215.
Illustrate with example:
(i) Lyophilic and Lyophobic sols.
(ii) Multimolecular and macromolecular colloids.
(iii) Homogeneous and Heterogeneous catalysis.
(i)
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property
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Lyophilic sols (suspensoid)
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Lyophobic sols (Emulsoid)
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Surface tension
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Lower than that of the medium
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Same as that of the medium
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Viscosity
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Much higher than that of the medium
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Same as that of the medium
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Reversibility
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Reversible
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Irreversible
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Stability
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More stable
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Less stable
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Visibility
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Particles can’t be detected even under ultramicroscope
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Particles can be detected under ultramicroscope.
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(ii) Multimolecular Colloids: When a large number of atoms or small molecules (having diameters of less than 1nm) of a substance combine together in a dispersion medium to form aggregates having size in the colloidal range, the colloidal solutions thus formed are called multimolecular colloids. The species (atoms or molecules) constituting the dispersed particles in multimolecular colloids are held together by Vander Waals’ forces.
The gold sol, sulphur sol etc. are some examples of multimolecular colloids. A gold sol may contain particles of various size composed of several atoms of gold. Similarly, sulphur sol consists of particles containing about a thousand of S8molecules.
Macromolecular Colloids: Certain substances form large molecules whose dimensions are comparable to those of colloidal particles. Such molecules have very high molecular masses and are termed as macromolecules. When such substances are dispersed in suitable dispersion medium, the resulting colloidal solutions are known as macromolecular colloids. Thus, in macromolecular colloids, the dispersed particles are themselves large molecules having very high molecular masses.
(iii)
When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis. The
following are some of the examples of homogeneous catalysis:
(i) Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process.
2SO2(g) + O2(g) 2SO3(g)
The reactants, sulphur dioxide and oxygen, and the catalyst, nitric oxide, are all in the same phase.
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the
examples of heterogeneous catalysis are given below:
(i) Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.
2SO2 (g) 2SO3(g)
The reactant is in gaseous state while the catalyst is in the solid state.
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