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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Postulates of Arrhenius Theory

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

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 ionisation. 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

where K is called the Dissociation constant.

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-

CuSO -- -- - > Cu2+ + SO 2-

H2SO-- -- - > 2H+ + SO 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.


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 (ionised) 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+ + 3SO 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.

(e.g.,) CH3COOH < -- --- > CH3COO-  + H+. Acetic acid is a weak

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


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