Surface Chemistry
MCQsAnswer: The phenomenon of accumulation of the molecules of a substance on a solid or liquid surface resulting in the increased concentration of the molecules on the surface is called adsorption. In absorption, the substance is uniformly distributed throughout the bulk of the solution. A distinction can be made by taking an example of water vapours. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel.
Answer: The extent of adsorption increases with increase in surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents.
Answer: Factors affecting adsorption of a gas on solids are:
Nature of the adsorbent: The same gas is adsorbed to different extents by different solids at the same temperature. Also, greater the surface area of the adsorbent, more is the gas adsorbed.
Nature of the adsorbate: Different gases are adsorbed to different extents by different solids at the same temperature. Higher the critical temperature of the gas, greater is its amount adsorbed.
Temperature: Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.
Specific area of the adsorbent: Surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. Greater the surface area, higher would be the adsorption therefore, porous or powdered adsorbents are used.
Pressure: At constant temperature, the adsorption of gas increases with pressure.
Activation of adsorbent: It means increasing the adsorbing power of anadsorbent by increasing its surface area. It is done by:
a. making the adsorbent’s surface rough
b. removing gases already adsorbed
c. subdividing the adsorbent into smaller pi
Answer: During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat. Adsorption therefore, is invariably an exothermic process. In other words, ΔH of adsorption is always negative to keep the value of ΔG negative for the reaction to be spontaneous as ΔS decreases during adsorption.
ΔG = ΔH − TΔS
Answer: Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.
Answer: Adsorption decreases with an increase in temperature because it is an exothermic process and according to Le Chatelier’s principle the reaction will proceed in backward direction with increase in temperature. However at a constant temperature, adsorption increases with pressure.
Answer: There are two types of colloidal sols:
a. Lyophilic sols: The word lyophilic means solvent loving. They are obtained by direct mixing the dispersed phase and the dispersion medium, e.g., sols of gum, gelatin, starch, etc. They are solvent attracting hence quite stable and cannot be coagulated easily.
b. Lyophobic sols: They cannot be prepared by direct mixing the dispersed phase and dispersion medium but are prepared by special methods, e.g., sols of metals. They are solvent repelling. Hydrophobic sols are easily coagulated due to repulsion between water and dispersed phase.
Answer: Depending upon the type of particles of the dispersed phase, colloids are classified as : multimolecular, macromolecular and associated colloids.
a. Multimolecular colloids: On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1nm). The species thus formed are called multimolecular colloids. For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur molecules.
b. Macromolecular colloids: Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples of naturally occurring macromolecules are starch, cellulose, proteins and enzymes; and those of man-made macromolecules are polythene, nylon, polystyrene, synthetic rubber, etc.
c. Associated colloids (Micelles): There are some substances which at low concentrations behave as normal strong electrolytes but, at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10–4 to 10–3 mol L–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.
Answer: Few impurities which are soluble in water and are adsorbed on the surface of the precipitate are removed by washing them with water.
Answer: Colloid is not a substance, but a state of substance because the same substance may exist as a colloid or crystalloid under different conditions e.g., sulphur. Colloidal solution of sulphur consists of sulphur molecules dispersed in water. In this state, sulphur atoms combine to form multimolecules whose size lies between 1 nm to 1000 nm and form colloidal state. Sulphur forms true solution in carbon disulphide. Similarly soap is a solution at low concentration but a colloid at higher concentration.
Answer:
Alcosol: The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.
Aerosol: The sol in which dispersion medium is gas and dispersed phase is either solid or liquid, the colloidal system is called aerosol e.g., fog, insecticides, sprays, etc.
Hydrosol: The sol in which dispersion medium is water is called hydrosol e.g., starch sol. a. Alcosol : The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.
Read the given passages and answer the questions that follow.
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. The interface or surface is represented by separating the bulk phase by a hyphen or a slash. For example, the interface between a solid and a gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases. The bulk phase that we come across in surface chemistry may be pure compounds or solutions. The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces. There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules remain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent. Adsorption of gases by solids depends on nature and surface area of the adsorbent, nature of the gas being adsorbed, temperature, pressure and activation of the solid adsorbent. When a gas is held on surface of a solid by van der Waals forces, it is called as physical adsorption while if a gas is held on the surface of a solid by chemical bond, it is called as chemical adsorption.
How do the size of particles of adsorbent influence the extent of adsorption of a gas on a solid?
Smaller the size of the particles of the adsorbent, greater is the surface area and greater is the adsorption.
Why is physical adsorption multilayered?
Physical adsorption involves van der Waals forces, so any number of layers may be formed one over the other on the surface of the adsorbent.
Which has a higher enthalpy of adsorption, physisorption or chemisorption?
Chemisorption due to the involvement of chemical bond formation.
How does an increase in temperature affect both physical as well as chemical adsorption?
With increase in temperature, physical adsorption decreases while chemical adsorption first increases and then decreases.
What is the importance of having clean surface in surface studies?
Clean surface eases the adsorption of adsorbate on the adsorbent.
A colloidal solution is a heterogeneous system in which a definite substance is distributed in the form of very small particles or dispersed phase in another substance called the dispersion medium. If water being the medium, colloids are called hydrosols. Colloids are of two types on the basis of nature of interaction between dispersed phase and dispersion medium: Lyophobic sols and Lyophilic sols. In Lyophobic sols, particles of dispersed phase have no affinity for dispersion medium rather they hate dispersion medium. They are not easily prepared and need stabilizing agents for their preservation. They are irreversible. They are also called extrinsic colloids while in Lyophilic sols, particles of dispersed phase have great affinity for the dispersion medium. They are self-stabilized because of strong attractive forces operating between the suspended particles and the dispersion medium. They are reversible in nature. They are also known as intrinsic colloids.
Write the main reason for the stability of colloidal sols.
All the particles of colloidal sol carry the same charge so they keep on repelling each and other and do not aggregate together to form bigger particles.
Write the dispersed phase and dispersion medium of smoke.
Dispersed phase—Solid; Dispersion medium—Gas.
What type of colloid is formed when a liquid is dispersed in a solid? Give an example.
Gel, e.g., cheese, jellies
In what way is a sol different from a gel?
Colloidal system in which solid is dispersed in liquid is called sol and that in which liquid is dispersed in solid is called gel.
Hydrophobic sol is easily coagulated. Give reason.
Hydrophobic sol are unstable because they do not undergo hydration. Hence, they are easily coagulated by addition of a small amount of electrolyte, by heating or by shaking.
