Question

A blood red color is obtained when ferric chloride solution reacts with ________.
(a)- $KCN$
(b)- $KSCN$
(c)- $K_4[Fe{(CN)}_6]$
(d)- $K_3[Fe{(CN)}_6]$

Answer 1

Hint: Ferric chloride is a compound having formula $FeC{l}_3$ and its solution is colorless to light brown. With all options given above, when reacted with ferric chloride forms complex compounds. So, the compound which gives blood-red color is $Fe{(SCN)}_3$.


Complete step by step answer:
Ferric chloride is a compound having formula $FeC{l}_3$ and its solution is colorless to light brown and it is also known as iron (III) chloride because the oxidation of iron in this compound is +3.
So, with $KCN$reacts with ferric chloride solution $FeC{l}_3$to form a coordination compound, in which iron is the central metal atom, cyanide is the ligand and potassium is the cationic part. The reaction is given below:
$FeC{l}_3+KCN\to K_3[Fe{(CN)}_6]+KCl$
And this in solution gives green-yellow color.
When $KSCN$reacts with ferric chloride solution $FeC{l}_3$it forms a complex compound having formula $Fe{(SCN)}_3$. The reaction is given below:
$FeC{l}_3+3KSCN\to Fe{(SCN)}_3+3KCl$
This solution is blood-red in color.
When $K_4[Fe{(CN)}_6]$reacts with ferric chloride solution $FeC{l}_3$it forms a coordination compound having formula $F{e}_4{[Fe{(CN)}_6]}_3$ in which the cyanide is the ligand and iron acts as both cationic part and central metal atom. The reaction is given below:
$FeC{l}_3+K_4[Fe{(CN)}_6]\to F{e}_4{[Fe{(CN)}_6]}_3$
This compound is known as Prussian blue and is a blue solution.
When $K_3[Fe{(CN)}_6]$reacts with ferric chloride solution $FeC{l}_3$it forms a coordination compound having formula $F{e}_3{[Fe{(CN)}_6]}_3$ in which the cyanide is the ligand and iron acts as both cationic part and central metal atom. The reaction is given below:
$3FeC{l}_3+3K_3[Fe{(CN)}_6\to F{e}_3{[Fe{(CN)}_6]}_3+9KCl$
This solution is red in color.

Therefore, the correct answer is an option (b)- $KSCN$.

Note: Don't get confused between option (b) and (d) because both of them give a compound with ferric chloride that is red, but specifically $KSCN$ gives a blood-red solution and $K_3[Fe{(CN)}_6]$only gives the red solution.