Scattering of light

\(\frac{x}{m}\) = K.p^1/n
log \((\frac{x}{m})\) = log K + \(\frac{1}{n}\), log p
y = c + mx
m = 1/n and c = log K

(i) In this method, the colloidal solution is taken in a bag made up of semipermeable membrane.
(ii) It is suspended in a trough of flowing water, the electrolytes diffuse out of the membrane and they are carried away by II water.

The colloidal solutions due to their different methods of preparation may contain impurities. If they are not removed, they may destablise and precipitate the colloidal solution.

2Cu + \(\frac{1}{2}\)O₂   → Cu₂O
It is an intermediate.
Cu₂O + H₂  → H₂O + 2Cu

2KClO₃ + 6MnO₂   →  6MnO₃ + 2KCl
It is an intermediate
6MnO₃ → 6MnO₂ + 3O₂

In chemical adsorption, \(\frac { x }{ m } \) increases with rise in temperature and then decreases. The increase illustrates the requirement of activation of the surface for adsorption is due to fact that formation of activated complex requires certain energy. The decrease at high temperature is due to desorption, as the kinetic energy of the adsorbate increases.

(i) Effective and efficient conversion is the special characteristic of enzyme catalysed reactions. An enzyme may transform a million molecules of reactant in a minute 
For Eg: \({ 2H }_{ 2 }{ O }_{ 2 }\longrightarrow { 2H }_{ 2 }O+{ O }_{ 2 }\)
For this reaction, the activation energy is 18k cal/mole without a catalyst  With colloidal platinum as a, catalyst the activation energy is 11.7kcal /mole. But with the enzyme catalyst the activation energy of this reaction is less than 2kcal/ mole.
(ii) Enzyme catalysis is highly specific in nature.
(iii) Enzyme catalysed reaction has maximum rate at optimum temperature
(iv) The rate of enzyme catalysed reactions varies with the pH of the system. The rate is maximum at a pH called optimum pH.
(v) Enzymes can be inhibited i.e. poisoned activity of an enzyme is decreased and destroyed by a poison. The physiological action of drugs is related to their inhibiting action.
(vi) Catalytic activity of enzymes is increased by coenzymes or activators.

(i) For a chemical reaction, catalyst is needed in very small quantity.
(ii) There may be some physical changes, but the catalyst remains unchanged in mass and chemical composition in a chemical reaction.
(iii) A catalyst itself cannot initiate a reaction. 
(iv) A solid catalyst will be more effective if it is taken in a finely divided form.
(v) A catalyst are specific in nature.
(vi) In an equilibrium reaction, presence of catalyst reduces the time for attainment of equilibrium and hence it does not affect the position of equilibrium and the value of equilibrium constant.
(vii) A catalyst is highly effective at a particular temperature called as optimum temperature.
(viii) Presence of a catalyst generally does not change the nature of products

The intermediate compound formation theory :
A catalyst acts by providing a new path with low energy of activation. In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or decompose to give products and the catalyst is regenerated.
Consider the reactions :
A+B➝AB; C is the catalyst .............(1)
A + C ➝ AC (intermediate) ..........(2)
AC + B ⟶ AB + C ...............(3)
Example 1:
The mechanism of Fridel crafts reaction is given below
\({ C }_{ 6 }{ H }_{ 6 }+{ { CH }_{ 3 }Cl\overset { anhydrous\\ { AlCl }_{ 3 } }{ \longrightarrow } }{ C }_{ 6 }{ H }_{ 5 }{ CH }_{ 3 }+HCl\) 
The action of catalyst is explained as follows
CH₃Cl + AlCl₃ ⟶ [CH₃]⁺ [AlCl₄]^-
It is an intermediate.
\({ C }_{ 6 }{ H }_{ 6 }+\left[ { CH }_{ 3 }^{ + } \right] \left[ { AlCl }_{ 4 } \right] ^{ - }\longrightarrow { C }_{ 6 }{ H }_{ 5 }{ CH }_{ 3 }+{ AlCl }_{ 3 }+Hcl\) 
Example 2:
\({ { 2KClO }_{ 3 }\overset {\\ { MnO }_{ 3 } }{ \longrightarrow } }{ 2KCl }+{ 3O }_{ 2 }\)
Thermal decomposition of KCIO₃ in the presence of MnO₂ proceeds as follows. Steps in the reaction
2KCIO₃ ⟶ 2KCI + 3O₂ Can be given as
2KClO₃ + 6MnO₂   →  6MnO₃ + 2KCl
It is an intermediate
6MnO₃ → 6MnO₂ + 3O₂
Example 3:
Formation of water due to the reaction of H₂ and O₂ in the presence of Cu can be given as
H₂ + 1/2O₂ → H₂O 
2Cu + \(\frac{1}{2}\)O₂   → Cu₂O
It is an intermediate.
Cu₂O + H₂  → H₂O + 2Cu
Advantages:
This theory describes
(a) The specificity of a catalyst and
(b) The increase in the rate of the reaction with increasc inthe concentration of a catalyst
Limitations:
(a) The intermediate compound theory fails to explain the action of catalytic poison and activators (promoters).
(b) This theory is unable to explain the mechanism of heterogeneous catalysed reactions.

