Electrochemistry

You will learn in this chapter about:

*        Electrolytes and non-electrolytes.
*        Electrolysis.
*        Electrolysis of molten NaCl.
*        Electrolysis of water.
*        Faraday's laws of electrolysis.
*        Electrochemical equivalent.
*        Uses of electrolysis.
*        Electroplating.
*        Electrochemical cells.
*        Batteries.

INTRODUCTION

Chemical energy is associated with chemical reactions and the electrical energy is obtained from electricity. Thus chemical and electrical energies are the two forms of energy which are inter convertible.

The branch of chemistry that deals with the relationship between electricity and chemical reactions i.e. with the conversion of electrical energy into chemical energy and chemical energy into electrical energy is defined as electrochemistry.

8.1 ELECTROLYTES AND NON-ELECTROLYTES

The chemical compound which conducts electricity in molten condition or through its aqueous solution with chemical change is called an electrolyte.

All acids, bases and salts are electrolytes (in aqueous solutions or fused state); Such as Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Sodium hydroxide (NaOH), Sodium chloride (NaCl), Copper sulphate (CuSO4) etc, are electrolytes

You cannot electrolyse something unless it conducts electricity. But all the conductors are not electrolytes you can pass electric current through a metal for years but it will not decompose. Metals are good conductors but are not electrolytes. Metals are used as electrodes through which electric current enters and leaves the electrolytes.

All electrolytes are ionic compounds or polar compounds like acids and bases. Salts are ionic compounds and are solids. These solids do not conduct "electricity because in the solid state the ions are very tightly packed or held together showing no movement of the ions. They cannot move. However an ionic solid is melted or dissolved in water, its ions become free to move. Thus an ionic compound in molten states or in aqueous solution, conduct electricity. This conduction is due to the free movement of ions.

Non - Electrolytes:

The chemical compounds which do not conduct electricity in, molten or in aqueous solutions are called non-electrolytes; such as Sugar, Petrol, Benzene etc.

 

8.2 ELECTROLYSIS OR ELECTROLYTIC CONDUCTION

A chemical process or reaction which does not have the capability to take place spontaneously can be made to take place by passing electric current. If appropriate quantity of electric current is passed under suitable condition through an electrolytic solution, the reactions occurs non-spontaneously i.e. under stress.

The cell used for the reaction to occur by passing electric current is called electrolytic cell. Thus electrolytic cell is a device which is used to convert electrical energy into chemical energy (a non-spontaneous chemical reaction takes place by the loss and gain of electrons) for example a device for electroplating or electrolysis of water in the presence of an acid. The process is called electrolysis or electrolytic conduction.

Electrolysis may be defined as a process in which movements of the ions take place towards their respective electrodes to undergo chemical changes under the influence of an applied electric field.

Electrolysis of Molten Sodium Chloride:

Sodium chloride, (Salt) does not conduct electricity in the solid state. To make it conduct electricity, either fuse or melt the salt or dissolve it in water. So there are two ways of doing electrolysis of sodium chloride. We are doing electrolysis of molten sodium chloride. Sodium chloride melts at 800C. It is easy to predict the result of electrolysing a molten electrolyte. The salt just splits into two parts, sodium (metal) positive ion (Na+) and chloride (non-metal) negative ion (Cl-). We can write the ionization reaction as:

NaCl(s) fused Na+ (l) + Cl- (l)

In solid sodium chloride, the ions are held together tightly in a regular lattice arrangements. In solid the ions cannot move, but when sodium chloride is melted, the ions are freed from their lattice and they can move freely to conduct electricity.

Some fused NaCl is taken in an electrolytic cell i.e.; in a glass vessel. Two platinum rods (electrodes) are dipped into the fused salt (NaCl). The two electrodes are connected to a source of electricity i.e. to a battery outside the cell by wires. The electrode connected to the negative terminal of the battery is called cathode and the electrode connected to the positive terminal of the battery is called anode.

When an electrical potential is passed through the molten sodium chloride salt, electrolysis starts. The positive ions i.e. cations (Na+) are attracted towards cathode and the negative ions i.e. anions (Cl-) are attracted towards anode. At the two electrodes chemical reactions take place.

