Question

When conc. $H_{2}S{O}_4$ was treated with $K_4\left[Fe{\left(CN\right)}_6\right]$, $CO$ gas was evolved. By mistake, somebody used dilute $H_{2}S{O}_4$ instead of conc. $H_{2}S{O}_4$ then the gas evolved was:

• A. $CO$
• B. $HCN$
• C. $N_2$
• D. $C{O}_2$

Answer 1

The correct option is B $HCN$

$K_4\left[Fe{\left(CN\right)}_6\right]$ when heated with concenterated $H_{2}S{O}_4$ gives $CO$.
$K_4\left[Fe\right(CN{)}_6]+6H_{2}S{O}_4+6H_{2}O\to 2K_{2}S{O}_4+FeS{O}_4+3(N{H}_4{)}_{2}S{O}_4+6CO$
When reacted with boiling dilute $H_{2}S{O}_4$ it gives $HCN$.
$2K_4\left[Fe\right(CN{)}_6]+3H_{2}S{O}_4\to 3K_{2}S{O}_4+K_{2}Fe[Fe\left(CN{)}_6\right]+6HCN$