Calculation of oxidation Number in Redox Reactions

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Calculation of oxidation Number in Redox Reactions

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Oxidation number of the element is defined as the residual charge which its atom has when all other atoms from the molecule are assumed to be removed as ions.

General oxidation number of different groups are as follows:

IA +1

IIA +2

IIIA +3/ +1

IV A – 4 to +4

V A – 3 to +5

VI A – 2 to +6

VII A – 1 to + 7

Oxidation Number / Oxidation state is never more than the group in which the element is present.

For neutral molecules like {{\rm{H}}_{\rm{2}}}{\rm{O}}(water) and {\rm{CO}} carbon mono oxide, Oxidation state is O.

If Hydrogen is attached with metal then its O. S is –1 and if it is attached with non – metal then its 0.5 is +1 oxygen can show 3 oxidation states.

Oxide  Peroxide Superoxide

\left( { - 2} \right)     \left( { - 1} \right)         \left( { - \frac{1}{2}} \right)

Use of Oxidation Number

1. Calculation of oxidation Number of elements underlined in the following species –

Let x be the oxidation number of underlined element.

(i) image

\begin{array}{l} x + {\rm{2}}\,\,\, \times \left( { - 2} \right)\\ x - 4\\ x = + 4 \end{array}

(ii) image

    image

(iii) image

\begin{array}{l} {\rm{1}} * 3 + x + \left( { - 2} \right) \times 4\\ 3 + x - 8\\ x - 5\\ x = + 5 \end{array}

(iv) image

\begin{array}{l} x + 4 \times \left( { - 2} \right) = - 2\\ x - 8 = - 2\\ x = - 2 + 8\\ x = + 6 \end{array}

(v) image

For this take {\rm{P}}{{\rm{O}}_4}^{ - - - }

\begin{array}{l} x + \left( { - 2} \right) \times 4 = - 3\\ x - 8 = - 3\\ x = - 3 + 8\\ x = + 5 \end{array}

(vi) image

image

In this 2 Peroxide links are present and one oxide

So,

\begin{array}{l} x + \left( { - 2} \right) + 4 \times \left( { - 1} \right)\\ x - 2 - 4\\ x - 6\\ x = + 6 \end{array}

(vii)image

image

We can write it as {{\rm{H}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}

So, 2 + 2x + 2 \times 0 + 6 \times \left( { - 2} \right)

\begin{array}{l} 2 + 2x - 12\\ \,\,\,\,\,\,\,\,2x - 10\\ \,\,\,\,\,\,\,\,\,\,x = + 5 \end{array}

2. In the Name of the compound

If the compound is in low oxidation state then suffix ‘ous’ is added

Ex – (I) image

image

(II) image

image

image

(III) If we get talk about mixing of 2 oxidation states