Three students, Manish, Ramesh and Rajni were determining the extra elements present in an organic compound given by their 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.
As we know that theLassaigne’s extract is prepared to detect the presence of N, S, Halogens and P.
As when the Lassaigne’s extract is prepared, it allows all the elements present in the particular compound to come into the elemental state, so that they can easily be detected.
Now, the reaction involved is for Nitrogen estimation:
Na + C + N→ NaCN
NaCN + FeSO4 + dilute sulphuric→ Fe4[FeCN6]3 (Prussian blue colour solution)
The reaction involved is for sulphur estimation:
Na + C + N + S → NaCNS
3NaCNS + FeSO4 + dilute sulphuric→ Fe(CNS)3 (Blood red colour) + 3Na+
Hence, the Ramesh has N as well as S present in his organic compound so, he was getting the Blood red colour of Fe(CNS)3 and Manish and Rajni were having only Nitrogen in their compound, hence they got Prussian blue colour.
Rate this question :
Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation?NCERT - Chemistry Part-II
Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.NCERT - Chemistry Part-II
Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.NCERT - Chemistry Part-II
Will CCl4 give white precipitate of AgCl on heating it with silver nitrate? Give reason for your answer.NCERT - Chemistry Part-II
Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?NCERT - Chemistry Part-II
In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:
(a) Na4[Fe(CN)6] (b) Fe4[Fe(CN)6]3
(c) Fe2[Fe(CN)6] (d) Fe3[Fe(CN)6]4NCERT - Chemistry Part-II