Class 11 Chemistry Notes Stocihiometry – Balancing of Equations

Balancing of Equations:

1. Ion-electron method
2. Oxidation number method
   [Concept involved that in any chemical reaction e^– cannot be produced so no. of e^– s in O.H. & R.H. should be same]

 Note : To predict the product of reaction remember:

• Free halogen on reduction gives halide ion (F₂ → F^–)
• Alkali metals on oxidation give metallic ion with +1 oxidation state.
• In alkaline medium KMnO₄ is reduced to K₂MnO₄ but in strong alkaline or neutral medium it reduces of  Mn⁺⁴
• In acid solution KMnO₄ is reduced to Mn²⁺
• H₂O₂ on reduction gives water and on oxidation gives oxygen.
• Dichromate ion in acid solution is reduced to Cr³⁺.

Nature of Oxides Based on Oxidation Number:

Lowest oxidation state                      →        Basic                     (MnO)
Intermediate oxidation state         →        Amphoteric       (Mn₃O₄, MnO₂)
Highest oxidation state                     →        Acidic                  (Mn₂O₇)

Rules For Assigning Oxidation Number:

1. Oxidation number of free elements or atoms is zero.
2. Oxidation number of allotropes is zero.
3. Oxidation number of atoms in homo-nuclear molecules is zero.
4. Oxidation number of mono-atomic ions is equal to the algebraic charge on them.
5. Oxidation number of F in compounds is – 1
6. Oxidation number of H in its compounds is +1, except in metal hydrides where it is-1
7. Oxidation number of O is-2in its compound, but in and in peroxides it is 1 and -0.5 in KO₂
8. Oxidation number of alkali metals in their compounds + 1.
9. Oxidation number of alkaline earth metals in their compounds is + 2.
10. The sum of oxidation number of all the atoms in a molecule should be zero and in an ion equal to its charge.

Average Oxidation Number : Find Oxidation Number of Fe in Fe₃O₄.

Fe₃O₄ is FeO. Fe₂O₃
O.N. of Fe in FeO is + 2         ;           O.N. of Fe in Fe₂O₃ is + 3.

Therefore average O.N. of three Fe atoms (+2 + 2x (+3))/3 = +(8/3)

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