Oxidation - Reduction Reactions Or Redox Reactions

Chemical Reaction And Equation of Class 10

OXIDATION – REDUCTION REACTIONS OR REDOX REACTIONS

Oxidation number for an element is the arbitrary charge present on one atom when all other atoms bonded to it are removed. The removal of other atoms (dissociation of bonds between them) is done by assuming that the bonds are either ionic or pure covalent. So when a bond between two identical atom is broken, the bonded electrons are distributed equally between these atoms. Similarly when a bond between two different atoms is broken, the bonded electrons are assumed to be retained by the more electronegative atom. For example, if we consider HCl molecule, the bonded electrons will go with more electronegative chlorine atom resulting in formation of H+ and Cl ions. So the oxidation number of H and Cl in HCl are +1 and –1 respectively.

Oxidation number of an element can be determined keeping the following points in mind.

Rules for assigning Oxidation Numbers

  • An atom in its elemental state has an oxidation number of 0. For example:

Oxidation - Reduction Reactions Or Redox Reactions

  • An atom in a monatomic ion has an oxidation number identical to its charge. For example:

Oxidation - Reduction Reactions Or Redox Reactions

  • An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. In the hydroxide ion (OH), for example, the oxygen atom has an oxidation number of –2, as if it were a monoatomic O2– ion, and the hydrogen atom has an oxidation number of + 1, as if it were H+.

Oxidation - Reduction Reactions Or Redox Reactions

  • Hydrogen can be either +1 or –1. When bonded to a metal, such as Na or Ca, hydrogen has an oxidation number of –1. When bonded to a nonmetal, such as C, N, O, or Cl, hydrogen has an oxidation number of + 1.

Oxidation - Reduction Reactions Or Redox Reactions

  • Oxygen usually has an oxidation number of –2. The major exception is in compounds called peroxides, which contain either the Oxidation - Reduction Reactions Or Redox Reactions ion or an O–O covalent bond in a molecule. Each oxygen atom in a peroxide has an oxidation number of –1.

Oxidation - Reduction Reactions Or Redox Reactions

  • Halogens usually have an oxidation number of – 1. The major exception is in compounds of chlorine, bromine, or iodine in which the halogen atom is bonded to oxygen. In such cases, the oxygen has an oxidation number of –2, and the halogen has a positive oxidation number. In Cl2O, for example, the O atom has an oxidation number of –2 and each Cl atom has an oxidation number of + 1.

Oxidation - Reduction Reactions Or Redox Reactions

  • The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. This rule is particularly useful for finding the oxidation number of an atom in difficult cases. The general idea is to assign oxidation numbers to the ‘‘easy’’ atoms first and then find the oxidation number of the ‘‘difficult’’ atom by subtraction. For example, suppose we need to know the oxidation number of the sulfur atom in sulfuric acid (H2SO4). Since each H atom is + 1 and each O atom is –2, the S atom must have an oxidation number of + 6 for the compound to have no net charge:

Oxidation - Reduction Reactions Or Redox Reactions

  • To find the oxidation number of the chlorine atom in the perchlorate anion Oxidation - Reduction Reactions Or Redox Reactions we know that each oxygen is –2, so the Cl atom must have an oxidation number of +7 for there to be a net charge of –1:

Oxidation - Reduction Reactions Or Redox Reactions

  • To find the oxidation number of the nitrogen atom in the ammonium cation Oxidation - Reduction Reactions Or Redox Reactions we know that each H atom is +1, so the N atom must have an oxidation number of –3 for there to be net charge of + 1:

Oxidation - Reduction Reactions Or Redox Reactions

  • In case of co-ordinate covalent bonds, two cases arise:

(a) For co-ordinate bonds between same atoms or dissimilar atoms in which the donor atom is less electronegative than the acceptor atom, an oxidation number of +2 is assigned to the donor atom and –2 to the acceptor atom.

(b) If the donor atom is more electronegative than the acceptor atom, neglect the contribution of the co-ordinate bond. For example, in Oxidation - Reduction Reactions Or Redox Reactions the contribution of co-ordinate bond is neglected because N atom is more electronegative than the acceptor atom, C. Thus oxidation number of N in Oxidation - Reduction Reactions Or Redox Reactions is –3 and that of C is + 2.

By applying these rules, the oxidation number of an element in a molecule or ion can be calculated.

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