English Medium Notes of 9th class Chemistry chapter 7 and 8 MCQs and short questions
Oxidation is addition of oxygen or removal of hydrogen or loss of electrons by an element and as a result oxidation number increases.
Reduction is addition of hydrogen or removal of oxygen or gain of electrons by an element and as a result oxidation number decreases.
Oxidation number is the apparent charge on an atom. It may be positive or negative.
Oxidizing agents are the species that oxidize the other element and reduce themselves. Non-metals are oxidizing agents.
Reducing agents are species that reduce the other elements and oxidize themselves. Metals are reducing agents.
Chemical reactions in which the oxidation state of species change are termed as redox reaction. A redox reaction involves oxidation and reduction processes taking place simultaneously.
Redox reactions either take place spontaneously and produce energy or electricity is used to drive the reaction.
The process in which electricity is used for the decomposition of a chemical compound is called electrolysis. It takes place in electrolytic cells such as Downs cell and Nelson’s cell.
Galvanic cells are those in which spontaneous reactions take place and generate electric current. They are also called voltaic cells.
Sodium metal is manufactured from fused sodium chloride in the Downs cell.
NaOH is manufactured from brine in Nelson’s cell.
Corrosion is slow and continuous eating away of a metal by the surrounding medium. The most common example of corrosion is rusting of iron.
The rusting principle is electrochemical redox reaction, in which iron behaves as anode. Iron is oxidized to form rust Fe2O3. nH2O.
Corrosion can be prevented by many methods. The most important is electroplating .
Electroplating is depositing of one metal over the other by means of electrolysis .
Iron can be electroplated by tin, zinc, silver or chromium.
Formation of cations of alkali and alkaline earth metals is due to their electropositive behavior.
The chemical reactivity of alkali and alkaline earth metals, is quite different.
Calcium and magnesium are less reactive than sodium.
Halogens form very stable compounds with alkali metals.
Mercury and gold exist in free elemental form in nature.