Chemistry Test - 5

Concept:

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.
• It is a surface phenomenon and occurs due to the unsaturation of forces on the surface of an adsorbate molecule.

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• There are two types of adsorption:
  • Physical or physisorption where there exist only weak van der Waals forces between adsorbent and adsorbate.
  • Chemical or chemisorption where new bonds are formed between adsorbent and adsorbate.

Explanation:

• 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.

Concept:

Adsorption is defined as the deposition of molecular species onto the surface.
The molecular species that gets adsorbed on the surface is known as Adsorbent and the surface on which adsorption occurs is known as Adsorbate.

Types of Adsorption

On the basis of interaction forces between adsorbate and adsorbent, adsorption is of two types. 

Physical adsorption:

This type of adsorption is also known as physisorption. It is due to weak Van der Waals forces between adsorbate and adsorbent. 

For example, H2 and N2 gases adsorb on coconut charcoal.

Chemical adsorption:

This type of adsorption is also known as chemisorption. It is due to strong chemical forces of bonding type between adsorbate and adsorbent.

We can take the example involving the formation of iron nitride on the surface when iron is heated in N2 gas at 623 K. 

Additional Information

Adsorption of gas on a solid is a spontaneous exothermic reaction.

Amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.

The process of removal of adsorbent from the surface of adsorbate is known as Desorption.

Concept:

1. Colloidal medicines:

Medicines are more effective in colloidal state because colloids have a larger surface area. Thus, they get easily assimilated, absorbed and digested.

Thus, option (a) is wrong.

2. Addition of alum:

Alum is used in the form of Aluminium Sulphate (usually) to remove particulates in water through flocculation to remove solids and some ions and allow them to be filtered or settled. This process is a part of water treatment process.

Excess of alum can make water unfit for drinking but in terms of excess is not mentioned in the option.

Thus, option (b) is wrong.

3. Electrophoresis:

The coagulation of lyophobic sols can be done by electrophoresis where the colloidal particles move towards oppositely changed electrodes which gets discharged and forms precipitate.

Thus, option (c) is correct.

4. Brownian motion:

The viscosity is also inversely proportional to the speed of Brownian motion. So, lesser the viscosity, faster the motion.

Thus, option (d) is wrong.

Concept:

Brownian motion:

• 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.

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• The existence of Brownian motion gives us two conclusions about the nature of matter :
  • That matter is made up of tiny particle, 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.

Explanation:

Factors affecting Brownian motion:

• Brownian motion is affected by any factor that affects the motion of particles.
• Sol particles in a colloid are continuously bombarded by the molecules of the dispersion medium and thus show zig-zag motion. This Zig-zag motion is called the Brownian motion.
• Brownian motion is observed in the particle size of 2-5 microns.
• Temperature influences the speed and kinetic energy of the particles, and thus Brownian motion is affected by temperature.
• The motion of particles depends on their size and thus influences Brownian motion.
• As the number of particles increases, the number of collisions also increases, and thus Brownian motion is disrupted.
• The energy of the light beam depends on the wavelength of the light as E = hc/λ, so as the energy of light wavelength changes, the energy of the sol particles also changes which in fact alters the Brownian motion.
• Thus, Brownian motion is dependent on the intensity of light wavelength used.
• The Brownian motion also depends on the density and viscosity of the dispersed phase and the density and viscosity of the dispersed medium.

Hence, the incorrect statement about the Brownian movement is it is independent of the wavelength of light used.

Important Points

• Brownian motion is inversely proportional to the viscosity of the medium.
• Gravity doesn't affect Brownian motion.

Concept:

Freundlich adsorption isotherm:

Freundlich gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature.

\(\frac{{\rm{x}}}{{\rm{m}}} = {\rm{K}}.{{\rm{p}}^{1/{\rm{n}}}}\)

Taking log on both sides, we get,

\(\log \frac{{\rm{x}}}{{\rm{m}}} = \log {\rm{K}} + \frac{1}{{\rm{n}}}\log {\rm{p}}\)

We know that, the formula of slope is:

\({\rm{m}} = \frac{{\rm{y}}}{{\rm{x}}}\)

In the formula, the slope is \(\frac{1}{{\rm{n}}}\)

Therefore,

\(\Rightarrow \frac{{\rm{y}}}{{\rm{x}}} = \frac{1}{{\rm{n}}}\)

