Question

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

Answer 1

Hint: The given molecule is the coordinate compound. We can also see that the molecule is the neutral molecule. Thus, the overall charge over it will be zero. Knowing the charges on other constituent atoms we can find the required answer.

Complete Solution :
Let us first know about the oxidation state;
Oxidation state-
The oxidation state, also known as oxidation number describes the degree of oxidation of an atom in a given chemical compound.

In short, if we consider any reaction of compounds with each other, oxidation state is the total number of electrons that atom would gain or lose to form a bond with another atom to form the compound.
Facts about oxidation numbers-
- It is always zero for pure elements.
- It is equal to the charge of the ion for monatomic ions.
- It is +1 for all alkali metal ions.
- It is +2 for all alkaline earth metal ions.
- It is +3 for all boron family metal ions.
- It is +1 for hydrogen in proton but is -1 in hydride.
- It is -2 for oxygen in oxide ion but is -1 in peroxide ion.

Calculation of oxidation number-
Consider a neutral molecule XYZ;
Oxidation state of XYZ = (Oxidation state of X + Oxidation state of Y + Oxidation state of Z) = 0
If the oxidation state of X and Y are given (as -1 and -2 respectively) and we need to find the oxidation state of Z as,
Oxidation state of X + Oxidation state of Y + Oxidation state of Z = 0
Oxidation state of Z = +3
Thus, now moving towards the given illustration, we get to know that;
The given molecule i.e. ${{K}_{4}}\left[ Fe{{\left( CN \right)}_{6}} \right]$ is a neutral molecule.
Thus, considering above stated rough example we can solve for Fe as;
Let the oxidation number of Fe as x.
We have an oxidation number of CN as -1 and of K as +1.

Thus,
$\begin{align}
  & {{K}_{4}}\left[ Fe{{\left( CN \right)}_{6}} \right]=\left( 4\times 1 \right)+x+\left( 6\times \left( -1 \right) \right)=0 \\
 & 4+x-6=0 \\
 & x=+2 \\
\end{align}$
Therefore, the oxidation state of Fe is +2.
So, the correct answer is “Option A”.

Note: The oxidation states play an important role while naming the complex compounds as these have been given a vital place in the rules for naming the same. The given compound can be named as Hexacyanoferrate (II).