Electrochemical Changes

ELECTROCHEMICAL CHANGES The chemical changes that involve the flow of electric current are called electrochemical changes. There are two types of electrochemical changes.

Electrolytic Cells: These cells constitute the electrochemical reactions in which electrical energy is converted into chemical energy. In these cells electric energy is used to drive redox reactions, which are nonspontaneous and have positive free energy (∆G) Electrolysis: It is a phenomenon in which chemical changes take place by the passage of electric current from the external source. The cells, which are used to carry out electrolysis, are called electrolytic cells. Electrolysis is used to extract many metals like Na, K, Ca, Sr, Mg, and Al. Example of electrolysis: When electric current is passed through molten sodium chloride it will dissociate into sodium and chloride ions. Sodium is produced at cathode and chlorine is liberated at anode.

Process of Electrolysis: This process is carried out by taking a solution of electrolyte in a suitable vessel that should be a bad conductor of electricity e.g. glass or any other non- reactive material. Two metallic rods are suspended in the electrolytic solution. These plates are connected to the terminals of a battery with the help of connecting wires. These plates or rods are called electrodes and allow the passage of current. The electrode, which is connected to the positive terminal of the battery, is called anode while that connected to the negative terminal is called cathode.

When electrolyte is dissolved in water it splits up into positive ion (cation) and negative ion (anion). When electric current is allowed to pass through the solution then oppositely charged electrodes attract the ions. As a result cation move towards cathode and anion moves towards anode. This movement of ions in the solution is called ionic conduction and constitutes flow of current through the solution. When anions reach at cathode they give up their electrons and these electrons flow out of the solution while cations gain the electrons from the cathode and get converted into neutral particles. This is known as a primary charge. The primary products may be collected as such or they undergo further changes to form molecules or compounds. These are called secondary products and change is called as secondary change. Example:

Mechanism of Electrolysis: This process was explained by the theory of ionization. According to this theory electrolytes are present as ions in solution and the function of electricity is only to direct these ions to their respective electrodes. This process is possible only when the substance (electrolyte) is in molten or dissolved state.

Examples: 1) Electrolysis of molten sodium chloride: Molten sodium chloride has Na+ and Cl-.

During the process of electrolysis, sodium is deposited at the cathode and chlorine is liberated at anode. On passing electric current through molten sodium chloride, the dissociated ions i.e. chloride ions are attracted towards anode here they give up one electron each to the anode and become chlorine atoms. The chlorine atoms are unstable and combine in pairs to form chlorine molecules. Similarly sodium ions move towards the cathode, take up one electron each and become neutral atoms. As a result, sodium metal is obtained at the cathode.

The reaction is expressed as:

2) Electrolysis of aqueous sodium chloride:

In water, sodium chloride ionizes as

Water also dissociates into ions, though to a very slight degree.

Thus the aqueous solution of sodium chloride contains Na+, H+,OH- and Cl- ions. When electric current is passed through it Cl2 gas is evolved at anode and H2 gas is evolved at cathode. Now the resulting solution contains only Na+ and OH- ions. At cathode both Na+ and H+ ions try to get reduced but the reaction having higher value of reduction potential ( Eo) is preferred. Therefore at cathode the reaction occurring is

So the reaction at cathode is

But H+ (aq) are produced by the dissociation of water as:

Therefore net reaction at cathode can be written as

On the other hand, at anode, the reaction that is possible

The reaction with higher value of Eo is preferred so,

The net reaction during the process of electrolysis can be summarized as

Overall reaction is represented as:

So during electrolysis of aqueous sodium chloride, H2 gas is liberated at cathode and O2 gas is liberated a anode and solution contains sodium hydroxide.

3) Electrolysis of water: We know pure water is a poor conductor of electricity but with the addition of small quantity of acid or alkalies it becomes a good conductor. The electrolytic cell where electrolysis of water takes place consists of two graphite electrodes into it. Connect these electrodes with ammeter and it will not give any deflection. This means water is a poor conductor. However, when a few drops of dilute H2SO4 are added, ammeter shows its deflection and electrolysis starts.

Presence of acid in water increases its degree of ionization.

Water will dissociate as

Since we have added Sulphuric acid in the water for electrolysis so, SO42- ions are also present in the electrolyte. And these are also formed at the anode.

In this case both OH- and SO42- ions move towards anode .But SO2-4 ions will not be released at because of their high discharge potential as compared to OHions. The gases H2 and O2 are collected at anode and cathode respectively. The volume of the gases collected is in the ratio of 2:1 respectively, thus the formula of the compound is H2O.

3) Electrolysis of aqueous Copper Sulphate solution: Copper Sulphate and water ionizes as

During electrolysis copper is deposited at cathode while oxygen is liberated at anode.

It may be noted that in the above example, the electrode used should be of other non reactive metal i.e. Pt (platinum) so that it should not be attacked by the copper ions.

5) Electrolysis of aqueous copper sulfate using copper electrode:

At anode: In this type of electrolysis no ions are produced. On the other hand, anode itself undergoes loss of electron to form Cu2+ ions, which go to solution.

So, the net result of electrolysis is that copper is deposited at the cathode from the solution and an equivalent amount of copper from the anode dissolves in the solution as Cu 2+.

Electrolysis of Sulphuric acid: During electrolysis, Sulphuric acid is dissociated into H+ ions and SO42- ions.

For dilute solutions, reaction (i) is preferred and O2 liberates at anode. But at higher concentration of sulfuric acid, reaction (ii) is preferred and solution of H2S2O8 (per oxo disulphuric acid) is obtained. -------------------------------------------------------------------------------------------