Download App

Organic Chemistry: Some Basic Principles and Techniques — Chemistry STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 ScienceChemistryOrganic Chemistry: Some Basic Principles and Techniques5 Marks
Question
Discuss the chemistry of Lassaigne's test.
✓

Answer

The elements nitrogen, sulphur and halogens are tested in an organic compound by Lassaigne’s test. The organic compound (N, S or halogens) is fused with sodium metal as to convert these elements into ionisable inorganic substances, i.e., nitrogen into sodium cyanide, sulphur into sodium sulphide and halogens into sodium halides.

$2\text{Na}+\text{S}\xrightarrow{\Delta}\text{Na}_2\text{S}$

$\text{Na}+\text{C}+\text{N}\xrightarrow{\Delta}\text{NaCN}$

$\text{Na}+\text{X}\xrightarrow{\Delta}\text{NaX}(\text{x}=\text{Cl},\text{Br},\text{I})$

Once the ions are formed, the inorganic tests can be applied to them and the compound can be analysed.

  1. Test for Nitrogen: The sodium fusion extract is boiled with iron(II) sulphate and then acidified with acid. The formation of Prussian blue colour confirms the presence of nitrogen.

6CN- + Fe2+ → [Fe(CN)6]4-

3[Fe(CN)6]4- + 4Fe3+ → Fe4[Fe(CN)6]3.xH2O

  1. Test for Sulphur: The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. Ablack precipitate of lead sulphide indicates the presence of sulphur.

$\text{S}^{2+}+\text{Pb}^{2+}\rightarrow\text{PbS}\downarrow \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Black}$

  1. Test for Halogens: The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the. presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.

X- + Ag+ → AgX

X represents a halogen -Cl, Br or I.

  1. Test for Phosphorus: The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric add and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.

Na3PO4 + 3HNO3 → H3PO4 + 3NaNO3

$\text{H}_3\text{PO}_4 \ \ \ \ + \ \ \ \ 12(\text{NH}_4)_2\text{MoO}_4 \ \ \ \ + \ \ \ \ 21\text{HNO}_3 \ \ \ \ \rightarrow \ \ \ \ (\text{NH}_4)_3\text{PO}_4.12\text{MoO}_3 \ \ \ \ + \ \ \ \ 21\text{NH}_4\text{NO}_3 \ \ \ \ + \ \ \ \ 12\text{H}_2\text{O} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Ammonium molybdate} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Ammonium phosphomoybdate} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(yellow ppt.)}$

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "5 Marks Questions" from Organic Chemistry: Some Basic Principles and Techniques→Organic Chemistry: Some Basic Principles and Techniques — all question types→Chemistry STD 11 Science question papers→Rajasthan Board STD 11 Science question papers→Rajasthan Board question paper generator→

Similar questions

Match the following prefixes with their multiples:
 
Prefix
Multiples
(i)
micro
106
(ii)
deca
109
(iii)
mega
10-6
(iv)
giga
10-15
(v)
femto
10
→
Give reasons for the following:
  1. Covalent bonds are directional bonds while ionic bonds are non- directional.
  2. Water molecule has bent structure whereas carbon dioxide molecule is linear.
  3. Ethyne molecule is linear.
→
Determine the molecular formula of an oxide of iron, in which the mass per cent of iron and oxygen are 69.9 and 30.1, respectively.
Note: Given that the molar mass of the oxide is 159.69g mol-1
→
What transition in the hydrogen spectrum would have the same wavelength as the Balmer transition n = 4 to n = 2 of He+ spectrum?
→
S. No
Column I
Column II
1.
[Mn(CO)5]-
+6
2.
[Mn2(CO)10]
+2
3.
K4[Mn(CN)6]
0
4.
K2MnO4
-1
→
Bromine monochloride, BrCl decomposes into bromine and chlorine and reaches the equilibrium :
$2 BrCl ( g ) \rightleftharpoons Br _2(g)+ Cl _2(g)$
for which $K_c=32$ at 500 K . If initially pure BrCl is present at a concentration of $3.3 \times 10^{-3} mol L ^{-1}$, what is its molar concentration in the mixture at equilibrium?
→
Write the significance/applications of dipole moment.
→
Consider the following equilibrium at 2773K
$\text{H}_2(\text{g})+\text{Cl}_2(\text{g})\rightleftharpoons2\text{HCl}(\text{g})$
Initially 0.25M H2 and 0.25MCl2 are introduced into a reaction vessel and the system in allowed to attain equilibrium. At equilibrium the concentrations of H2(g) and Cl2(g) became 0.0314M. Calculate Kc and Kp.
→
  1. The density of the water at room temperature is 0.1g/ mL. How many molecules are there in a drop of water if its volume is 0.05mL?
  2. An alloy of iron (53.6%), nickel (45.8%) and manganese (0.6%) has a density of 8.17g cm-3

Calculate the number of Ni atoms present in the alloy of dimensions 10.0cm × 20.0cm × 15.0cm

→
Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.
→