Theory of electrolytic dissociation

Theory of electrolytic conduction. Arrhenius theory of electrolytic conductance is also known as Arrhenius theory of ionization since electrolytic dissociation into ions is considered here.

Postulates of Arrhenius theory In aqueous solution, the molecules of an electrolyte undergo spontaneous dissociation to form positive and negative ions.

The assumption made was that when an acid, base or salt is dissolved in water, a considerable portion becomes spontaneously dissociated into positive and negative ions. The Arrhenius assumption was based on degree of ionization, so it valid on weak electrolytes (incomplete ionization) but it failure with respect to strong electrolytes (complete ionization in either strong and weak electrolytes).

The decreasing of equivalent conductance at high concentration as Arrhenius suggest belong to decreasing of degree of ionization of electrolyte. The Arrhenius interpretation concentrate about number of ions and he ignore the mobility of ions. The Arrhenius suggestion are only apply on weak electrolytes because the strong electrolytes are completely dissociate at high concentration [27, p. 1].

Postulates of Arrhenius Theory:

1. When dissolved in water, neutral electrolyte molecules are split up into two types of charged particles.

These particles were called ions and the process was termed ionization. The positively charged particles were called cations and those having negative charge were called anions.

The theory assumes that the ions are already present in the solid electrolyte and these are held together by electrostatic force. When placed in water, these neutral molecules dissociate to form separate anions and cations.

A⁺ B^-→ A⁺ + B^-

For that reason, this theory may be referred to as the theory of electrolytic dissociations.

2. The ions present in solution constantly reunite to form neutral molecules. Thus there is a state of equilibrium between the undissociated molecules and the ions.

AB ↔ A⁺ + B^-

Applying the Law of Mass Action to the ionic equilibrium we have,

[ A ⁺ ][ B ^- ] / [AB] = K (1)

where K is called the Dissociation constant (1).

3. The charged ions are free to move through the solution to the oppositely charged electrode. This is called as migration of ions. This movement of the ions constitutes the electric current through electrolytes. This explains the conductivity of electrolytes as well as the phenomenon of electrolysis.

4. The electrical conductivity of an electrolyte solution depends on the number of ions present in solution. Thus the degree of dissociation of an electrolyte determines whether it is a strong electrolyte or a weak electrolyte.

We know that electrolytes dissociate in solution to form positive ions (cations) and negative ions (anions).

AgNO₃→ Ag⁺+ NO₃^-

CuSO₄ → Cu²⁺ + SO₄^2-

H₂SO₄ → 2H⁺ + SO₄^2-

As the current is passed between the electrode of the electrolytic cell, the ions migrate to the opposite electrodes. Thus in the electrolytic solution of AgNO₃, the cations (Ag⁺) will move to the cathode and anions (NO₃^- ) will move to the anode. Usually different ions move with different rates. The migration of ions through the electrolytic solution can be demonstrated by the following experiments (figure 9.1).

9.1.png

Figure 9.1 Nitration of ions through electrolytic solution to opposite electrodes

5. The properties of solution of electrolytes are the properties of ions. The solution of electrolyte as a whole is electrically neutral unless an electric field is applied to the electrodes dipped into it. Presence of hydrogen ions (H⁺) renders the solution acidic while presence of hydroxide ions (OH^-) renders the solution basic.

6. There are two types of electrolytes. Strong electrolytes are those when dissolved in water are completely dissociated (ionized) into ions of positive and negative charges. The total number of cations and anions produced are equal to those in the formula of the electrolyte.

Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄^2-

NaCl, KCl, AgNO₃ etc., are few examples of strong electrolytes.

In the case of weak electrolytes, there is partial dissociation into ions in water and an equilibrium exists between the dissociated ions and the undissociated electrolyte.

CH₃COOH ↔ CH₃COO^- + H⁺ Acetic acid is a weak

electrolyte in water and unionized acetic acid molecules are in equilibrium with the acetate anions and H⁺ions in solution.

Evidences of Arrhenius theory of electrolytic dissociation

1. The enthalpy of neutralization of strong acid by strong base is a constant value and is equal to -57.32 kJ. gm. equiv ^-1. This aspect is well explained by adopting Arrhenius theory of electrolytic dissociation. Strong acids and strong bases are completely ionized in water and produce H⁺ and OH^- ions respectively along with the counter ions. The net reaction in the acid-base neutralization is the formation of water from H⁺ and OH^-ions.