From graph, the value of slope is:

\(\Rightarrow \frac{2}{3} = \frac{1}{{\rm{n}}}\)

\(\therefore {\rm{n}} = \frac{3}{2}\)

The Freundlich equation is:

\(\frac{{\rm{x}}}{{\rm{m}}} = {\rm{K}}.{{\rm{p}}^{1/{\rm{n}}}}\)

\(\Rightarrow \frac{{\rm{x}}}{{\rm{m}}} \propto {{\rm{p}}^{1/\left( {3/2} \right)}}\)

\(\Rightarrow \frac{{\rm{x}}}{{\rm{m}}} \propto {{\rm{p}}^{2/3}}\)

Concept:

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. Common examples of adsorbents are clay, silica gel, colloids, metals etc.

Fig. 1. Adsorption

Adsorption is a surface phenomenon. The process of removal of adsorbent from the surface of adsorbate is known as desorption.

Difference between Absorption and Adsorption

Explanation:

From the above order of adsorption power; the increasing order is

Concept:

In heterogeneous catalytic reactions, physical state of reactants and that of catalyst(s) used are different.

The hydrogenation of vegetable oils and Ostwald's process all are heterogeneous process. Combustion of coal is not heterogeneous catalytic reaction.

Haber's process:

\(N_2(g)+3H_2 \xrightarrow[]{FE(s),Mo(s)}2NH_3(g)\)

Catalyst is used.

Hydrogenation of vegetable oils:

\(vegetable oil(l)(Unsaturated) \xrightarrow[]{[(Ph_3P)]Cl orNi(s)}Vanaspathi(s)\)

Catalyst is used.

Ostwald's process:

\(4NH_3(g)+50_2 \xrightarrow[]{Pt(s), V_30_5(s)}4NO(g)+6H_2O(g)\)

Catalyst is used.

Combustion of coal:

C + O₂ ⟶ CO₂

(coal)

Concept:

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.

Additional Information

Pressure: Freundlich proposed an empirical relationship between the amount of gas adsorbed by a unit mass of adsorbent (x/m) and pressure (P) at a particular temperature. The following equation was proposed for freundlich adsorption isotherm:

x/m = kP1/n, n > 1

Therefore, the amount of adsorbed gas (x) increases with pressure (P).

Specific area (Surface area): Specific area of an adsorbent is the surface area available for adsorption per gm of the adsorbent. Greater the specific area of an adsorbent, greater will be the adsorption. The specific area of an adsorbent can be increased by making the surface rough.

Concept:

(A) V₂O₅ - Preparation of H₂SO₄ in contact process

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum, as a good catalyst, is susceptible to reacting with arsenic hence Vanadium oxide (V₂O₅) is now preferred.

(B) TiCl₄ + Al(Me)₃ - Polyethylene (Ziegler – Natal catalyst used for Fin polymerisation)

Ziegler–Natal catalyst is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). A typical ZN catalyst system usually contains two parts: a transition metal (Group IV metals, like Ti, Zr, Hf) compound and an organo aluminum compound (co-catalyst). The common examples of ZN catalyst systems include TiCl₄ + Et₃Al and TiCl₃ + AlEt₂Cl.

(C) PdCl₂ - Ethanol formation (Wacker’s Process)

Wacker’s process is the method of preparation of aldehydes and ketones from alkenes. The Wacker Oxidation is an industrial process, which allows the synthesis of ethanol from ethene by palladium-catalyzed oxidation with oxygen. Copper serves as redox cocatalyst.

(D) Iron oxide - NH₃ in (Haber’s Process)

Haber’s Process is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today.

Thus, the correct representation of the catalyst with their products is:

Explanation:-

• Freundlich Isotherm is an Adsorption Isotherm. It explains the concentration of the gas adsorbed into a given surface or the amount of solute adsorbed.

Important Points

•  Adsorption- The accumulation of molecular species at the surface rather than in the bulk of a solid or a liquid is termed adsorption.
• Adsorbate- The molecular species or substances, which concentrate or accumulate at the surface is called adsorbate. Here, p
• Adsorbent- The material on the surface of which the adsorption takes place is called adsorbent. Here, θ
  • Freundlich Equation- x/ m = k .P^1/n  ( n > 1)
  • Log x/m = Log k + 1/n Log P

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