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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 FeSO_(4) 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. Teacherasked 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. |
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Answer» Solution :If the organic compound contains both N and S, then during fusion it may form either sodium thiocyanate (NaSCN) or a mixture of sodium cyanide (NaCN) and sodium SULPHIDE `(Na_(2)S)` depending upon the amount of sodium metal used. If the sodium metal used. If the sodium metal used is less, only NaSCN is produced. This then reacts with `Fe^(3+)` ions (produced by oxidation of `Fe^(2+)` ion during preparation of Lassaigne's extract) to give red colouration due to the FORMATION of ferric thiocyanate. `Fe^(2+) overset("AERIAL oxidation")RARR Fe^(3+), Fe^(3+) + 3NaSCN rarr underset("(Red colouration)")underset("Ferric thiocyanate")(Fe(SCN)_(3))+3Na^(+)` However, if excess of sodium metal is used, the initially formed sodium thiocyanate decomposes to form a mixture of sodium cyanide and sodium sulphide. `underset("Sod. thiocyanate")(NaSCN)+2Naoverset(Delta)rarr underset("Sod. cyanide")(NaCN)+underset("Sod. sulphide")(Na_(2)S)` NaCN thus formed then reacts with `FeSO_(4)`, more NaCN and `Fe^(3+)` ions to give ultimately prussian blue colour due to the formation of ferric ferocyanide or IRON (III) hexacyanoferrrate (II). `2NaCN + FeSO_(4) rarr Na_(2)SO_(4) + Fe(CN)_(2)` `Fe(CN)_(2) + 4NaCN rarr underset("Sod. hexacyanoferrate (II)")(Na_(4)[Fe(CN)_(6)])` `3Na_(4)[Fe(CN)_(6)]+4 Fe^(3+) rarrunderset("(Prussian blue)")underset("Iron (III) hexacyanoferrate (II)")(Fe_(4)[Fe(CN)_(6)]_(3)+12 Na^(+)` From the above discussion, it follows that Manish and Rajni used excess sodium and hence formed NaCN in the Lassaigne's extract which gave prussian blue colour due to the formation of iron (III) hexacyanoferrate (II) while Ramesh used less sodium and hence formed NaSCN in the Lassaigne's extract which gave red colouration due to the formation of ferric thiocyanate. |
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