H⁺ + OH^-→ H₂O, DH_r^o = -57.32 kJ.mol ^-1

2. The colour of certain salts or their solution is due to the ions present. For example, copper sulphate is blue due to Cu²⁺ ions. Nickel salts are green due to Ni²⁺ ions. Metallic chromates are yellow due to CrO₄^2- ions.

3. Ostwalds dilution law, common ion effect and solubility product and other such concepts are based on Arrhenius theory.

4. Chemical reactions between electrolytes are almost ionic reactions. This is because these are essentially the reaction between oppositely charged ions. For example,

Ag⁺ + Cl^- → AgCl↓

5. Electrolytic solutions conduct current due to the presence of ions which migrate in the presence of electric field.

6. Colligative properties depend on the number of particles present in the solution. Electrolytic solution has abnormal colligative properties. For example, 0.1 molal solution of NaCl has elevation of boiling point about twice that of 0.1 molal solution of non-electrolyte. The abnormal colligative properties of electrolytic solutions can be explained with theory of electrolytic dissociation.

Ostwald's dilution law for weak electrolytes. According to Arrhenius theory, weak electrolytes partially dissociate into ions in water which are in equilibrium with the undissociated electrolyte molecules. Ostwald's dilution law relates the dissociation constant of the weak electrolyte with the degree of dissociation and the concentration of the weak electrolyte. Consider the dissociation equilibrium of CH₃COOH which is a weak electrolyte in water.

CH₃COOH ↔ CH₃COO^- + H⁺

K_a = [ H ⁺ ][CH ₃COO ^- ] / [CH₃COOH]

a is the degree of dissociation which represents the fraction of total concentration of CH₃COOH that exists in the completely ionized state. Hence (1 - a) is the fraction of the total concentration of CH₃COOH, that exists in the unionized state. If 'C' is the total concentration of CH ₃COOH initially, then at equilibrium Ca, Ca and C (1 - a) represent the concentration of H⁺, CH₃COO^- and CH₃COOH respectively.

Then K_a = (Ca .C a) / C (1-a) / a² C / (1-a)

If a is too small, then Ka = a²C

a = root(Ka/C) also [H⁺] = [CH₃COO^-] = Ca

[H⁺] = root (Ka.C)

K_a= a²C / (1-a) is known as the Ostwalds dilution law. For weak bases,

K_b= a²C / (1-a) and a = rt (K_b/C) at a = small values.

K_b = dissociation constant for weak base.

This law fails for strong electrolytes. For strong electrolytes, a tends to 1.0 and therefore K_a increases tremendously [26, p. 1].

Strong and Weak Electrolytes. Solutes giving conducting solution in a suitable solvent are called electrolytes. On the basis of degree of ionization, these electrolytes have been divided into two categories.

a) Strong electrolytes

b) Weak electrolytes

Strong Electrolytes. Substances, which are highly dissociated and give solutions with high conductance in water, are called strong electrolytes. Due to the high degree of dissociation of strong electrolytes these substances are good conductor of electricity i.e., aqueous solutions of these substances have high value of molar conductance and on dilution the increase in their molar conductance is very small. This is due to the fact that such electrolytes are completely ionized at all dilutions therefore on further dilution the number of current carrying particles does not increase in the solution. Thus, solutions of electrolytes that have high molar conductance, and increases very slowly on dilution has a high degree of dissociation is called strong electrolyte.

During the passage of an electric current through solutions, flow of electricity is associated with the movement of particles, which are called ions. The ions carrying positive charges and moving in the direction of the current, i.e., towards the cathode, are referred to as cations and those carrying a negative charge and moving in the opposite direction, i.e., towards the anode, are called anions [28, p. 2].

Weak Electrolytes. Weak acids and weak bases, e.g., amines, phenols, most carboxylic acids and some inorganic acids and bases, such as hydrocyanic acid and ammonia, and a few salts, e.g., mercuric chloride and cyanide, are dissociated only to a small extent at reasonable concentration; this group of compounds in general are called as weak electrolytes.

The molar conductance of the solutions of these electrolytes increases rapidly on dilution. The reason of this is that more molecules ionize on dilution inspite of this they are never completely ionized. For these electrolytes, the nature of the solvent is also important; a particular compound may be strong electrolyte, being dissociated to large extent, in one solvent, but may behave as weak electrolyte in other solvent due to low degree of dissociation [28, p. 3].