Surface Chemistry MCQ Quiz - Objective Question with Answer for Surface Chemistry - Download Free PDF

CONCEPT:

Micelle Formation and Krafft Temperature

• Micelles are aggregates of surfactant molecules that form in a solution above a certain concentration and temperature.
• For ionic surfactants, micelle formation occurs only above a specific temperature called the Krafft temperature.
• The Krafft temperature is the temperature below which the surfactant molecules remain in crystalline form and do not form micelles, even if the concentration is above the critical micelle concentration (CMC).

EXPLANATION:

• Surfactants are amphiphilic molecules, meaning they have both hydrophobic and hydrophilic parts.
• At temperatures below the Krafft temperature:
  • The solubility of the surfactant is too low to allow micelle formation.
  • The surfactant remains in a solid crystalline state.
• At temperatures above the Krafft temperature:
  • The solubility increases, and micelle formation becomes possible when the surfactant concentration exceeds the CMC.
  • Micelles act as carriers for hydrophobic substances in aqueous solutions.
• The Krafft temperature is specific to the surfactant and its chemical structure.

Therefore, micelles from ionic surfactants can be formed only above a certain temperature called the Krafft temperature.

CONCEPT:

Physical Adsorption

• Physical adsorption occurs due to weak intermolecular forces like Van der Waals forces between the adsorbate and adsorbent.
• This type of adsorption is generally reversible and is influenced by temperature and pressure.
• Desorption in physical adsorption refers to the process where the adsorbed particles are released from the surface of the adsorbent.

EXPLANATION:

• Physical adsorption is highly dependent on temperature:
  • When the temperature increases, the kinetic energy of the adsorbed molecules increases, leading to the breaking of weak intermolecular forces between the adsorbate and adsorbent.
  • This results in desorption of the adsorbed particles.
• Pressure also plays a role in physical adsorption:
  • Increasing pressure usually favors adsorption, but it does not directly cause desorption.
• Thus, desorption occurs primarily when the temperature increases.

Therefore, the correct answer is Temperature increases.

CONCEPT:

Coagulation of Colloidal Sols

• Coagulation (or flocculation) is the process by which colloidal particles aggregate to form larger particles and settle out of the dispersion medium.
• The charge on the colloidal particles determines the type of ion required for coagulation. In this case, arsenic(III) sulphide sol carries a negative charge.
• According to the Hardy-Schulze Rule, the effectiveness of ions in coagulating a sol is directly proportional to the charge of the ion.
• The higher the charge on the cation, the more effective it is in coagulating a negatively charged sol.

EXPLANATION:

• To coagulate the negatively charged arsenic(III) sulphide sol, we need positive ions (cations).
• The given options are ionic substances that dissociate to produce cations with the following charges:
  • KCl: Produces K⁺ (charge = +1)
  • MgCl₂: Produces Mg²⁺ (charge = +2)
  • Al₂(SO₄)₃: Produces Al³⁺ (charge = +3)
  • Na₃PO₄: Produces Na⁺ (charge = +1)
• According to the Hardy-Schulze Rule, Al³⁺ from Al₂(SO₄)₃ has the highest charge (+3), making it the most effective ion for coagulating the negatively charged sol.

Therefore, the correct answer is Al₂(SO₄)₃.

CONCEPT:

Hardy-Schulze Rule

• The Hardy-Schulze rule states that the coagulating power of an ion is directly proportional to the valency (charge) of the ion carrying a charge opposite to that of the colloidal particles.
• This means that ions with higher charges are more effective in causing coagulation (precipitation) of colloidal particles compared to ions with lower charges.

EXPLANATION:

• Colloidal particles are charged and tend to remain stable because of their charge, which prevents them from aggregating.
• To destabilize the colloid and cause coagulation, ions with an opposite charge are required.
• According to the Hardy-Schulze rule, ions with higher valency have a stronger ability to neutralize the charge on the colloidal particles, leading to coagulation.
• For example:
  • For a negatively charged sol (such as arsenic sulfide sol), the coagulating power of cations follows the order: Al³⁺ > Ba²⁺ > Na⁺.
  • For a positively charged sol, the coagulating power of anions follows the order: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > Cl⁻.
• In this context, the coagulating power of an effective ion carrying the charge opposite to the sol particles is best explained by the Hardy-Schulze rule.

