Electrolysis - Part II

2. Conductor versus electrolyte
We have seen what are conductors and insulators, in the chapter on Electricity. Electrolyte, as studied in the last section, is a solution that can conduct electricity due to ions present in it.  Now the question you might ask is what is the difference between a conductor and an electrolyte?  

The figure below shows two circuits : one for a conductor and one for an electrolyte. The differences between the two are as follows:

• In a conductor, electrical current is flowing through the material in its solid form. Conductors are generally metals. On the other hand, an electrolyte can carry current in its molten state or in a solution form. Electrolytes are generally made from ionic compounds and not covalent compounds (HCl and NH₃ are a few of the exceptions).  
  When a current passes through a conductor, it may get heated up because of its inherent resistance. Other than this physical change, there is no other change in the conductor. Once, the current stops flowing, the conductor returns to its original state. On the other hand, in an electrolyte, the cations and anions go in opposite directions and get neutralized at the positive and negative electrodes respectively. The chemical change thus produced is irreversible. After the current stops, the electrolyte may change in strength than what it had started with.
  

• The current through a conductor is due to free electrons that flow and complete the circuit. The current through an electrolytic cell is because of the flow of ions.
  

• For a given conductor and a cell, the current flowing through the circuit is constant. In an electrolytic cell, the current depends on the strength of the electrolyte, which may change over time.

3. Electrolysis of water
The apparatus or the electrolytic cell, required for performing electrolysis of water is shown in the figure below. The cell is called Hoffman’s voltameter.

Since water is a covalent compound, pure or distilled water is a non-electrolyte. A few drops of ionic compound like dilute sulphuric acid are enough to make the water become an electrolyte. The Hoffman voltameter consists of platinum electrodes. The anode and cathode are connected to a battery. The cell produces a small current of the order of a few milliamps and you will see bubbles appearing in the two arms of the voltameter. The anode collects oxygen and the cathode arm collects hydrogen gas.

The overall reaction that takes place is :

H₂SO₄                         2H⁺      +   SO₄^--   (dissociation, reversible reaction)

In the presence of these ions, water also becomes capable of dissociation.

H₂O                              H⁺      +    OH^-    (dissociation, reversible reaction)

Reaction at the cathode   

H⁺         +   e^-             H ,                           H   + H              H₂

Reaction at the anode

OH^-                    OH       +   e^-

 4OH                  H₂O  +   O₂ 

At the anode OH^-   ions are released in preference to SO₄^--   ions. This is because it is easier for an OH^-   ion to give up an electron quickly than for the SO₄^--   ion to do so.  Since the sulphuric acid itself does not participate in the chemical reaction that is taking place, it can be called as a catalyst of the reaction. 

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