Question

Al(s) + 3Ag+(aq) -> Al3+(aq) + 3Ag(s)
Which is oxidising and which is reduducing.

Explain please with the help of OXIDATION no.

Answer 1

Ag is the oxidising agent.Al is the reducing agent.Oxidation numbers are numbers that denote the number of atoms that can form compounds.Eg: oxidation number of oxygen as a element is 0Oxidation number of oxygen in H2O2 hydrogen peroxides is -1Oxidation number of oxygen in O2F2 is +2.This difference is due to ionic bonding.
oxidation number of  Aluminum = 0   and  Silver = +1 .In the product side oxidation numbers are  Al = +3  and Silver = 0.Oxidation number of Al is increased.  So it is oxidized and so It is reducing agent.  Aluminum caused reduction of Silver.Oxidation number of Silver is decreased, so it is an oxidation agent.

Answer 2

Oxidation number is same as the charge present on the element or ion.  It is also equal to valency. it is also the number equal to the number of atoms that will determine the number of atoms of other elements it will combine with.

In the reactants
     oxidation number of  Aluminum = 0    and of  Silver = +1
In the products of reaction
     oxidation numbers are  Al = +3        Silver = 0

Oxidation number of Al is increased.  So it is oxidized. So  It is reducing agent.  Aluminum caused reduction of Silver.

Oxidation num of Silver is decreased, it is reduced, so it is an oxidation agent.  Silver caused oxidation of Al.