ΔS is -ve

Pyroxylin(nitro cellulose)

As₂S₃ is a negatively charged colloid. It will be most effectively coagulated by the cation with greater valency. i.e., Al³⁺.

dispersion medium-gas
dispersed phase-liquid

Adsorption leads to decrease in randomnes (entropy).i.e, ΔS < 0 for the adsorption to occur, ΔG slhould be -ve. We know that ΔG = ΔH - TΔS if ΔS is -ve, TΔS is +ve.It means that ΔG will become negative only when ΔH is -ve and ΔH > TΔS.

(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

Factors affecting adsorption:
The adsorption is well understood by considering I the various factors affecting it. Qualitatively, the extent of surface adsorption depends on
(i) Nature of adsorbent  
(ii) Nature of adsorbate
(iii) Pressure
(iv) Concentration at a given temperature.
(i) The adsorption is a surface phenomenon: it depends on the surface area of adsorbent.  i.e., higher the surface area, higher is the amount adsorbed.
(ii) Nature of adsorbate:
The nature of adsorbate can influence the adsorption. Gases like SO₂, NH₃, HCI and  CO₂ are easily liquefiable as have greater vander waal's force of attraction. On the other hand, permanent gases like H₂, N₂ and O₂ cannot be liquefied easily. These permanent gases are having low critical temperature and adsorbed slowly, while gases with high critical temperature are adsorbed readily.
(iii) Effect of temperature:
Chemical adsorption is fast with increase pressure, it cannot alter the amount. In Physisorption, when pressure increases the amount of adsorption increases.
(iv) Effect of pressure:
When pressure increases, adsorption becomes steadily increase in case of chemisorption and increases in case of physisorption.

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.

S.No	Physical adsorption or vander waals adsorption or Physisorption	Chemical adsorption or Chemisorption or Activated adsorption
1.	It is instantaneous	It is very slow
2.	It is non-specific	It is very specific depends on nature of adsorbent and adsorbate
3.	In Physisorption, when pressure increases the amount of adsorption increases	Chemical adsorption is fast with increase pressure, it can not alter the amount.
4.	Physisorption decreases with increase in temerature	When temperature is raised chemisorption first increases and then decreases
5.	No transfer of electrons	Chemisorption involves transfer of electrons between the adsorbent and adsorbate
6.	Heat of adsorption is low in the order of 40kJ/ mole.	Heat of adsorption is high i.e., from 40- 400kJ/mole.
7.	Multilayer of the adsorbate is formed on the adsorbent.	Monolayer of the adsorbate is formed
8.	It occurs on all sides	Adsorption occurs at fixed sites called active centres. It depends on surface area
9	Activation energy is insignificant.	Chemisorption involves the formation of activated complex with appreciable activation energy.

Gold number is defined as the number of milligrams of hydrophilic colloid that will just prevent the precipitation of 10 ml of gold sol on the addition of 1 ml of 10 % NaCl solution. Smaller the gold number greater the protective power.

(i) The Continuous rapid Zig - Zag, Chaotic, random and ceaseless motion executed by a colloidal sol particles in the dispersion medium is called Brownian movement.
(ii) The colloidal sol particles are continuously bombarded with the molecules of the dispersion medium and hence they follow a zigzag, random, continuous movement.
Ex: Movement of pollen grains suspended in water.

(i) The surface of colloidal particle adsorbs one type of ion due to preferential adsorption.
(ii) This layer attracts the oppositely charged ions in the medium and hence at the boundary separating the two electrical double layers are setup. This is called as Helmholtz electrical double layer.

(a) Blood is a colloidal sol. When we rub the injured part with moist alum, coagulation of blood takes place.
(b) Coagulation stops bleeding. Moist alum is an electrolyte.

By addition of suitable electrolytes precipitated particles can be brought into colloidal state. The process is termed as peptisation and the electrolyte added is called peptising or dispersing agent.
\(\underset { Precipitate }{ AgCl } \overset { HCl }{ \longrightarrow } \underset { Colloid }{ AgCl } \)

1. Distilling of one component
2. Adding an electrolyte to destroy the charge.
3. Destroying the emulsifier using chemical methods.
4. Using solvent extraction to remove one component.
5. By freezing one of the components.
6. By applying centrifugal force.
7. Adding dehydrating agents for water in oil (W/O) type.
8. Using ultrasonic waves.
9. Heating at high pressures.

