Electrochemistry

REDOX

1. Redox Reaction: Oxidation and reduction process which occur simultaneously in a chemical reaction.

2. . Oxidation or reduction can be can defined as following

   (a) loss or gain of oxygen          (b) loss or gain of hydrogen            
   (c) transfer of electrons             (d) changes in oxidation  number

3. Substance which is to be oxidized is called reducing agent while substance which is to be reduce is called oxidizing agent.

Oxidation	Reduction
v  Addition of O2	v  Removal of O2
v  Removal of H2	v  Addition of H2
v  Loss of electron	v  Gain of e-
v  Increase in oxidation number	v  Decrease in oxidation number

OXIDATION NUMBER

Oxidation numbers of elements can be calculated based on the ‘rules in assigning oxidation numbers’.

Two examples are given below.

Examples:

a)    Determine the oxidation number for the underlined elements

(i)             H₃PO₄

(1×3) + P + (-2×4) = 0

    3    + P + (-8) = 0

   P = 8 – 3 = +5  \ the oxidation number for phosphorus is +5

        (ii)   N₂H₄

2N + (1×4) = 0

2N +     4    = 0

                N = – 4/2 = –2        \ the oxidation number for nitrogen is –2

Note:

1. Element oxidation number = 0  such as Mg , C , Cl₂ and etc

2. Oxidation number for ion are usually according to their respective charge,

 example H⁺ = +1 , Mg²⁺ = +2, O^2- = -2

3. Oxidation number or oxidation state of an element is the charge that the atom of the  element would have if complete transfer of electrons takes place. Chemists assign the numbers according to a set of rules.