CBSE Class 11 Chemistry Redox Reactions Notes Set 03

Oxidation number denotes theoxidation state of an element in a compound ascertained according to a setof rules formulated on the basis thatelectron in a covalent bond belongsentirely to more electronegative element.

Calculation of oxidation number-

1. O. S. of all the elements in their elemental form (in standard state) is taken as zero O. S. of elements in C_l2, F₂, O₂, P4, O₃, Fe(s), H₂, N₂, C(graphite) is zero.

2. Common O. S. of elements of group one (1st) is one. Common O. S. of elements of group two (2nd) is two.

3. For ions composed of only one atom, theoxidation number is equal to the chargeon the ion.

4. The oxidation number of oxygen in most compounds is –2.While in peroxides (e.g., H₂O₂, Na₂O₂), eachoxygen atom is assigned an oxidationnumber of –1, in superoxides (e.g., KO₂,RbO₂) each oxygen atom is assigned anoxidation number of –(½).

5. In oxygendifluoride (OF₂) and dioxygendifluoride (O₂F₂), the oxygen is assignedan oxidation number of +2 and +1,respectively.

6. The oxidation number of hydrogen is +1 but in metal hydride its oxidation no. is–1.

7. In all its compounds, fluorine has anoxidation number of –1.

8. The algebraic sum of the oxidation numberof all the atoms in a compound must bezero.

9. In polyatomic ion, the algebraic sumof all the oxidation numbers of atoms ofthe ion must equal the charge on the ion.

Stocknotation:     the oxidationnumber is expressed by putting a Romannumeral representing the oxidation numberin parenthesis after the symbol of the metal inthe molecular formula. Thus aurous chlorideand auric chloride are written as Au(I)Cl andAu(III)Cl₃. Similarly, stannous chloride andstannic chloride are written as Sn(II)Cl₂andSn(IV)Cl₄.

Oxidation: An increase in the oxidationnumber

Reduction: A decrease in the oxidationnumber

Oxidising agent: A reagent which canincrease the oxidation number of an elementin a given substance. These reagents are calledas oxidants also.

Reducing agent: A reagent which lowers the oxidation number of an element in a givensubstance. These reagents are also called asreductants.

Redox reactions: Reactions which involvechange in oxidation number of the interactingspecies.

 Balancing of redox reactions: Oxidation Number Method: Write the net ionic equation for the reaction of potassium dichromate(VI), K₂Cr₂O₇ with sodium sulphite,Na₂SO₃, in an acid solution to give chromium(III) ion and the sulphate ion.

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