NEET Study Notes on Surface Chemistry; Surface chemistry can be defined as the phenomenon of study of processes that occur on the substance surfaces. The interaction of these surfaces causes the chemical phenomenon. The two surfaces can be liquid-gas, gas-vacuum, solid-liquid, solid-gas, etc. Some of the common phenomena that occur at interfaces include Corrosion, Crystallization, Heterogeneous Catalysis, Dissolution, Adsorption, etc.

• The concepts of Surface chemistry also help us in understanding the applications of Surface Engineering.
• NEET Chemistry Syllabus is divided into topics from Class 11th and 12th. Surface Chemistry forms a part of the Class 12th Physics Chemistry Syllabus. The overall weightage of Physics Chemistry is around 32%. Understanding these topics will help you to secure marks from this unit in NEET 2022.

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NEET Study Notes for Surface Chemistry: Important Topics

NEET Study Notes on Adsorbption, Physisorption, Chemisorption

Adsorbption

When molecular species accumulate in higher concentration on the surface of a substance rather than in the bulk of liquids or solids is the phenomenon of absorption. For example- Gases like Hydrogen, Nitrogen, and Oxygen get absorbed on activated charcoal.

• The substance that accumulates on the surface is called the adsorbate whereas the substance that adsorbent is the solid or liquid surface on which the adsorbate accumulates.
• Molar heat of adsorption is the heat that evolves on the adsorption of 1 mole of gas. On adsorption, a decrease in enthalpy and entropy occurs.
• On the basis of the interaction between intermolecular forces between adsorbate and absorbent, adsorption can be classified into the following:

Physical Adsorption or Physisorption

The heat adsorption in Physisorption is small- around 20-40 kJ/mol. Due to the weak forces involved in Physisorption, also known as Van der Waal’s forces, the gas gets concentrated on the solid surface.

• Physisorption is an exothermic process and reversible in nature. Physisorption of gas through solid can also be reversed to solid through the gas.
• The factors that mainly affect Physiorption are surface area and the nature of the adsorbent. The surface area of the adsorbent is directly proportional to the extent of adsorption.
• Physisorption and temperature are indirectly proportional to each other.
• Examples of Physiorption include adsorption of gases like Hydrogen, Oxygen, and nitrogen on adsorbent charcoal.

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Chemical Adsorption or Chemisorption

This process occurs due to the chemical bond between adsorbent and adsorbate. Chemisorption is a specific process as it only occurs on the existence of a chemical bond between absorbent and adsorbate.

• The heat adorned during Chemisorption is high - around 80-240 kJ/mol. At a higher temperature, the gases which underwent physisorption at a lower temperature may be transformed to chemisorption.
• Chemisorption is irreversible in nature.
• Surface area and chemisorption are directly proportional to each other. On an increase in the surface area, chemisorption also tends to increase.
• Chemisorption ad temperature is directly proportional to each other.

Example- adsorption of gases like Hydrogen, nitrogen on the adsorbent catalyst of ferrous at high temperature.

Factors that affect Adsorption of Gases on solids:

• Nature of Gas- Gases that easily undergo the process of liquefaction get easily adsorbed. Due to the specific nature of chemisorption, it only takes place when a chemical reaction between adsorbent and adsorbate occurs.
• Adsorbent Nature- The nature of adsorbents plays a large role in adsorbent efficiency as metals like nickel and platinum that are fine and charcoal, silica gel due to their porous nature acts as better adsorbent due to the large surface area they provide.
• Pressure- On an increase with pressure, the extent of adsorption increases, however, due to the limited absorption sites, after a point the extent of adoption becomes independent of the pressure.
• Temperature- The temperature effect in adoption is different for physical adsorption and chemical adsorption. In physical adsorption, the temperature increase leads to slower Physiorptuon whereas the temperature increase in chemical adsorption leads to higher chemisorption.
• Surface Area- Large surface area leads to a greater extent of adsorption.
• The heat of adsorption- the energy that liberates when 1g gas molecule undergoes adsorption on the solid surface is called heat of adsorption. Adsorption is an exothermic process.

Adsorption Isotherms

At a constant temperature and pressure, the variation in the quantity of gas, that is adsorbed by adsorbent is called Adsorption Isotherms. The two adsorption Isotherms are

• Freundlich Adsorption Isotherm- At a specific temperature and pressure, the relationship between quantities of gas that is adsorbed by unit mass of solid adsorbent is called Freundlich Adsorption Isotherm.
• Langmuir Adsorption Isotherm- In this Isotherm, the interaction between adjacent adsorbed molecules does not occur. Molecule thicker is the adsorbed gas on the solid adsorbent.

Read NEET Study Notes on Chemical Kinetics

Catalysis

In addition to a substance, known as the catalyst the rate of chemical reaction increases, this process is called Catalysis.

Catalyst can also be defined as the substance that does not undergo chemical change however its addition in a chemical reaction, transforms the rate of the reaction. Catalysis can be classified into

• Homogeneous catalysis- The presence of a catalyst in the same state of matter as that of reactants and products is known as Homogeneous catalysis. Some of the examples of homogeneous catalysis include when hydrogen and chlorine are combined in steam presence, Ester Hydrolysis in acid/alkali presence, etc.
• Heterogeneous Catalysis- The presence of catalysts is different than those of reactants and products is known as heterogeneous catalysis. Examples- Ammonia manufacturing by Haber process, dehydrogenation of ethanol, etc.