At Cathode:

Sodium ions (Na+) which are positively charged. Move towards cathode and gain electrons to get neutralized. Thus Na+ ions are discharged at cathode and form neutral molten sodium metal.

We can represent the happening at cathode as:

Sodium ion + electron  ------à  Sodium metal

Na+ (l) +e-          ------à  Na (l)

At Anode:

Chloride ions (Cl-) which are negatively charged move towards anode; Cl- ions have one electron in excess. The anode has a shortage of electrons and is positively charged. When Cl- ions arrive at anode, the anode attracts their electrons and CI ions lose their extra electrons at anode, forming neutral chlorine (Cl-) atoms but we know Cl- atoms do not exist in the free state. They combine to form Cl2 molecules. So Cl- ions are discharged at anode to liberate Cl2 gas by the loss of electrons at anode.

Chloride ion     ------à    Chlorine atom + e-

Or                Cl-(l)               ------à    Cl (g) +e

Then             Cl (g) + Cl (g) ------à  Cl2 (g)

Over all it amounts to

2Cl-(g)            ------à  Cl2 (g) +2e-

The overall reaction can be described as:

2(Na+ (l) + e           ------à         Na (l))          Cathode reaction

2Cl- (g)                   ------à         Cl2 (g) +2e-       Anode reaction

On adding the two reactions, the loss and gain of electrons cancel and we get final result.

2Na+ (l) +2e-                 ------à         2Na (l)

2Cl-(l)                     ------à         Cl2 (g) +2e-

2Na+ (l) + 2C1- (l)    ------à         2Na (l) + Cl2 (g)

The electrolysis of molten sodium chloride shows that we get sodium metal at cathode and Cl2 gas is liberated at anode. The following fig 8.1 describes the electrolysis of molten NaCl.

2. Electrolysis of Water:

Pure water on its own hardly conducts electricity because it ionizes feebly into ions. But when few drops of acid or base or very small quantity of a salt is added in water, it makes water to conduct electricity and then water can be electrolysed to produce hydrogen gas at cathode and oxygen gas at anode. Consider an electrolytic cell containing acidulated water. Two electrodes are dipped in the acidulated water. Water in the presence of few drops of acid ionizes as:

2H2O (l)     acid     H3O+ (aq)      +     OH-(aq)

                                                   Hydronium ion          hydroxide ion

When electricity is passed from the battery, the positive ions (H30+) move towards cathode. They gain electrons and get neutralized to liberate H2 gas.

At Cathode:

2H3O+ (aq) + 2e- ----- > (g) + 2H2O (1)

Hydroxide ion (OH-) moves towards anode. OH- ions lose electrons at anode and get neutralized to liberate O2 gases.

At Anode:

4OH- (aq) ----- > O2 (g) + 2H2O(1) + 4e-

The over all reaction should be balanced according to gain and loss of ' '     electrons at the two electrodes so the cathode reaction is multiplied by 2 and then added in the anode reaction.

 

2(2H3O+ + 2e- ----- > H2 (g)   + 2H2O(1))      Cathode reaction

Or       4H3O+ 4e- ----- > 2H2 (g) + 4H2O(1)

40H-    ----- > 02(g)   + 2H20(1) + 4e-Anode reaction

 

4H3O+ + 4OH- ----- > 2H2 (g) + O2 (g) + 6H2O (1)

4H3O+ and 40H- together will be equivalent 8H2O, so

8H2O ----- > 2H2 (g) + O2 (g) + 6H2O

 

Or       8H2O - 6H2O ----- > 2H2 (g) + O2 (g)

Or       2H2O ----- >  2H2 (g) + O2 (g)

For simplicity of the electrolysis of water, we write the equation as:

 

2H2O (1) Electric current>   2H2 (g)  +   O2 (g)

                                                    Cathode product   anode product

 

It is observed that on electrolysis of water, we get two volumes of hydrogen gas for each volume of oxygen gas. Humphry Davy, who first did the electrolysis of water, confirmed the formula of water is H2O (2:1 ratio of Hydrogen and Oxygen).

 
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