Question

Which one of the following combines with $Fe\left(II\right)$ ion to from a brown complex?

• A. $N_{2}O$
• B. $NO$
• C. $N_2{O}_3$
• D. $N_2{O}_5$

Answer 1

The correct option is B $NO$

We can perform brown ring test by adding iron(II) sulphate to a solution of a nitrate and then adding concentrated $H_{2}S{O}_4$ slowly without disturbing by which it forms a layer below the aqueous solution. At the junction of the two layers, brown ring will form indicating the presence of nitrate ion.

$2HN{O}_3+3H_{2}S{O}_4+6FeS{O}_4\to 3F{e}_2(S{O}_4{)}_3+2NO+4H_{2}O$

(Remaining)

$\left[Fe\right(H_{2}O{)}_6]S{O}_4+NO\to [Fe\left(H_{2}O{)}_5\right(NO\left)\right]S{O}_4+H_{2}O$

In the overall reaction, there is reduction of nitrate ion by iron (II) which is reduced to iron(I) and formation of nitrosoium complex where nitric oxide is oxidised to $N{O}^{+}$.

As NO is combined with $F{e}^{2+}$ ion to form a brown complex.

The correct option is B.