Short note on Ostwald Dilution Law.

OSTWALD’S DILUTION LAW:-

 According to the Arrhenius Theory of dissociation, an electrolyte dissociates into ions in water solutions. These ions are in a state of equilibrium with the undissociated molecules. This equilibrium is called the Ionic equilibrium. Ostwald noted that the Law of Mass Action can be applied to the ionic equilibrium as in the case of chemical equilibria.

 Let us consider a binary electrolyte AB which dissociates in solution to form the ions A+ and B–.

                                                  AB     --->    A+   +   B–

Let C moles per litre be the concentration of the electrolyte and α (alpha) its degree of dissociation. The concentration terms at equilibrium may be written as : [AB] = C (1 – α) mol litre^-1 [A+]= C α mol litre^-1

                        [B–] = C α mol litre^-1

Applying the Law of Mass Action :

Rate of dissociation = k1 × C (1 – α)

 Rate of combination = k2 × C α × C

Ionic Equilibria Solubility Product

OSTWALD’S DILUTION LAW Experimental Verification of Ostwald’s Law Limitation of Ostwald’s Law THEORY OF STRONG ELECTROLYTES Ghosh’s Formula Debye-Huckel Theory DEGREE OF DISSOCIATION THE COMMON–ION EFFECT FACTORS WHICH INFLUENCE THE DEGREE OF DISSOCIATION (1)   Nature of Solute (2)   Nature of the solvent (3)   Concentration (4)   Temperature SOLUBILITY EQUILIBRIA AND THE SOLUBILITY PRODUCT APPLICATION OF SOLUBILITY PRODUCT PRINCIPLE IN QUALITATIVE ANALYSIS Selective Precipitation Separation of the Basic ions into Groups

At equilibrium :

k1 × C (1 – α)=k2 × C α × C α

or

1 2(1 ) c kCC K Ck α × α == −α

or

2

–1mol litre

(1 )c C

K

α

=

−α

...(1) The equilibrium constant Kc is called the Dissociation constant or Ionization constant. It has a constant value at a constant temperature. If one mole of an electrolyte be dissolved in V litre of the solution, then 1 C V = V is known as the Dilution or the solution. Thus the expression (1) becomes

2 (1 )cK V α = −α

...(2)

This expression which correlates the variation of the degree of dissociation of an electrolyte with dilution, is known as Ostwald’s Dilution Law. For Weak Electrolytes For weak electrolytes, the value of α is very small as compared to 1, so that in most of the calculation we can take 1 – α 1. Thus the Ostwald’s Dilution Law expression becomes 2 cK V α = It implies that the degree of dissociation of a weak electrolyte is proportional to the square root of the dilution i.e., cK Vα∝ or K V ′α= For Strong Electrolytes For strong electrolytes, the value of α is large and it cannot be neglected in comparison with 1. Thus we have to use the original expression (2). That is, 2 (1 )cK V α = −α or 2 cc K V K V α = −α which gives a quadratic equation α2   +   α KcV   –   KcV   =   0 from this equation the value of α can be evaluated. Experimental Verification of Ostwald’s Law The Ostwald’s Dilution law can be verified if the values of α, the degree of dissociation, at different dilutions are known. The values of α are determined experimentally by using the relation.

ν ∞ λ

α=

λ where λν and  λ∞ are the equivalent conductances at dilution V and infinite dilution respectively. Their values are found by conductance measurements and Kohlrausch’s law. The value of  α at various dilutions thus determined are inserted in the expression :