Surface Chemistry: Colloids, Emulsions, Adsorption and Catalysis

The study of the phenomenon which takes place on the surfaces of any two substances is known as surface chemistry. It involves observation of a phenomenon that occurs at the phenomenon of any two surfaces. The two surfaces can be solid-gas, solid-liquid, liquid-gas, solid-vacuum, etc. Catalysis and Chromatography are few processes that work on the principles of surface chemistry. A lot of phenomenon takes place at the surfaces of two substances.

Key Terms: Catalysis, Chemical Reactions, temperature, Chromatography, emulsions, crystallization

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Phenomena taking place at the surface

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Adsorption - The process of attraction and aggregation of the molecules of a substance on the surface of a solid is known as adsorption.

          Example - N₂ adsorbs on the surface of activated charcoal.

N₂ adsorbs on the surface of activated charcoal

There are two types of adsorption based on the forces of interaction between the adsorbent and the adsorbate. The types are

• Chemisorption: It is also known as Chemical Adsorption. In this type of adsorption, there is a presence of strong chemical forces between the adsorbent and the adsorbate. It is a single-layered process. It is also irreversible and highly specific in nature.
• Physisorption: It is also known as Physical Adsorption. In this type of adsorption, there is a presence of weak van der Waals force between the adsorbent and the adsorbate. It is a multi-layered process. It is also reversible and is not specific in nature. 

Types of Adsorption

1. Corrosion - The process through which the refined metals convert themselves into more stable compounds is known as corrosion. 
2. Crystallization - The type of technique that is used in order to purify the substances to separate Solids from liquids is known as crystallization
3. Heterogeneous Catalysis - The process wherein a catalyst is used in order to increase the rate of a chemical reaction is known as catalysis. The catalyst does not undergo any transformation and can be recovered in a chemically unchanged state. 

There are two types of catalysis:-

• When the catalyst involved and the reacting substances are same states of matter or in the same phase, it is known as Homogeneous Catalysis
• When the catalyst involved and the reacting substances are in different states of matter or different phases, it is known as Heterogeneous Catalysis.

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Emulsions in Surface Chemistry

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Emulsions are known as those colloidal solutions where both the dispersion medium and the dispersed phase are in a liquid state. Milk and paint are examples of emulsions.

There are two types of emulsions based on the dispersion medium. They are-

• Water in Oil emulsions: In this type of emulsion, the droplets of water are dispersed in the oil. An example of this type of emulsion is Vanishing Cream.
• Oil in Water emulsions: In this type of emulsion, the droplets of oil are dispersed in the water. An example of this type of emulsion is Milk.

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Applications of Surface Chemistry

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Various applications of surface chemistry are-

• Catalysis

When reactants combine, in order to form products, they have to overcome a certain level of energy, this is known as activation energy. It refers to the difference of energy that is between the reactant species and the transition state. Sometimes, when the reactants don’t have enough energy and they can’t overcome the barrier, we add a substance to increase the rate of reaction by lowering the activation energy. This substance is known as a catalyst and this process is known as catalysis.

• Chromatography

The technique where the compounds are separated, purified, and tested is known as chromatography. Here, the mixture that is to be separated is applied on a stationary phase, and a pure solvent moves over it slowly. As per their Solubility, the components are carried away in a pure solvent such as water.

• Colloid Formation

A colloid is a heterogeneous mixture. Here, the infinitesimal particles of a substance are scattered in another medium which is known as the dispersion medium.

1. It is used in electrode Reactions.
2. Microchips in computer.
3. In automobile exhausts.

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Things to Remember

• Chemical changes which takes place at the surface of two mediums is called surface chemistry.
• Some of the surface chemical changes are adsorption, corrosion, crystallization, heterogeneous catalysis.
• Emulsions are known as those colloidal solutions where both the dispersion medium and the dispersed phase are in a liquid state. 
• Emulsions are classified into oil in water and water in oil emulsions.
• Surface chemistry is used in chromatography, catalysis, colloidal formation, etc.

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Previous Year Questions

1. What is the half-life period of the reaction in minutes?
2. What is the rate of the reaction (in Ms−1) during this interval?
3. The correct statement with regard to this relation is​
4. In a first order reaction 80% of the reactant at an instant was reduced to 8% in 4606 seconds. The rate constant of the reaction is​
5. The rate law expression for the sugar inversion can be written as
6. ​The correct rate law of the reaction is
7. What is the total order of the reaction?​
8. The time required to reduce initial concentration by a factor of 1/16 is​
9. If the temperature is increased from 10∘C to 100∘C, the rate of the reaction will become .​
10. The rate of production of C during this perio​d is
11. Which of the following is the rate law for this reaction?​
12. The time taken for 75% conversion in minutes is​
13. The unit, molL−1s−1 is meant for the rate constant of the reaction having the order​
14. The half-life of a reaction is inversely proportional to the square of the initial concentration of the reactant. Then the order of the reaction is
15.  When the pressure is 40kPa , the half-life period is 175s . The order of the reaction is​
16. Rate constant kk is equal to collision frequency A if Ea=∞E 44. In kvsTkvsT is a straight line 55. In kvs1/Tkvs1/T is a straight line Correct statements are
17. What is the value of rate constant?​
18. For a zero order reaction, the plot of concentration of reactant vs time is (intercept refers to concentration axis)
19. Half-lives of a first order and a zero order reactions are same. Then, the ratio of the initial rates of first order reaction to that of the zero order reaction is
20. The potential energy diagrams for four reactions are given below Which one of the following statements about these diagrams is incorrect?​​