Surface Chemistry Test - 40

Hydrophillic colloid particles are attracted toward water. They are also called reversible sols.

Aerosol contains small particles of liquid or solid dispersed in a gas.

These are opposite in nature to hydrophilic colloids. The colloid particles are repelled by water. They are also called irreversible sols.

Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid.

A micelle or micella is an aggregate of surfactant molecules dispersed in a liquid colloid.

This observation of Hardy and Schulze are known as Hardy Schulze law. It can be defined as: Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation. 

• When the reactants, products and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis.
  
• In Heterogeneous catalysis, the reactants, products, and catalyst are in different phase. It is a process occurring at the interface.
• Corrosion and crystallization are the processes which occur at the interface of phases.
• Hence, option C is the right answer.

Gas gas

Gas-gas interface cannot be obtained as they are completely miscible in nature. 

For example, air is a mixture of various gases such as, $$O_2,  N_2,  CO_2$$ etc.

• The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.
  
• During adsorption, there is always a decrease in residual forces of the surface, i.e., there is a decrease in surface energy which appears as heat. Hence, adsorption is an exothermic process.
• So the value of $$\Delta H$$ is -ve
• On the basis of the equation, $$\Delta G=\Delta H-T\Delta S$$ can be negative if $$\Delta H$$ has sufficiently high negative value as $$\Delta H-T\Delta S$$ is positive. Thus, in an adsorption process, which is spontaneous, a combination of these two factors makes $$\Delta G$$ negative. As the adsorption proceeds,  $$\Delta H$$ becomes less and less negative ultimately  $$\Delta H$$ becomes equal to $$T\Delta S$$ and $$\Delta G$$  becomes zero. At this state, equilibrium is attained. 
• Thus at equilibrium, $$\Delta H=T\Delta S$$

Involved van der waals forces are universal.

This phenomenon involves the use of weak Van der Waal forces by means of which gas molecules get adsorbed on a solid surface.

There is no specificity as any gas can be adsorbed onto the surface.

• During adsorption, there is always a decrease in residual forces of the surface, i.e., there is a decrease in surface energy which appears as heat. Adsorption, therefore, is invariably an exothermic process. In other words, $$\Delta H$$ of adsorption is always negative.
  
• The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases (i.e., with higher critical temperatures) are readily adsorbed as van der Waals’ forces are stronger near the critical temperatures.
  
• In adsorption, more of gas is adsorbed when pressure is increased as the volume of the gas decreases (By Le–Chateliers’s principle) and the gas can be removed by decreasing pressure. Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature.
  
• Hence, option D is the right answer.