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Introduction |
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In any chemical reaction, the existing chemical bonds are broken and new chemical bonds are formed. Hence, all chemical reactions are fundamentally electrical in nature since electrons are involved in some way or the other in all types of chemical bonding. Many chemical reactions utilize electrical energy, whereas others can be used to produce electrical energy. As electrical energy involves the flow of electrons, these reactions are concerned with the transfer of electrons from one substance to the other. |
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Ionisation |
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The process of conversion of a neutral atom into charged ions to complete its octet is known as ionization. |
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Electrolytic Dissociation |
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Electrovalent substances are made up of ions in the solid state. The oppositely charged ions are held together by strong electrostatic (coulombic) force of attraction. Due to these forces the ions cannot move. |
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Theory of Electrolytic Dissociation |
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The ions carry an electric charge and also allow the flow of electric current through it. The flow of electricity is due to the flow of the ions. The strength of conductivity of these ions is proportional to the concentration of these ions. |
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Electrolytes and Non-electrolytes |
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An ionic or electrovalent compound that conducts electricity in molten (fused) or aqueous (solution) state can be classified as an electrolyte. However, if the compound is unable to ionise it does not conduct electricity it is called a non-electrolyte. In general, the extent to which an electrolyte can break up into ions categorises an electrolyte. This gives a measure of the degree of dissociation (a) of an electrolyte. Based on this degree the electrolytes can be classified as strong or weak electrolyte and non-electrolyte. |
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Other Classes of Electrolytes (Acids, Bases and Salts) |
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Acid, bases and their salts are all compounds that can undergo electrolytic dissociation. Hence these substances can also be classified as electrolytes. |
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Introduction to Electrolysis |
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"Electrolysis is the electrolytic dissociation and decomposition of an electrolyte (electrovalent substance), by the passage of a direct current or electricity through its aqueous or molten form." The process of electrolysis is not a simple chemical reaction of mix and match. As electricity is involved in this process, care is taken to understand and set up the apparatus as required. In view of this, basic requirements for theoretical, experimental understanding and set up procedures are to be studied. |
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Important Terms Related with Electrolysis |
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Conductors
All metals possess a large number of free electrons. Under ordinary conditions, these electrons move randomly in such a way that the net flow of charge across any section of the metal conductor is zero. Thus a conductor is void of any charge as such. But when the free electrons are made to flow in one direction, the electric current is said to flow through the conductor. |
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Simple Voltaic Cell |
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In order to study the working of an electrolytic cell and the phenomena of electrolysis it is important to know the working of a galvanic or a voltaic cell that generates electrical energy from its chemical reaction. |
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Mechanism of Electrolysis |
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The process of electrolysis is exactly opposite to one that takes place in a voltaic cell. A voltaic cell or a battery uses its chemical energy to produce electrical energy. While in electrolysis, electrical energy is used to bring about a chemical change. Humphrey Davy (1807) showed this in his experiments, when he isolated potassium by passing electricity through molten potassium hydroxide. |
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The Process of Electrolysis |
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According to the theory of electrolytic dissociation, electrolytes can be electrolysed only in the dissolved or molten state. These charged ions are directed towards the respective electrodes by the electricity supplied. |
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Electrochemical Series of Metals |
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Different metals get ionised at different rates. For e.g., metals like sodium when exposed to air combine almost instantly with the oxygen present therein (sodium atom gives up an electron almost as soon as it is in contact with oxygen). On the other hand, metals like gold if exposed to air even for a very long period, do not react with air. Even if gold is dropped into an acid, it remains unaffected. |
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Selective Discharge of Ions |
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Some electrolytes contain more than one type of cations or anions. When one type of ions is discharged in preference to the other, under certain conditions of electrolysis, it is known as selective discharge of ions. |
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Characteristics of Electrolysis |
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Following are the characteristics of the electrolysis process:
1) Electrolytic reactions involve the gain of electrons at the cathode and loss of electrons at anode.
2) The cathode reaction is a reduction reaction as the cations accept electrons, while the anode reaction is an oxidation reaction as the anions give up or lose electrons. |
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Electrolysis of some common electrolytes |
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Let us study various characteristics of electrolysis of some common electrolytes.
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Applications of Electrolysis |
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Electrolysis has various applications. Some of them are mentioned below:
a) Electroplating or coating of metals.
b) Electrorefining or purification of metals. |
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Electroplating |
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Electroplating is a process of depositing a thin layer of a fine and superior metal (like chromium, zinc, nickel, gold etc.) over the article of a baser and cheaper metal (like iron, copper, brass), with the help of electric current. |
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Electrorefining of Metals |
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Electrorefining of metals is a process of obtaining pure metal from the impure one by the process of electrolysis. |
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Electrometallurgy |
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The extraction of certain metals from their ores by the process of electrolysis is called electrometallurgy. |
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Summary |
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Passage of electricity through a metal or graphite is different from passage of electricity through aqueous solution of electrolyte in that during the process the metal does not decompose but the electrolyte decomposes. |
