Redox Reactions Class 11 Notes

   Redox Reactions MCQs   

Oxidation Number

Oxidation number is defined as residual charge that is present on atom when the atom is in combined state with other atoms. Oxidation number may be positive, negative, or zero. If the oxidation number is positive, it means the atom loses electrons, and if it is negative, it means the atom gains electrons. If it is zero, then the atom neither gains nor loses electrons.

Rules to assign and calculate oxidation number

Some important rules and exceptions to assign the oxidation numbers-
1. The oxidation number of atoms in their elemental state (pure element) is taken as zero.
2. The oxidation number of Group IA and IIA are +1 and +2 respectively.
3. The oxidation number of Hydrogen is +1 when with non metals and -1 with metals.
4. The oxidation number of oxygen is -2 in most of the compounds but in peroxides it is -1 and in superoxide it is -0.5. In the compound of oxygen difluoride, oxidation number of oxygen is +2.
5. The oxidation number of fluorine in any compound is -1 but other halogens may be +ve or -ve.
6. The metals always have oxidation number in positive and non metal in negative when present together in ionic compounds.
7. In compounds that have two atoms with different electro negativities, the oxidation number of more electronegative is taken as –ve and for less electronegative it is taken as positive. For example: In NaBH₄ the oxidation number of boron will be in positive and oxidation of hydrogen will be in negative.
8. The oxidation number of an ion is the same as its charge.
9. In complex ions, the sum of oxidation states of all the atoms is equal to the charge present on the complex.
10. In neutral compounds, the sum of all oxidation numbers is equal to zero.

Assign the Oxidation Number of the Highlighted Atoms

1. NaBH₄
2. CaOCl₂
3. CrO₅
4. K₂Cr₂O₇
5. CH₂Cl₂
6. LiH
7. Cr₂(SO₄)₃
8. [CrO₈]⁻³
9. KO₂
10. OF₂

How to Balance Chemical Equations