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CHAPTER EIGHT
REDOX REACTIONS
8.2 What are the oxidation number of the underlined elements in each of the
following and how do you rationalise your results ?
(a) KI3 :- ON of Iodine in KI3 = +1+ (-3x) =
X=1/3 . Therefore the ON of iodine in KI# = 1/3 which is not possible. As ON cannot be
fractional.We can consider the structure of KI3.
In I3 ion a co-ordinate bond is formed between I2 and I- ion.The ON of two iodine atoms is therefore
zero.The ON of iodine forming the co-ordinate bond is –1. Therefore the ON of iodine in KI3 is –1
(b) H2S4O6 :- Oxidation number of S in H2S4O6 is 2(+1)+4(x)+6(-2)= 0
= x = +2.5. The oxidation number cannot be in fraction .Therefore the oxidation number of S is
calculated from its structure.
The O.N of Sulphur atoms in the middle is zero. The O.N for the remaining two S atoms
bonded with O is +5
(c) Fe3O4:-. In the conventional method of calculation the oxidation state of Fe in Fe2O3 is 8/3.The
oxidation state cannot be in fractions. Fe2O3 is a mixture of two oxides. FeO and Fe2O3. Therefore
we can calculate the ON of Fe in each one of them.
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FeO= x + (-2) = 0
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X= +2
Fe2O3 = 2x + 3* (-2) = 0
= 2x + (-6) = x = +3
Therefore the oxidation number of Fe is +2 and +3
(d) CH3CH2OH: - There are two carbon atoms in CH3CH2OH. One directly attached the the
functional group and the other is CH3 group attached to the functional group carbon atom. There
are two ways of calculating the ON of Carbon. One is the conventional method which calculates the
average ON of all the carbons present and the other is by chemical bonding method in which the
ON of each carbon is calculated.
Conventional method :- CH3CH2OH = 2x+ 6(+1) + 1(-2) = - 2
Chemical bonding method :- The structure of ethanol is given below.
The C-1 is more electronegative than C-2.The ON of C-1 = +1+2(+1)+x+1(-1)=0 = x=-2
The C-2 is less electronegative . The ON of C-2 = 3(+1)+x+1(-1)=0
x=-2
(e) CH3COOH:- In the conventional method average ON of C= 2x+4-4=0
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x=0
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In the Chemical bonding method the the C-2 is attached to 3 hydrogen atoms and
C- 1 is COOH groupwhich is attached to one oxygen atom and a OH group.
ON of c-2 = 3(+1) +x+1(-1)=0
X=-2
ON no C-1 = +1+x+1(-2)+1(-1) =0
X=-2
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