Ncert Exercises & Solutions

Three students, Manish, Ramesh and Rajni were determining the extra elements present in an organic compound given by thei teacher. They prepared the Lassaigne's extract (L.E.) independently by the fusion of the compound with sodium metal. Then they added solid FeSO₄ and dilute sulphuric acid to a part of Lassaigne's extract. Manish and Rajni obtained prussian blue colour but Ramesh got red colour.

Ramesh repeated the test with the same Lassaigne's extract, but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation. Also, write the chemical equations to explain the formation of compounds of different colours.

If the organic compound contains both N and S, then while fusion it may for form either a mixture of sodium cyanide (NaCN) and sodium sulphide (Na₂S) or sodium thiocyanate (NaSCN) depending on the amount of Na metal used.

If Less sodium metal is used, only NaSCN is obtained.

This then gives red colour on reacting with Fe³⁺ ions (produce by oxidation of Fe²⁺ ions while preparing Lassaigne's extract) due to the formation of ferric thiocyanate.

{Fe}^{2 +} \overset{Aerial oxidaiton}{\to} {Fe}^{3 +}

{Fe}^{3 +} + 3 NaSCN \to \underset{Ferric thiocyanate}{Fe {(SCN)}_{3}}

(red) + 3 {Na}^{+}

In case, excess of sodium metal is used, the initially formed sodium thiocyanate decomposes as follows:

\underset{Sodium}{NaSCN}

thiocyanate + 2 Na \overset{∆}{\to} \underset{Sodium}{NaCN}

cyanide + \underset{Sodium}{{Na}_{2} S}

sulphide

This NaCN then reacts with FeSO₄, Fe³⁺ ions and NaCN, it gives prussian blue colour due to the formation of ferric ferrocyanide or iron (III) hexacyanoferrate (II).

2 NaCN + {FeSO}_{4} \to {Na}_{2} {SO}_{4} + Fe {(CN)}_{2}

Fe {(CN)}_{2} + 4 NaCN \to \underset{Sodium hexacyanoferrate (II)}{{Na}_{4} [Fe {(CN)}_{6}]}

3 {Na}_{4} [Fe {(CN)}_{6}] + 4 {Fe}^{3 +} \to \underset{Iron (III) hexacyanoferrate}{{Fe}_{4} {[Fe {(CN)}_{6}]}_{3}}

(II) (prussian blue) + 12 {Na}^{+}

On the basis of above results, it is clear that Ramesh used less sodium and NaSCN formed in the Lassaigne's extract which gave red colouration due to

Fe {(SCN)}_{3}

formation while Manish and Rajni used excess sodium and hence NaCN formed in the Lassaigne's extract which gave prussian blue colour of

{Fe}_{4} [Fe {(CN)}_{6}]

NEET Information

courses

Company

Botany Questions

Chemistry Questions

Physics Questions

Zoology Questions