question_answer 1)

  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 solidFeS04 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.

Answer:

  In the Lassaigne's test for nitrogen in an organic compound, the prussian blue colour is obtained due to the formation of ferric ferrocyanide. \[6NaCN+Fe{{(OH)}_{2}}\xrightarrow{{}}N{{a}_{4}}[Fe{{(CN)}_{6}}]+2NaOH\]\[3N{{a}_{4}}[Fe{{(CN)}_{6}}]+2F{{e}_{2}}{{(S{{O}_{4}})}_{3}}\xrightarrow{{}}F{{e}_{4}}{{[Fe{{(CN)}_{6}}]}_{2}}+6N{{a}_{2}}S{{O}_{4}}\] In compounds containing nitrogen and sulphur together, the sodium metal should be in slight excess otherwise in Lassaigne's rest, sodium thiocyanate \[(NaCNS)\] is formed which gives red colour with \[F{{e}^{3+}}\] ions and decomposes as follows: \[NaCNS+2Na\xrightarrow{{}}NaCN+N{{a}_{2}}S\] \[3NaCNS+FeC{{l}_{3}}\xrightarrow{{}}\underset{\begin{smallmatrix}  Ferric\,sulphocyanide \\  (Blood\,red\,in\,colour) \end{smallmatrix}}{\mathop{Fe{{(CNS)}_{3}}}}\,+3NaCl\]