partial positive charge on hydrogen. Accordingly the water molecules can form strong electrostatic attractions with other water molecules, polar molecules and ions

16.3 Dissociation of Water

Water is frequently regarded as a substance that practically does not dissociate into ions. It is observed that it will dissociate to a small extent according to the following equation.

The hydrogen ion in water always forms hydroxonium H₃O⁺. For the sake of simplicity, we consider only the hydrogen ion H⁺ since this will not influence the results.

In solutions containing acids, the hydrogen ions formed will affect the equilibrium in the above equation. A similar effect could also be seen in solutions containing bases which form the hydroxyl ions (OH^-). The equilibrium is determined by the dissociation constant which is expressed in the following equations.

In the water dissociation reaction equilibrium which greatly shifts the direction of undissociated water but it is readily established and this imparts considerable importance to the role played by the reaction of water dissociation and also in many properties of aqueous solutions. Dissociation of water being very slight is considered that either the activity of undissociated water molecule or its concentration is regarded as constant. We can now combine the dissociation constant and rearrange the above two formulae as follows

Where the constant Kw = Kd.a × aH₂O or

Kw = Kd. CH₂O

This is called the ionic product of water

Table 16.1 Ionic product kw , concentration C_H+ and C_OH- of ions in water and pH of water at various temperatures

At 25°C the ionic product of water kw = 1.008 x 10-14

Since pure water as in any other neutral medium aH⁺ = aOH^- ( or C_H⁺ = C_OH^-) for 25°C

aH⁺ = aOH^- = √ Kw = 1.004 x 10^-7 g ions per litre and hence the degree of dissociation

Where 55.5 =CH₂O i.e the number of moles of water per litre at 25^oC

In non neutral media the aH⁺ and aOH^- are not equal to each other. As it could be observed from the above equations these two ions are intimately connected and are inversely proportional to each other. Thus upon adding acid to water we increase the concentration of hydrogen ions and therefore augment the value of aH⁺ but this accelerates the opposite side of the reaction , some of the added H⁺ ions bind an equal amount of OH^– to form H₂O and reduction in acid aOH- ions occur. Equilibrium is established again when the product of ion activities once more acquires the value it had prior to addition of the acid. Hence, any increase in hydrogen ion concentration causes a corresponding decrease in the hydroxyl ion concentration and vice versa.