Test: Adsorption - 1 - Chemistry MCQ

Physisorption is exothermic in nature. Therefore in accordance with Le-Chatliers principle, it decreases with increase in temperature. This means physisorption occurs at a low temperature more readily.

Adsorption is accompanied with decrease in entropy and enthalpy of system. The value of  TΔS is negative.

Adsorption decreases randomness and thus, ΔS=−ve and also, TΔS = −ve.

• Physical adsorption is due to van der Waals forces which are weak forces, 
• Physical adsorption decreases at high temperature and low pressure
• Physical adsorption is reversible
• Adsorption energy for chemical adsorption is generally more than that for physical adsorption. It is not lesser than that for physical adsorption.

For the adsorption of a gas on a solid x/m ​= kp^1/n. This equation is known as Freundlich Adsorption Isotherm.

On taking logarithm on both sides, we obtain, logx/m ​= logk+ 1/n​logp
∴ plot of log(x/m) versus log p will be linear with slope = 1/n​ .

If the accumulation of gas on the surface of a solid occurs on account of weak van der Waals’ forces, the adsorption is termed as physical adsorption or physisorption.

Hence, Physisorption can also be called as Van der Waals adsorption.

Physical adsorption of a gas by a solid is generally reversible. We can say by Le-Chatelier's principle that more of gas is adsorbed when pressure is increased as the volume of the gas decreases and since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature.

So the correct case is when it is Low temperature and high pressure the Physisorption tends to happen.

According to Langmuir Adsorption isotherm, the amount of gas adsorbed at very high pressures reaches a constant limiting value. The amount of gas adsorbed will not increase with the further increase in pressure

• Physical adsorption decreases with an increase in temperature because an increase in temperature leads to an increase in kinetic energy of molecules and hence, their desorption.
• Since physical adsorption is an exothermic process, it occurs more readily at lower temperatures and decreases with an increase in temperature (Le-Chatelier's Principle).
• If the adsorption is a spontaneous phenomenon, it should be exothermic, at least for adsorption from the gaseous phase. 
• A molecule in the gas phase will have an entropy higher than in the adsorbed state, then the ΔS should be negative, the only way to have a ΔG negative (spontaneous) is to have a ΔH negative (exothermic process).
• Therefore, the process will be spontaneous (ΔG=−ve) if the temperature is low.

• Adsorption of gas on the solid metal surface is spontaneous and exothermic, then ΔS decreases.
• Since for spontaneous and exothermic process, ΔG = −ve, ΔH = −ve at all temperatures.
• Therefore, from ΔG = ΔH − TΔS,ΔS should be −ve.
• Also, the adsorption of a gas on a solid surface gives a more orderly arrangement.

Surface area available for adsorption per gram of catalyst is called specific surface area.
Specific surface area is important to measure to evaluate the activity and adsorption capacity of materials. (e.g. catalysis and adsorbent).

The specific surface area is increased as the particle size becomes small. The specific surface area is also increased if the particle has pores. The surface activity and adsorption volume are changed according to the specific surface area. 

The Langmuir adsorption isotherm is deduced using the assumption that the adsorption sites are equivalent in their ability to absorb the particles.

Langmuir adsorption isotherm shows the following characteristics:

1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.
2. Adsorbed molecules do not interact.
3. Adsorption at all sites occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed. Molecules of adsorbate do not deposit on others, already adsorbed but only on the free surface of the adsorbent.

Given data
p = 1.896cm³g^-1 bar^-1
b = 0.146 bar^-1
 

Substitute in the corresponding equation

Brownian movement:

• Sometimes, when the beam of sunlight enters a room, we can see tiny dust particles suspended in the air which is moving rapidly.
• The tiny dust particles move here and there because they are constantly hit by the fast-moving particles of air.
• The zig-zag movement of the small particles suspended in a liquid (or gas) is called Brownian motion.

• The existence of Brownian motion gives us two conclusions about the nature of matter :
  • That matter is made up of tiny particles, and
  • That the particles of matter are constantly moving.​
• The best evidence for the existence and movement of particles in liquids was given by Robert Brown in 1827.

Hence, the continuous zig-zag movement of colloidal particles in a dispersion medium is called Brownian Motion.

So, the correct option is the Brownian movement.

• The process of adsorption includes the following steps:
  • Diffusion of reactants on the surface of the solid catalyst.
  • Adsorption of the reactants on the surface.
  • Reaction on the surface
  • Desorption of the products from the surface and
  • Diffusion of products away from the surface.
• During the process of adsorption, the molecules which were free get adhered to the surface of the catalyst and become bound by attraction.
• From a free state, they go into a restricted state, and thus the entropy of particles decreases.
• As entropy decreases, the molecules become more stable or less in energy, and thus the excess energy is released in the form of heat, which makes the process exothermic in nature.

Hence, the adsorption of gases on solid surface is exothermic because the entropy decreases.

Classification of colloids based on the type of phases:

• The colloidal system is heterogeneous in nature and consists of two phases called the dispersion medium and the dispersed phase.
• The component that is present in a small amount and behaves like a solute is called the dispersed phase and the component present in excess is called dispersion medium.
• Depending on the nature of dispersed phase and dispersion medium, colloids can be classified as:

• From the above table, it is clear that smoke contains solid particles suspended in the gaseous phase.
• Hence, smoke is a dispersion of solid in gas.

Adsorption:

• It is the process of deposition of molecules of liquid or gases onto the surface of a solid particle.
• Silica and aluminium gels are used to adsorb moisture to reduce humidity.
• Misty windows: Water molecules cling to the window glass.
• Painting: molecules of liquid paint cling to wood, metals, and other materials.
• The chromatographic analysis is based on the phenomenon of adsorption.

• Adsorption is defined as the deposition of molecular species onto the surface.
• The molecular species that gets adsorbed on the surface is known as adsorbate and the surface on which adsorption occurs is known as adsorbent. 
• As adsorption is accompanied by release of heat energy, so in accordance with Le-Chatelier’s principle, the increase of temperature should decrease the extent of adsorption.
• Le-Chatelier’s principle: It states that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint.
• Adsorption is exothermic, so heat can be assumed as product. Now if product (heat or temperature) is increased, equilibrium will shift towards left side (reverse adsorption). Hence, increase in temperature decreases the amount of gas adsorbed.

A sponge will absorb or take in water from another area and put it inside of itself. A dry sponge can hold more water than a wet sponge is closer to saturation and as such cannot hold more water. Sponges with more tiny holes can absorb more water than the sponges with less tiny holes and thus leads to the absorption process.

The Freundlich isotherm was the first isotherm model proposed for sorption processes. It can be applied for non ideal sorption on heterogeneous surfaces, as well as, multilayer sorption. A variation in the slope between 0 and 1 is associated with a chemisorption process, which is more heterogeneous as the value gets closer to 0. Due to the lack in fundamental thermodynamic basis, since there is no approach to Henry’s law at vanishing concentrations, this represents a limitation of this isotherm model.

Chemisorption involves formation of chemical bonds between adsorbate and adsorbent molecules. Once the valency is satisfied, the adsorbent molecules can’t form bond with more adsorbate molecules. Thus only one layer is formed.