(i) Zeolites are microporous, crystalline, hydrated, alumino silicates, made of silicon and aluminium tetrahedra.
(ii) There are about 50 natural zeolites and 150 synthetic zeolites.
(iii) As silicon is tetravalent and aluminium is trivalent, the zeolite matrix carries extra negative charge.
(iv) To balance the negative charge, there are extra framework cations for example H⁺or Na⁺ons. Zeolites carrying protons are used as solid acids, catalysis and they are extensively used in the petrochemical industry for cracking heavy hydrocarbon fractions into gasoline, diesel, etc.,
(v) Zeolites carrying Na⁺ ions are used as basic catalysis.
(vi) One of the most important applications of zeolites is their shape selectivity.
(vii) In zeolites, the active sites namely protons are lying inside their pores. So, reactions occur only inside the pores of zeolites.
Reactant selectivity:
When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal, this selectivity is called reactant shape selectivity.
Transition state selectivity:
If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed.
Product selectivity:
It is encountered when certain product molecules one too big to diffuse out of the zeolite pores.

Active centres:
The surface of a catalyst is not smooth. It bears steps, cracks and corners. Hence the atoms on such locations of the surface are co-ordinatively unsaturated. So, they have much residual force of attraction. Such sites are called active centres. So, the surface carries high surface free energy. The presence of such active centres increases the rate of reaction (activity) by adsorbing and activating the reactants.
The adsorption theory explains the following:
(i) Increase in the activity of a catalyst by increasing the surface area. Increase in the surface area of metals and metal oxides by reducing the particle size increases the rate of the reaction.
(ii) The action of catalytic poison occurs when the poison blocks the active centres of the catalyst.
(iii) A promoter or activator increases the number of active centres on the surfaces
Selectivity:
A Catalyst can catalyse a particular type of reaction. Hence they are said to the specific (selectivity) in nature. Enzyme catalysis is highly specific in nature.
\(\mathrm{NH}_{2} \mathrm{CONH}_{2}+\mathrm{H}_{2} \mathrm{O} \stackrel{\text { Unease }}{\longrightarrow} 2 \mathrm{NH}_{3}+\mathrm{CO}_{2}\)
The enzyme urease which catalyses their reaction of Urea does not catalyse the reaction of methyl Urea.
\(\mathrm{NH}_{2} \mathrm{CONH} \mathrm{CH_3}+\mathrm{H}_{2} \mathrm{O} \stackrel{\text { Urease }}{\longrightarrow} \text { No reaction }\)
Intermediate compound formation theory explains the specificity of a catalyst.

(i) A negative ion causes the precipitation of positively charged sol and vice versa.
(ii) When the valency of ion is high, the precipitation power is increased. For example, the precipitation power of some cations and anions varies in the following order.
\({ Al }_{ 3 }+>Ba^{ 2+ }+>Na^{ + },Similarly[Fe(CN)_{ 6 }]^{ 3- }>SO_{ 4 }^{ 2- }>{ Cl }^{ - }\)
(iii) The precipitation power of electrolyte is determined by finding the minimum concentration (millimoles/lit) required to cause precipitation of a sol in 2 hours.
(iv) This value is called flocculation value. The smaller the flocculation value greater will be precipitation

(i) Using a colloid mill, the solid is ground to colloidal dimension.
(ii) The colloid mill consists of two metal plates rotating in opposite direction at very high speed of nearly 7000 revolution I minute.
(iii) The colloidal particles of required colloidal size is obtained by adjusting the distance between two plates.
(iv) By this method, colloidal solutions of ink and graphite are prepared.