Question

a) Explain bragg's spectrometer method.
b) Write about the preparation of colloids by chemical methods.

Answer 1

In order to answer this question, you must explain the Bragg’s Spectrometer method in detail and also go through the preparation of colloids by chemical methods. Basically, there are $4$ methods of preparation of colloids by chemical methods. Go through them one by one and then write each of them in brief.

Complete step-by-step answer: Step 1. In this step we will explain the Bragg’s Spectrometer method:
Bragg's spectrometer: This method is used for studying crystals using X-rays.
The apparatus consists of $X - Ray$ tube from which a narrow beam of X-rays is allowed to fall on the crystal mounted on a rotating table. The rotating table is provided with scale and vernier from which the angle of incidence θ can be measured.
The arm which is rotating about the same axis as the crystal table, carries an ionization chamber. The rays reflected from the crystal enter into the ionization chamber and ionize the gas present inside. Due to the ionization, current produced is measured by electrometer.
The current of ionization is a direct measure of intensity of reflected beam from the crystal. For different angles of incidence, the corresponding ionization current is measured from the electrometer. These values are plotted in the form of graphs.
For $NaCl$ , the maximum reflection for peaks for $100$ plane occurred at $\theta = {5.9^ \circ }$ , ${11.85^ \circ }$ and ${18.15^ \circ }$ . The sines of these angles are $0.103$ , $0.205$ and $0.312$ which are in the ratio $1:2:3$ . These peaks represent the first, second and third order reflection. The ratio confirms the correctness of Bragg's equation.
Step 2: Preparation of colloids by chemical methods:
(i) Double decomposition: An arsenic sulphide sol is prepared by passing a slow stream of hydrogen sulphide gas through a cold solution of arsenious oxide. This is continued till the yellow colour of the sol attains maximum intensity.
$A{s_2}{O_3} + 3{H_2}S\,\, \to \,\,A{s_2}{S_3} + 3{H_2}O$
Excess ${H_2}S$ is removed by passing in a stream of hydrogen.
(ii) Oxidation: A colloidal solution of Sulphur is made to pass through an aqueous solution of sulphur dioxide. It can also be obtained by passing the gas through a solution of an oxidization agent such as bromine water as well as nitric acid. A colloidal solution of sulphur is obtained by passing ${H_2}S$ Into a solution of $S{O_2}$ :
$2{H_2}S + \,S{O_2}\,\, \to \,\,2{H_2}O\, + \,3S$
${H_2}S\,\, + \,[O]\,\, \to \,\,{H_2}O\,\, + S$
(iii) Reduction: Another technique of preparing colloidal solutions of metals such as silver, gold as well as platinum involves the use of reducing agents for reduction of the salt solutions of these metals. Examples of reducing agents include stannous chloride. Silver sol and gold sol can be obtained by treating dilute solution of silver nitrate or gold chloride with organic reducing agents like tannic acid or formaldehyde.
$AgN{O_3} + \,\,\operatorname{Tannic}\,Acid\,\, \to \,\,AgSol$
$AuC{l_3} + \,\,\operatorname{Tannic}\,Acid\,\, \to \,\,AuSol$
(iv) Hydrolysis: Colloidal solutions of the hydroxides of $Fe,Cr,Al$ etc. can be prepared by hydrolysis of their salts. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.
$FeC{l_3} + \,3{H_2}O\,\, \to \,\,Fe{(OH)_3} + \,3HCl$

Note: Bragg's law, or Wulff–Bragg's condition, a special case of Laue diffraction, gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move, as does any electromagnetic wave.