SS 2 Chemistry Notes with Assignment

REDUCTION AND OXIDATION REACTION

Reduction and Oxidation reaction are two reactions that take place simultaneously, in the sense that one substance participating in the reaction undergoes reduction while the other undergoes oxidation. One cannot take place without the other. This tow types of reaction are collectively called Redox reaction.

To understand this Redox reaction, we must understand the meaning of oxidation state/oxidation number.

Oxidation state/ Oxidation number

Oxidation state of a substance is the number of electrons lost or gained by the substance during combination. It is also defined as the number of bonds formed by an atom during chemical bonding. Oxidation state or number of an atom is also called the valency.

Rules for determining the oxidation state of element in their compounds

1. The oxidation state of hydrogen (H) is +1 except in hydride where it is -1. Hydrides are binary compounds of hydrogen and metals. Examples: NaH, CaH₂, etc.
2. The oxidation state of oxygen (O) is -2 except in peroxides where it is -1. Peroxides are higher oxides of metals. That is an oxide of a metal which has a higher proportion of oxygen. Example: Na₂O₂ instead of Na₂O, CaO₂ instead of CaO, etc.
3. The sum of the oxidation number of a charged substance is equal to the number of charge on it. Example: oxidation state of SO₄^2- is equal to -2, the oxidation state of MnO₄^– is equal to -1, etc.
4. The sum of the oxidation number of a neutral compound is equal to zero (0). That is the oxidation state of MnO₂ is zero.
5. The oxidation state of any element at its free or uncombined state is zero.

 

 

Worked example

Determine the oxidation state of Mn (manganese) in MnO₄^–.

Solution

MnO₄^–

The IUPAC name of the substance of MnO₄^– is Tetraoxomanganate (vii) ion

2. Determine the oxidation state of Cr (chromium) in Cr₂O₇^2-

Solution

The IUPAC name is therefore heptaoxodichromate (vi) ion

 

Assignment

1. Determine the oxidation sate of S in S₂O₃^2- and give the IUPAC name of S₂O₃^2-.
2. Determine the oxidation state of P in Na₃PO₄ and give the IUPAC name of Na₃PO₄

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