Question

# The oxidation state of iron in ${{\rm{K}}_4}\left[ {{\rm{Fe}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]$ is: A ) 1B ) 4C ) 3D ) 2

Hint: Oxidation State is the total number of electrons either gained or lost by an atom to form a chemical bond with another atom.The sum of the oxidation states of all the atoms for a neutral compound.

Oxidation state is also known as the oxidation number. It is the total number of electrons either gained or lost by an atom to form a chemical bond with another atom.
Iron usually forms compounds in which it has either +2 oxidation state or +3 oxidation state. Thus, in compounds such as ferrous chloride and ferrous sulphate, the oxidation state of iron is +2 as it loses 2 electrons during compound formation. On the other hand, in compounds such as ferric chloride and ferric sulphate, the oxidation state of iron is +3 as it loses 3 electrons during compound formation.

Let X be the oxidation state of iron in ${{\rm{K}}_4}\left[ {{\rm{Fe}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]$. The oxidation states of potassium and cyanide ions are +1 and -1 respectively. In a neutral molecule, the sum of the oxidation states of all the atoms / ions is zero.
Calculate the oxidation state of iron in ${{\rm{K}}_4}\left[ {{\rm{Fe}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]$.

$\begin{array}{c} 4\left( { + 1} \right) + X + 6\left( { - 1} \right) = 0\\ 4 + X - 6 = 0\\ X - 2 = 0\\ X = + 2 \end{array}$

Hence, the oxidation state of iron in ${{\rm{K}}_4}\left[ {{\rm{Fe}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]$. is +2.

The correct answer is option D )

Note:
Do not forget to include the oxidation state of potassium cation in the calculation for the oxidation state of iron.