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Enzyme Catalysts

Nitrogenous, Non-living catalysts that are responsible for reactions like oxidation, reduction, hydrolysis are called Enzyme Catalysts. The process of increasing the rate of reaction with these enzyme catalysts is called enzyme catalysis. Also known as biochemical catalysis, such catalysts are of a highly specific nature

These are the most efficient catalysts which require the appropriate temperature and ph medium for catalysis. Enzyme catalysts act with more efficiency in the presence of coenzymes or activators like Co²⁺, Mn²⁺

Mechanism of Enzyme Catalysis

Enzyme catalysts increase biochemical rate reaction. Enzymes act on substrates The product is formed by dissociating the enzyme-substrate complex.

Colloids

The heterogeneous combination of matter in which particles of the dispersed phase get dispersed into particles of the dispersion medium. The dispersion medium is the colloidal system's continuous phase.

Examples of colloidal systems with their dispersed phase and medium are given below:

Properties of Colloids

• Colloid particles are small and their size ranges between 1-1000 nanometers.
• Heterogeneous mixtures are called colloids.
• Colloid particles can be separated through centrifugation.
• Colloids demonstrate the Tyndall effect as well as the Brownian movement.
• Even if colloid particles are not disturbed, they don't settle.

Colloids can be divided into lyophilic and lyophobic colloids on the basis of dispersion medium and dispersed phase interaction.

• Lyophilic Colloids or Sols- These are solvent or liquid loving and thus the attraction between dispersion medium and dispersion phase is strong. Example- egg albumin sol, start dissolved in water. The viscosity of Lupohilic sols is very high whereas the surface tension is low. Tyndall effect is not demonstrated in Lyopohhilic Sols. These are more stable in comparison to Lyophobic Sols.
• Lyophobic Colloids or Sols- These are solvent or liquid hating colloids that result in less or no affinity between dispersed medium and dispersed phase. Example- ferric hydroxide sol. The viscosity and surface tension is equal to the dispersion medium. Tyndall effect is seen in Lyophobic Colloids. These are less stable.

On the basis of size, colloids can be classified into multimolecular colloids and macromolecular colloids.

• The colloidal particles with a combination of atoms, molecules with diameters less than 1nm are called multimolecular colloids. Examples are- sulfur sol, gold sol.
• Colloidal particles whose size is colloidal size are called macromolecular colloids. Examples- are starch, protein polymers, synthetic polymers, cellulose. Etc.

Check NEET Study Notes on Coordination Compounds

Emulsions

The homogeneous mixture of liquids in which the liquids exhibit property of immiscibility is called Emulsion. When dispersion medium and dispersed phase are of liquid nature, the Emulsion process is used. Examples- Latex, Mayonnaise, Milk, Aerosols, etc.

Properties of Emulsion include:

• Tyndall and Brownian movement is observed in the emulsion.
• With a dispersion medium, dilution can take place.

Emulsions are classified into;

• Simple Emulsions- Simple Emulsions are formed by dispersion of waters in oil. Examples are- Oil, Cold Cream, Butter in water.
• Complex Emulsions- They are formed by the dispersion of oil in water and water in oil. Hence these are also known as multiple emulsions. Surfactants can be used for stabilization.

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What is Tyndall Effect?

Discovered by John Tyndall, the Tyndall Effect demonstrates that when a beam of light is made to pass through a colloid, the colloidal particles scatter the beam of light. The intensity with which this light scatters is dependent on colloidal particle intensity. And incident light frequency.

As the wavelength of blue light is greater, it scatters to a large extent Tyndall effect is caused by colloidal particles which have a diameter ranging from 40 to 900 nanometer.

Examples of the Tyndall Effect include milk, lighting torch in fog, Opalescent glass.

Brownian Movement

When the movement of colloidal particles is in a random zig-zag pattern it is called the Brownian movement. This movement occurs in the dispersion medium. The movement is caused due to collision of dispersed medium particles on dispersed phase particles.

Brownian Movement is affected by the size of particles and temperature. Small particles show rapid movement and an increase in temperature causes more movement.

Brownian movement helps in stabilizing colloidal solutions, Avogadro number determination.

NEET Sample MCQs on Surface Chemistry

Question: The—- of light is demonstrated by Tyndall Effect?

1. Dispersion
2. Refraction
3. Scattering
4. Reflection

Answer: Scattering

Question: What is not a colloid?

1. Boric Acid
2. Mud
3. Butter
4. Milk

Answer: Boric Acid

Question: What does emulsion consist of?

1. Two liquids
2. Two solids
3. One liquid and one gas
4. One liquid and one solid

Answer: two liquids

Question: What will happen to the surface tension when detergent is added to water?

1. Increases
2. Remains the same
3. Decreases
4. Decreases then increase

Answer: decreases

Question: Which is a liquid hating colloid?

1. Hydropihillic
2. Lyophobic
3. Lyophilic
4. None of the above

Answer: Lyophobic

Question: What is the colloid with liquid dispersed phase and solid dispersion medium?

1. Emulsion
2. Sol
3. Foam
4. Gel

Answer: Gel

Question: What does an emulsifier do?

1. Homogenisis of emulsion
2. Coagulates emulsion
3. Stabilization of emulsion
4. Reduces dispersion

Answer: stabilization of emulsion

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