Therefore, the correct answer is Hardy-Schulze Rule.

CONCEPT:

Types of Mixtures

• Mixtures can be classified based on the state of the components involved (solid, liquid, or gas).
• Examples of such mixtures include:
  • Solid in liquid: A solid dissolved in a liquid (e.g., sugar in water).
  • Gas in solid: A gas trapped in a solid (e.g., foam-like materials such as sponge or cake).
  • Liquid in solid: A liquid dispersed in a solid (e.g., gel).
  • Solid in solid: A mixture of solids (e.g., alloys like bronze).

EXPLANATION:

• Cake is a foam-like material where air (a gas) is trapped in the solid structure of baked ingredients (such as flour, sugar, and eggs).
• During the baking process:
  • Air or gases (e.g., carbon dioxide from baking powder) get incorporated into the batter.
  • When baked, these gases expand, forming bubbles that are trapped as the batter solidifies.
• As a result, cake is a classic example of a "gas in solid" mixture.

Therefore, the correct answer is Gas in solid.

A colloid is a mixture in which one substance of microscopically dispersed insoluble particles are suspended throughout another substance.

• Dispersed phase: The phase that is scattered or present in the form of colloidal particles is known as dispersed phase.
• Dispersion medium: The medium in which the colloidal particles are dispersed is called dispersion medium.

Classification of colloids:

Hence we can conclude that 'Clouds' are examples of Aerosol.

The correct answer is Coagulation.

Key Points

• Alum helps in purifying water by coagulating the mud particles.
• The mud particles thus formed a gelatin-like precipitate, sp the particle size increases.
• Hence, the heavy particles settle down as precipitates and water becomes purified.

Additional Information

• In absorption, the substance you add to the water would fully absorb the contaminants.
• In adsorption, contaminants are trapped on the surface of the substance, but not ingested into the substance.
• Activated carbon filters work by the principle of adsorption.
• Dialysis is a process for the removal of waste and excess water from the blood, and is not typically used in water purification.

A sol is a colloidal suspension of very small solid particles in a continuous liquid medium. Sol particles typically have a charge that can be positive or negative, depending on the type of sol and the conditions of its preparation.

Among the options you provided:

• Starch - Typically, starch sols are negatively charged.
• Gum - Gum-based sols are generally not characterized by a specific charge without more context, but they are usually not particularly known for carrying a positive charge.
• Gold Sol - Gold sols are negatively charged. However, without specifying the stabilizing agent or the preparation method, we cannot definitively say it's positively charged. Historically, negatively charged gold sols are more commonly mentioned in literature due to the stabilizing agents commonly used.
• Blood is a positively charged colloid whereas sols of starch, gum & gold are negatively charged. 

Concept:

• A catalyst is a substance that can be added to a reaction to enhance the reaction rate without getting consumed in the process.
• Catalysts typically speed up a reaction by lowering the activation energy or changing the reaction mechanism.
• Enzymes are proteins that act as catalysts in biochemical reactions.
• A stoichiometric amount of one reactant can mean the amount that will react completely with a given amount of another reactant, leaving no excess of either one.
• Catalyst does not take part in balancing the chemical reaction hence catalysts are not required in Stoichiometric amounts.
• Most common catalysts are - Aluminosilicates, Iron, Vanadium, Nickel, Platinum + Alumina.

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​Shape selective catalysts:

• Some catalysis reactions depend upon the pore size of the reactant and the shape of catalyst used.
• The pore size and shape of catalysts is similar to the shape and size of the reactants then they can fit into each other and the reaction takes place.
• Zeolites are for example are shape-selective catalysts. They can catalyze only selective reactions.
• The pore size of Zeolites do not permit all transition state complex to pass through it, thus becomes selective in nature.
• Zeolites are large network of aluminosilicates having the network type Al-O-Si. Molecules like water and alkali metals are trapped inside the pores of the networks.

Explanation:

• Zeolites, a shape selective catalyst or HZSM-5 are the catalysts used in conversion of alcohols to gasoline.
• The temperature range is 300-400^oC.
• The alcohols first undergo dehydration to give unsaturated alkenes.
• The alkenes are then converted to higher alkenes via hydrogenation.
• The catalyst used in Haber's process synstesis of ammonia is Fe.
• It increases the rate of reaction between nitrogen and hydrogen.
• Finely divides solid Fe provides a surface for the reaction to occur.
• Gases nitrogen and hydrogen get adsorbed on the catalyst surface, a reaction occurs, and then the products diffuse out.
• catalysts used for synthesis of methanol from CO and H2 are copper and nickel.
• The catalyst used in polymerisation of ethelene is Zeigler-Natta catalyst.

