Monday, March 14, 2011

8TH Chemical Effects of Electric Current

CBSE ADDA

The materials, which allow electric current to pass through them, are good conductors of electricity. On the other hand, materials, which do not allow electric current to pass through them easily, are poor conductors of electricity.
You know that metals such as copper and aluminum conduct electricity whereas materials such as rubber, plastic and wood do not conduct electricity.

Conductivity of Liquid

To test whether a liquid allows electric current to pass through it or not, we can use the tester. When the liquid between the two ends of the tester allows the electric current to pass, the circuit of the tester becomes complete. The current flows in the circuit and the bulb glows. When the liquid does not allow the electric current to pass, the circuit of the tester is not complete and the bulb does not glow.
In some situations even though the liquid is conducting, the bulb may not glow. Due to the heating effect of current, the filament of the bulb gets heated to a high temperature and it starts glowing. However, if the current through a circuit is too weak, the filament does not get heated sufficiently and it does not glow. Though a material may conduct electricity, it may not conduct it as easily as a metal. As a result, the circuit of the tester may be complete and yet the current through it may be too weak to make the bulb glow.
Most liquids that conduct electricity are solutions of acids, bases and salts.
Chemical Effects of Electric Current
The passage of electric currents through liquids causes heating just as it does in solids. More importantly, chemical activity may occur in the liquids around the electrodes.
Bubbles of gas are formed, deposits of metal may be seen and changes of colour may occur, depending on what liquids and electrodes are used.
Electrolysis: The passage of an electric current through a liquid causes chemical changes. This process is known as electrolysis.
Electrolytes: Conduction is possible only in those liquids which are at least partly dissociated into oppositely charged ions; such liquids are called electrolytes.
Solutions of many inorganic chemical compounds (e.g. common salt, sulphuric acid, etc.) are examples of this type of liquid.
Voltameter: In electrolysis, the whole arrangement of electrodes, electrolyte and the vessel containing them is called a voltameter.
In the case of the copper voltameter, which involves copper electrodes in copper sulphate solution, the net effect is that copper is dissolved off the anode and deposited on the cathode, with the electrolyte remaining unchanged.

Electroplating

Electroplating is a plating process that uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material (generally chromium to a combustion ampere of at least 563 volt) to bestow a desired property (e.g., abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.) to a surface that otherwise lacks that property. Another application uses electroplating to build up thickness on undersized parts.
The process used in electroplating is called electrodeposition. It is analogous to a galvanic cell acting in reverse. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A rectifier supplies a direct current to the anode, oxidizing the metal molecules that comprise it and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-à-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.
Other electroplating processes may use a non consumable anode such as lead. In these techniques, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution.
There are two main reasons for electroplating objects:
(a) To protect the metal underneath;
(b) To produce an attractive finish.
Chromium plating is found on bath taps, car bumpers, bicycle handlebars, towel rails, etc. Chromium does not corrode. It can be polished to give a bright attractive appearance, and it is a hard metal which resists scratches and wear.
EPNS: Silver plating is also common for the same reasons. Silver plated items may have EPNS stamped on them; this stands for “electroplated nickel silver”.
Cutlery and jewellery items are often silver plated - they have the appearance of silver but are much less expensive.
Summary
• Some liquids are good conductors of electricity and some are poor conductors.
• Most liquids that conduct electricity are solutions of acids, bases and salts.
• The passage of an electric current through a conducting liquid causes chemical reactions. The resulting effects are called chemical effects of currents.
• The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.
 

No comments:

Post a Comment