Ionic Product of Water for IIT JEE

Water is the very essential molecule for the normal functioning of the body in all living organisms. Water in a pure form is made up of two molecules of hydrogen and one molecule of oxygen, which in the molecular formula is denoted as H2O. Pure water possesses a very weak electrolyte property which can easily undergo the Self ionization process and spit into two molecular ion parts.

1. Hydrogen ion (H+)

2. Hydroxide ion (OH-)

The product of concentrated H+ and OH- ions in the water at a certain temperature is termed as an ionic product of water. Ions vary with the temperature. 

The process where water undergoes self-ionization is also known as Autoprotolysis. The equation of the autoprotolysis (self-ionization) is shown as:

H2O OH- + H+

The hydrogen ion formed from the ionization of pure water is highly reactive in nature which further reacts with the other molecules of pure water and forms hydronium ion (H3O+)

H2O + H+ H3O+

The Law of mass action for the self-ionization reaction of pure water at chemical equilibrium, the value of the dissociation constant K is the relationship between the concentration of reactants and product, for this ionization of pure water reaction; equation of dissociation constant K written as:

\[K=[H+][OH-]/[H_{2}O]\]

Or   

\[[H^{+}][oH^{-}]=k[H_{2}o]\]

\[[H^{+}][oH^{-}]=kw\]

Here, the constant (Kw), is termed as an ionic product of water.

During the self-ionization process, the pure water can dissociate the water molecule to form one hydroxyl OH- ion (which is alkaline in nature) and one hydroxide H+ ion (which is acidic in nature), hence the autoprotolysis of pure water can act both as an acid as well as a base. 

Water can be converted to the acidic state by such components/ substances which increase the concentration of Hydrogen (H+) ions or decrease the concentration of Hydroxide ion (OH-) ions. In the same way, water can also be converted to the basis by such components/ substances which increase the concentration of Hydroxide ion (OH-) or decrease the concentration of Hydrogen (H+) ions.

In pure water, the concentration of the hydrogen H+ ion (hydronium H₃O+ ion) is always equivalent to the concentration of the hydroxide OH- ion. In simple terminology, it is also stated that the generation of every single hydrogen H+ ion (hydronium H3O+ ion) equivalent ratio of hydroxide OH- ion is also generated as well. This statement also states reversely as the number of generated hydroxide OH- ions is equal with the number of generated hydrogen H+ions (hydronium H3O+ ions).

Effect of Temperature on KW

Concentrations of H+ and OH- ions in the pure water of a product at a specific temperature is normally known as an ionic product of water. The value of Kw (ionic product of water) rises with the rise of temperature, i.e., the concentration of H+ and OH- ions rises with temperature rises. Some constant values of KW at different temperatures are shown in below table:

At 25°C temperature, the value of KW of pure water is 1 x 10-14. Although pure water is neutral in nature, hence the concentration of the H+ ion and OH- ion must be equal.

So, [H+] = OH˜

 = X

Put the value of [H+] and OH˜

into the KW equation

\[[H^{+}][oH^{-}]\]=KW= 1 x 10-14(at 25°C temperature)

\[[H^{+}][oH^{-}]\]=X2= 1 x 10-14

Hence, X = 1 x 10-7 M

So from the equation value of

\[[OH^{-}]=[H^{+}]\]= X = 1 x 10-7 M

which is also written as\[[OH^{-}]=[H^{+}]\]= 1 × 10-7 mol litre-1

This interprets that at 25°C temperature; in 1 litre solution of pure water only 10-7 moles of water is present in the ionic form out of total moles of pure water (i.e. approximately 55.5 moles).

When the addition of an acid or the base happens in the pure water, then the concentration of the ionic product concentration of the pure water can be calculated by using the ionic product constant equation \[[H^{+}][oH^{-}]=kw\], where the value of KW is constant at specific temperature but the concentration of the H+ and OH- ions do not remain the same. 