An electrical arc is struck between electrodes dispersed in water surrounded by ice. When a current of 1 amp 100 V is passed an arc produced forms vapours of metal which immediately condense to form colloidal solution. Ex copper, silver, gold, platinum, etc. can be prepared.

Homogeneous catalysis :
In a homogeneous catalysed reaction, the reactants, products and catalyst are present in the same phase.
Illustration (1):
\(2 \mathrm{SO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}+[\mathrm{NO}]_{(\mathrm{g})} \rightarrow 2 \mathrm{SO}_{3(\mathrm{~g})}+[\mathrm{NO}]_{(\mathrm{g})}\)
In this reaction the catalyst NO, reactants, SO₂ and O₂, and product, SO₃ are present in the gaseous form.
Illustration (2):
In the decomposition of acetaldehyde by I₂ catalyst, the reactants and products are all present in the vapour phase.
\(\mathrm{CH}_{3} \mathrm{CHO}_{(\mathrm{g})}+\left[\mathrm{I}_{2}\right]_{(\mathrm{g})} \rightarrow \mathrm{CH}_{4(\mathrm{~g})}+\mathrm{CO}_{(\mathrm{g})}+\left[\mathrm{I}_{2}\right]_{(\mathrm{g})}\)
Let us consider some examples in which the reactants, products and catalyst are present in aqueous solution.
(1) Hydrolysis of cane sugar with a mineral acid as catalyst.
\(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11(l)}+\mathrm{H}_{2} \mathrm{O}_{(l)} \stackrel{\mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6(l)}+\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6(l)}\\ Sucrose \quad \quad \quad \quad \quad \quad \quad \quad \quad Glucose \quad \quad Fructose\)
(2) Ester hydrolysis with acid or alkali as catalyst.
\( \mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5(l)}+\mathrm{H}_{2} \mathrm{O}_{(l)} \stackrel{\mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH}_{(l)}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}_{(l)}\\ Ethyl \ acetate \quad \quad \quad \quad \quad \quad \quad \quad acetic \ acid \quad \quad ethyl alcohol\)

(i) Positive catalyst: A catalyst which enhances the speed of the reactions called positive catalyst and the phenomenon is known as positive catalysis Eg: Decomposition of KClO₃ in the presence of manganese dioxide (MnO₂).
Eg: Decomposition of KCIO₃ in the presence of manganese dioxide (MnO₂).
\(2KCl{ O }_{ 3 }\overset { Mn{ O }_{ 2 } }{ \underset { \Delta }{ \longrightarrow } } 2KCl+{ 3O }_{ 2 }\uparrow \) 
(ii) Negative catalyst: A catalyst which retard (or) reduce the reaction rate is known as negative catalyst or inhibitors and the phenomenon is known as negative catalysis.
Eg: Decomposition of H₂O₂ decreases in the presence of glycerine.
\({ 2H }_{ 2 }{ O }_{ 2 }\overset { Glycerine }{ \longrightarrow } { 2H }_{ 2 }O+{ O }_{ 2 }\)

(i) Certain substances when added to a catalysed reaction decreases or completely destroys the activity of catalyst and they are often known as catalytic poisons.
For example,
(ii) In the reaction, 2SO₂ + O₂ ⟶ 2SO₃ with a Pt catalyst, the poison is As₂O₃
(iii) i.e., As₂O₃ destroys the activity of Pt. As₂O₃ blocks the activity of the catalyst. So, the activity is lost.

A sol is electrically neutral. Hence the medium carries an equal but opposite charge to that of dispersed particles. When sol particles are prevented from moving, under the influence of electric field the medium moves in a direction opposite to that of the sol particles. This movement of dispersion medium under the influence of electric potential is called electro osmosis.

(i) In lyophillic colloids or sols definite attractive force or affinity exists between dispersion medium and dispersed phase. Examples: sols of protein and starch. They are more stable and will not get precipitated easily. 
(ii) In a lyophobic colloids, no attractive force exists between the dispersed phase and dispersion medium. They are less stable and precipitated readily, but cannot be produced again by just adding the dispersion medium. 
Examples: sols of gold, silver, platinum and copper.