Oxidation numbers

So far we have defined redox (reduction/oxidation) reactions in terms of electron transfer. It is easy to determine the oxidant and reductant in reactions such as in the one below between magnesium and oxygen.


It is easy to see that magnesium atoms have been oxidised (lost electrons) to form magnesium ions with a charge of 2+. While oxygen atoms have taken electrons to form oxygen ions with a charge of 2-. Here we can decide which species is the oxidant and which is the reductant according to the flow of electrons.

What about reactions that do not involve ions? Reactions such as the formation of water


In this reaction electrons are shared not transferred and there is no clear formation of charge. The reaction between hydrogen and oxygen is clearly a redox reaction. Clearly we need to broaden our definition of redox reactions. This broader definition involves the assignment of oxidation numbers to atoms taking part in the reaction.

A few rules apply.

Rule 1) The oxidation number of all substances in their elemental form is zero.


The oxidation number of all the atoms above is zero.

Rule 2) The oxidation number of all monoatomic(single atom) ions is the charge on the ion.

Rule 3) The oxidation number of oxygen is -2. Exceptions are peroxides where it is -1 and where it is +2.

Rule 4) Hydrogen is +1 except in metal hydrides where it is -1.

Rule 5) In polyatomic ions and molecules the sum of the oxidation numbers of each atom is equal to the charge on the ion or molecule.

Example

1)What is the oxidation number of manganese in The overall charge is -1

Mn + 4 X -2 = -1
Mn = 8 -1 =7

2) What is the oxidation number of Cr in (scan the formula for more information)
The oxidation number of K=1 and for O = -2. The overall charge is 0.

2 X 1 + Cr + 4 X -2 = 0
Cr = 6

3) What is the oxidation number of manganese in (scan the formula for more information)

2X Mn + 3 X -2 =0
2Mn = 6
Mn =3

Exercises.

1) Find the oxidation number of carbon in Carbon has an oxidation number of 2+. and Carbon has an oxidation number of 4+.
2) Find the oxidation number of Mn has an oxidation number of 2+
3) Find the oxidation number of the sulfur in Sulfur has an oxidation number  of 4+ and Sulfur has an oxidation number  of 6+
4) Find the oxidation number of Cl in Chlorine has an oxidation number  of 5+and Chlorine has an oxidation number  of 0

Scan the formulae for the answer

Exercises

Continue

 

Balance for charge by adding electrons tot he most positive side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for oxygen by adding water to the oxygen deficient side. Balance for Ag atoms Balance for oxygen by adding water to the oxygen deficient side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for charge by adding electrons tot he most positive side. Home
Each oxygen atom has an oxidation number of -2 Each oxygen atom has an oxidation number of -2 Each potassium atom has an oxidation number of =1