ICSE Class 10 Chemistry Chapter 06 Electrolysis

6 Electrolysis

Syllabus

(i) Electrolytes and non-electrolytes. Definitions and examples.

(ii) Substances containing molecules only, ions only, both molecules and ions. Substances containing molecules only, ions only, both molecules and ions. Examples: relating their composition with their behaviour as electrolyte (strong and weak), non-electrolyte. Definition and explanation of electrolysis, electrolyte, electrode, anode, cathode, anion, cation, oxidation and reduction (on the basis of loss and gain of electrons).

(iii) An elementary study of the migration of ions, with reference to the factors influencing selective discharge of ions, illustrated by the electrolysis of: molten lead bromide; acidified water with platinum electrodes and aqueous copper (II) sulphate with copper electrodes; electron transfer at the electrodes. The above electrolytic processes can be studied in terms of electrolyte used, electrodes used, ionization reaction, anode reaction, cathode reaction, use of selective discharge theory wherever applicable.

(iv) Application of electrolysis: electroplating with nickel and silver; purification of copper; choice of electrolyte for electroplating. Reasons and conditions for electroplating, names of the electrolytes and the electrodes used should be given. Equations for the reactions at the electrodes should be given for electroplating, refining of copper.

(v) Acids, bases and salts as electrolytes: reference should be made to the activity series as indicating the tendency of metals, e.g., Na, Mg, Fe, Cu, to form ions.

6.1 Introduction

This chapter deals with a process called "Electrolysis" which establishes a relationship between electrical energy and chemical change.

The word electrolysis is made up of two words 'electro' meaning flow of electrons or electricity and 'lysis' meaning separating, i.e., bringing about a chemical change in a substance (chemical) change by passage of electricity.

Many substances allow electricity to pass through them and many do it. It has also been found that all the substances allowing passage of electricity do not undergo chemical change.

Most metallic elements like copper, aluminium, iron and all alloys, whether in solid state or in molten state, allow an electric current to pass through them without undergoing any chemical change. Non-metals (except graphite) like sulphur, phosphorus, etc., do not conduct electricity. Therefore, metals and alloys are called conductors and non-metals are called non-conductors (insulators).

The conduction of a current by a conductor is due to the flow of electrons. Insulators do not possess mobile electrons.

Chemical compounds behave differently when an electric current is passed through them. Some compounds conduct electric current in aqueous state or in molten state but not in solid state and are decomposed by the passage of electric current. For example, the passage of electric current through copper chloride solution decomposes it to metallic copper and chlorine. Such chemical compounds which conduct electric current in molten or aqueous state are called electrolytes. Some compounds do not conduct electric current at all whether in solid state or in aqueous state. They are called non-electrolytes, e.g., sugar solution.

Chemical decomposition of a chemical compound can be brought about by using electric current. Such a chemical process is called electrolysis.

Teacher's Note

Electrolysis is used daily in industries to purify metals and in electroplating to coat objects with a thin layer of valuable metals like silver and gold.

6.2 Some Important Terms

(i) Electrolysis

It is the process of decomposition of a chemical compound in aqueous solutions or in molten state accompanied by a chemical change by using direct electric current.

(ii) Electrolytes

Are compounds which either in aqueous solution or in molten state allow electric current to pass through them. Electrolytes can be electrovalent compounds, composed of metals and non-metals, or polar covalent compounds, that form ions in water.

Examples of electrolytes

Acids - \(\text{H}_2\text{SO}_4, \text{HNO}_3, \text{H}_3\text{PO}_4, \text{HCl}\)

Bases - \(\text{NaOH}, \text{KOH}, \text{NH}_4\text{OH}, \text{Ca(OH)}_2\)

Salts - \(\text{NaCl}, \text{CuSO}_4, \text{PbBr}_2\)

Electrolytes are of two types, based on their conductivity and ability to dissociate into ions.

1. Strong electrolyte 2. Weak electrolyte

(iii) Non-electrolyte

It is a compound which neither in solution nor in the molten state allows an electric current to pass through it. It also does not decompose at the electrodes. Non-electrolytes do not have ions even in solution. They contain only molecules.

Examples of non-electrolytes

Distilled water, alcohol, kerosene, carbon disulphide, cane sugar, benzene, glucose and urea.

(iv) Electrolytic cell

A non-conducting vessel in which electrolysis is carried out, is called an electrolytic cell or voltameter. It converts electrical energy into chemical energy (Fig. 6.1).

(v) Electrochemical cell

It is a device used to convert chemical energy into electrical energy.

Examples: Simple voltaic cell, Daniel cell,

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