Hence, a shape selective catalyst used in conversion of alcohol to gasoline.

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:

Types of Mixtures:

Homogeneous Mixture:

• Those mixtures, in which the substances are completely mixed together and are indistinguishable from one another, are called homogeneous mixtures.
• For example Soda water, Soft drinks, lemonade, salt or sugar solution, etc.

Heterogeneous Mixture:

• Those mixtures in which the substances remain separate and one substance is spread throughout the other substance as small particles, droplets, or bubbles are called heterogeneous mixtures.
• All the suspensions and colloids are heterogeneous mixtures.
• A mixture is heterogeneous and shows Tyndall effect because of the floating particle it behave like colloid solution or scatter light but actually its heterogeneous mixture the particle will settle down after some time and the tyndel effect will diminish hence it doesnt scatter light when particle settle down.
• For Example: - Sugar and sand mixture, muddy river water, soap solution. 

​Explanation:

Colloids:

• A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase or colloidal particles) in another substance which is called a dispersion medium.

Properties of colloids:

• A colloid appears to be homogeneous but actually, it is heterogeneous.
• The particles of most of the colloids cannot be seen even with a microscope.
• It scatters a beam of light passing through it (Tyndall effect).
• The size of the solute particle is in the range of 1 to 100 nm.
• Since they are heterogeneous in nature, centrifugation techniques can be used to separate the particles.

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.

The correct answer is Ultra-filtration.

Key Points

• The method in which the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles is known as Ultra-filtration.
• The blood is filtered through three layers i.e., the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule (podocyte cells), and a basement membrane between these two layers.
• This type of filtration is called Ultra-filtration. 

Additional Information

• Vacuum filtration is a mechanical method of dewatering sludge and Incineration units are used for the disposal of dewatered sludge.
• Dialysis works on the principle of the diffusion of solutes and ultra-filtration of fluid across a semi-permeable membrane.
  • Diffusion is a property of substances in water; substances in water tend to move from an area of high concentration to an area of low concentration.
• Electro Dialysis (ED) is a membrane process, during which ions are transported through a semi-permeable membrane, under the influence of an electric potential.

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 Adsorbent and the surface on which adsorption occurs is known as Adsorbate.
• It is a surface phenomenon and occurs due to the unsaturation of forces on the surface of an adsorbate molecule.

​

• 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: 

• In adsorption isotherms, the fraction of moles of adsorbate (x) by the grams of adsorbent(m) versus P is plotted.
• The Freundlich isotherm gives us the empirical relationship between the amount of gas adsorbed and its equilibrium pressure P.
• It is seen that the amount of adsorption increases with an increase of pressure when the pressure is in the lower range.
• As the pressure increases, the rate of adsorption increases but reaches saturation after some point.
• After this point, the amount of adsorption doesn't change even when pressure is increased.
• The equation of Freundlich isotherm is given as follows:

\(\frac{{\rm{x}}}{{\rm{m}}} \propto {\rm{\;}}{{\rm{P}}^{\frac{1}{{\rm{n}}}}}\) where k and n are constant for a particular adsorbent and adsorbate at a fixed temperature.

• The graph is represented as follows.

• When the temperature is fixed and pressure is low, the adsorption becomes directly proportional to the pressure of the gas.
• The relation reduces to
• x=kP.

This signifies, 1/n = 1 or n = 1.

• At a higher range of pressure, the adsorption becomes independent of pressure and the equation reduces to

x/m= k.

We can infer from this that 1/n = 0.

Hence, in Freundlich adsorption isotherm, the value of 1/n is 0 to 1.

Explanation:

An emulsion is a type of colloid in which one liquid is dispersed in another immiscible liquid in the form of small droplets.

Hair cream is an example of an emulsion where oil droplets are dispersed in water. Paint can also be classified as an emulsion or a suspension depending on the type of paint.

Cheese, on the other hand, is a type of colloid where solid particles are dispersed in a liquid. Smoke is an aerosol, which is a type of colloid where solid or liquid particles are suspended in a gas.