Chapter 7 Electrochemistry – 9th Class Chemistry Notes

Chapter 7 Electrochemistry – 9th Class Chemistry Notes

 

Question#1: Define electrochemistry? Also write a detailed note on oxidation reduction reactions?


ELECTROCHEMISTRY:

Electrochemistry is the branch of Chemistry that deals with the relationship between electricity and chemical reactions.

OXIDATION AND REDUCTION REACTIONS ON THE BASIS OF ADDITION OR REMOVAL OF OXYGEN:

Oxidation: Oxidation is defined as addition of oxygen during a chemical reaction.

Reduction: The removal of oxygen during a chemical reaction is called reduction.

Example: A reaction between zinc oxide and carbon takes place by the removal of oxygen (reduction) from zinc oxide and addition of oxygen (oxidation) to carbon. It is represented as:

2ZnO +  C ——>  2Zn + CO2

 

 

OXIDATION AND REDUCTION REACTIONS ON THE BASIS OF ADDITION OR REMOVAL OF HYDROGEN:

Oxidation: The removal of hydrogen during a chemical reaction is called oxidation.

Reduction: The addition of hydrogen during a chemical reaction is called Reduction.

Example: A reaction between hydrogen sulphide and chlorine takes place by oxidation of hydrogen sulphide and reduction of chlorine. Hydrogen is being removed from H2S and added to chlorine. It is represented as:

 

H2S + Cl2 ——> S +  2HCl

 

Oxidation and Reduction in Terms of Loss or Gain of Electron: Reactions which involve ‘loss or gain of electrons’ are also called oxidation and reduction reactions.

Oxidation: Loss of electrons by an atom or an ion is called Oxidation.

Example:

 

Na —–> Na+ + 1e

Reduction: Gain of electrons by an atom or ion is called reduction.

 

Cl + 1e ——–> Cl

 

Summary of all the Oxidation and Reduction reactions

Oxidation Reduction
Addition of oxygen Removal of oxygen
Removal of hydrogen Addition of hydrogen
Loss of electrons Gain of electrons

Question#2: Describe the rules for assigning the oxidation state?

Answer: OXIDATION STATE OR OXIDATION NUMBER:

Oxidation state or oxidation number (O.N.) is the apparent charge assigned to an atom of an element in a molecule or in an ion. For example: in HCl, oxidation number of H is +1 and that of CI is -1.

Rules for assigning oxidation numbers (O.N.)

  1. The oxidation number of all elements in the free state is zero.
  2. The oxidation number of an ion consisting of a single element is the same as the charge on the ion.
  3. The oxidation number of different elements in the periodic table is: in Group 1 it is +1, in Group 2 it is +2 and in Group 13 it is +3.
  4. The oxidation number of hydrogen in all its compounds is +1. But in metal hydrides it is -1.
  5. The oxidation number of oxygen in all its compounds is -2. But it is -1 in peroxides and +2 in OF2.
  6. In any substance, the more electronegative atom has the negative oxidation number.
  7. In neutral molecules, the algebraic sum of the oxidation numbers of all the elements is zero.
  8. In ions, the algebraic sum of oxidation number equals the charge on the ion.
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