The addition of the acid leads to the rise in the concentration of the hydrogen H+ ion or hydronium H3O+ ion or in other terms we can say that it leads to a decrease in the concentration of the hydroxyl OH- ion.

i.e. \[[H^{+}] or [H_{3}O^{+}] > [OH^{-}]\]; (Acidic solution)

Similarly, the addition of the base leads to the rise in the concentration of the hydroxyl OH- ion or in other terminology it decreases the concentration of the hydrogen H+ion or hydronium H3O+ ion

i.e. \[[OH^{-}] > [H^{+}]\]; (Alkaline or basic solution)

The Concentration of Ions in Neutral Acidic and Basic Condition

In a Neutral solution, the concentration of the H+ ion and OH- ion are equivalent to each other hence the value of the hydrogen ion and hydroxyl ion are the same.  

\[[H^{+}] = [OH^{-}]\] = 1 x 10-7 M

But in an Acidic solution, the concentration of H+ ion is higher than the concentration of the OH- ion, hence the value of the H+ ion is always greater than the value of OH- ion. This shows that the concentration of H+ ion is always higher than the 1 x 10-7 M and similarly the concentration of OH- ion is always lesser than the 1 x 10-7 M as shown below:

\[[H^{+}] > [OH^{-}]\]

Where, \[[H^{+}]\] > 1 x 10-7 M

Or, \[[OH^{-}]\] < 1 x 10-7 M

In Alkaline (basic) solution, the concentration of OH- ion is higher than the concentration of the H+ ion, hence the value of the OH- ion is always greater than the value of H+ ion. That shows that the concentration of OH- ion is always higher than the 1 x 10-7 M and similarly the concentration of H+ ion is always lesser than the 1 x 10-7 M as shown below:     

\[[OH^{-}] > [H^{+}]\]

Where, \[[OH^{-}]\]> 1 × 10-7 M

Or, \[[H^{+}]\] < 1 x 10-7 M

Hence, the value shows that the solution of pure water must be acidic in nature if the concentration of the hydrogen H+ ion is higher than 1 x 10-7 M and alkaline or basic in nature if the concentration of the H+ ion is lesser than 1 x 10-7 M.

The table shows the type of nature exhibited by water due to the amount of H+ ion concentration it consumes:

Similarly, the value shows that the solution of pure water must be basic in nature if the concentration of the hydroxyl OH- ion is higher than 1 x 10-7 M and alkaline or basic in nature if the concentration of the OH- ion is lesser than 1 x 10-7 M.

The table shows the type of nature exhibited by water due to the amount of OH- ion concentration it consumes:

It is concluded that each aqueous solution whether it is acidic, basic or neutral possesses both hydrogen H+ ion and hydroxyl OH- ion that are present in the same or different proportions. But the ionic product concentration KW of the water always remains constant 1 × 10-14 at a temperature of 25°C. Hence, when the concentration of one ion increases, accordingly the concentration of the other ion is decreased but the overall concentration of the product remains constant.

Importance of the Ionic Product of Water  

The ionic product of water is very important in knowing the balance of Acid and Base and in calculating the disorder of acid-base.

Human plasma is a solution that is Aqueous in nature and has to run by chemical rules known as electrical neutrality and ionic product constancy of water. 

In water, some molecules form hydronium ion (H3O+) which is also known as hydronium, with Hydroxide ion (OH-) by accepting a second water molecule. When this happens, both the ions react together to make water once again. 

The electroneutrality principle clearly states that plasma should be neutral electrically and the total sum of anions must match the sum of cations. The ionic product of water should be constant. Hence, the concentration of plasma of hydrogen ions totally depends on the concentration of ionic plasma. 

When Plasma anions become out of proportion of cations, their accumulation induces electrical imbalance with a fall of hydroxide ions that causes a rise in hydrogen ions. To protect the plasma pH from serious deviations, the concentration of plasma bicarbonate should be adjusted to these changes. 

Characteristics of Ions After Dissolved in Water

Ionic compounds when dissolved in water, break apart through a process into the ions. When in water, the ions attract to the molecules of water, a polar charge within each of them. If the force between water molecules and the ions gets strong enough and breaks the bond between these ions, makes the compound dissolve. The ions separate in the solution, each surrounded by the water molecules to prevent them from reattaching. The ionic solution then becomes an electrolyte, which means that it can conduct electricity. 

Covalent compounds

Covalent compounds when dissolved in water, break into molecules, not individual atoms. Water as we know is a polar solvent but covalent compounds are mostly nonpolar. Covalent compounds don't always dissolve in water, making a different layer on the surface of the water.