Question

Which of the following statements is not correct about the given reaction?
\[{K_4}[Fe{(CN)_6}]\xrightarrow{{oxidation}}F{e^{3 + }} + C{O_2} + N{O_3}^ - \]
(A) Fe is oxidised from \[F{e^{2 + }}\] to \[F{e^{3 + }}\]
(B) Carbon is oxidised from \[{C^{2 + }}\] to \[{C^{4 + }}\]
(C) N is oxidised from \[{N^{3 - }}\] to \[{N^{5 + }}\]
(D) Carbon is not oxidised

Answer 1

Hint: In \[{K_4}[Fe{(CN)_6}]\] complex cyano group has (-1) charge on them. One can find the oxidation number of any species by following the formula.
Overall charge on the species = Sum of the oxidation numbers of all the participating atoms.

Complete answer:
Let’s see the changes in oxidation states of the atoms involved in this reaction.
In \[{K_4}[Fe{(CN)_6}]\] complex, Potassium ions have +1 charge and complex is neutral hence we can say that charge on hexacyanoferrate complex is -4 and can write it as \[{[Fe{(CN)_6}]^{4 - }}\].
Now, we have a simple formula for relation between overall charge on the compound and the oxidation states of the atoms involved in its structure. For \[{[Fe{(CN)_6}]^{4 - }}\], it is
Overall charge = oxidation number of Fe + 6(overall charge on cyano group)
(-4) = oxidation number of Fe + 6(-1)
(-4) = oxidation number of Fe - 6
Oxidation number of Fe = 6-4 = +2
So, it is clear that Fe is oxidised from +2 to +3 oxidation state.
On the left hand side, carbon is in the form of cyanide group, let’s calculate its oxidation number.
Overall charge of CN group = oxidation state of C + oxidation state of N
 (-1) = oxidation state of C + (-3)
oxidation state of C = 3-1 = +2
In Carbon dioxide,
Overall charge on Carbon dioxide molecule = oxidation number of C + 2(oxidation number of O)
0 = oxidation number of C + 2(-2)
Oxidation number of C = +4
So, it is clear that carbon is oxidised from +2 to +4 oxidation state.
With the same formula applied, it is clear that nitrogen atoms are also oxidised from (-3) to +5 oxidation state. So, it is a false statement that carbon is not oxidised.
So, correct answer is (D) Carbon is not oxidised.

Additional Information:
- In the process of finding oxidation numbers, if we have some ligands for example, then we can put its combined charge in the formula for finding the oxidation number. That makes the process easier.
- Remember that oxygen is mainly found in (-2) oxidation state except it is in peroxide linkage or in its superoxide form.
- Nitrogen can have either(-3) or +5 oxidation states, that depends upon the atom that is bonded to nitrogen.

Note:
In a cyano group, do not assume positive charge on nitrogen atom and negative charge on carbon atom as it is not possible because of electronegativity. Same thing is for nitrate ion, where oxygen always has a negative oxidation number as it is a more electronegative atom.