Question

$F{{e}^{2+}}(aq)+N{{O}_{3}}^{-}(aq)+{{H}_{2}}S{{O}_{4}}(conc.)\to Brown\text{ }Ring$
The oxidation number of iron in the brown ring complex is:

Answer 1

Hint: Think about the nitrate test in inorganic qualitative analysis of anions. Write the complete reaction and find out the complex formed in this reaction. Recollect the oxidation state concept and then calculate the oxidation state of iron in the complex.

Complete step by step solution:
In the chemical test known as the nitrate test, it is used to determine the presence of nitrate ion in the solution. Usually, the presence of nitrate is very hard to determine since almost all nitrates are soluble in water. Many common ions give insoluble salts through nitrate ion being an oxidizer, many tests for the nitrate anion are based on this property. A common nitrate test is known as the brown ring test which can be performed by adding iron (II) sulphate to a solution of nitrate while slowly adding concentrated sulphuric acid to the reaction so that the acid forms a layer below the aqueous solution. A layer of the brown ring is formed at the junction of the two layers indicating that nitrate ion is present.
The brown ring complex that is formed from the chemical reaction $F{{e}^{2+}}(aq)+N{{O}_{3}}^{-}(aq)+{{H}_{2}}S{{O}_{4}}(conc.)\to Brown\text{ }Ring$ is \[\text{ }\!\![\!\!\text{ Fe(}{{\text{H}}_{\text{2}}}\text{O}{{\text{)}}_{\text{5}}}\text{N}{{\text{O}}^{\text{+}}}\text{ }\!\!]\!\!\text{ S}{{\text{O}}_{\text{4}}}\].
NO in the brown ring complex has +1 oxidation state. We know water is a neutral ligand, so its oxidation state is 0.
Let us consider that the oxidation state of Fe is x.
Since the overall charge on the coordination sphere, \[\text{ }\!\![\!\!\text{ Fe(}{{\text{H}}_{\text{2}}}\text{O}{{\text{)}}_{\text{5}}}\text{N}{{\text{O}}^{\text{+}}}\text{ }\!\!]\!\!\text{ }\] is +2, the sum of oxidation states of all elements in it should be equal to +2.
Therefore, x + 1 = +2
x = +1

Hence, the oxidation state of Iron or Fe in the brown ring complex is +1.

Note: The oxidation number is also known as Oxidation State, is basically the number of electrons transferred between atoms during the formation of a chemical bond, assuming that the electrons were completely transferred. We can find the oxidation number of an atom by counting the number of electrons that an atom in a molecule can share, lose or gain resulting in the formation of chemical bonds with some other atom of a different element. An oxidation number or state helps define the transfer of electrons.