JEE Chemistry Surface Chemistry MCQs Set 14

Answer: (b) Lower temperature

Explanation:
1. Physical adsorption is exothermic and rests on weak van der Waals forces.
2. By Le Chatelier's principle it is favoured at low temperature and falls as temperature rises.
3. Answer: (b)

Teacher's Note:
1. Think of adsorption as a release of heat.
2. Cooling the adsorbent increases the amount of gas held.

Answer: (c) Increase of pressure and decrease of temperature

Explanation:
1. Adsorption is exothermic, so a lower temperature favours it.
2. Higher pressure pushes more gas molecules onto the surface (Freundlich isotherm).
3. Answer: (c)

Teacher's Note:
1. Both lower temperature and higher pressure help physical adsorption.
2. The isotherm flattens at high pressure.

Answer: (d) All of the above are correct

Explanation:
1. Chemisorption (Langmuir type) is specific to particular gas-surface pairs.
2. Physisorption is reversible.
3. Both kinds of adsorption release heat, so all three statements are correct.
4. Answer: (d)

Teacher's Note:
1. Adsorption always has a negative ΔH.
2. Specificity and reversibility separate chemisorption from physisorption.

Answer: (d) All of these

Explanation:
1. Gas molecules become less free on the surface, so ΔS is negative and TΔS is negative.
2. The process is exothermic, so ΔH is negative.
3. All three statements hold.
4. Answer: (d)

Teacher's Note:
1. For a spontaneous process ΔG = ΔH − TΔS must be negative.
2. Since ΔS is negative, adsorption is spontaneous only when |ΔH| is large enough, i.e. at lower temperature.

Answer: (b) Physical adsorption in general increases with temperature

Explanation:
1. Physical adsorption is exothermic, so it decreases as temperature rises.
2. Statement (b) says the opposite and is wrong.
3. Answer: (b)

Teacher's Note:
1. Read the word 'wrong' carefully in the stem.
2. Surface-only means the substance does not enter the bulk, unlike absorption.

Answer: (c) Both adsorption and absorption take place

Explanation:
1. Sorption is the combined term for adsorption and absorption occurring together, as in hydrogen on palladium.
2. Answer: (c)

Teacher's Note:
1. Adsorption is a surface effect; absorption is uniform through the bulk.
2. McBain introduced the word sorption.

Answer: (d) All of these

Explanation:
1. Easily liquefiable gases (higher critical temperature) are adsorbed more.
2. Adsorption rises with pressure and falls with temperature, and also depends on the adsorbent surface.
3. Answer: (d)

Teacher's Note:
1. Nature of gas, nature and area of adsorbent, pressure and temperature all matter.
2. Activating the adsorbent increases its surface area.

Answer: (a) Physical adsorption

Explanation:
1. Physical adsorption involves weak forces that act beyond the first layer, so many layers can build up.
2. Chemisorption stops at one layer because it needs chemical bonds with the surface.
3. Answer: (a)

Teacher's Note:
1. Physisorption is multilayer, chemisorption is unimolecular.
2. At high pressure physisorption deviates from the simple Langmuir picture.

Answer: (a) 2

Explanation:
1. A colloid is a heterogeneous system of two phases: the dispersed phase and the dispersion medium.
2. Answer: (a)

Teacher's Note:
1. A sol of gold in water has gold as dispersed phase and water as medium.
2. Colloids are heterogeneous even though they look clear.

Answer: (b) Irreversible

Explanation:
1. Lyophobic (solvent-hating) sols cannot be re-formed by simply remixing the coagulated solid with the medium, so they are irreversible.
2. Lyophilic sols are reversible and solvent loving.
3. Answer: (b)

Teacher's Note:
1. Lyophobic sols need a stabilising agent.
2. Gold and As₂S₃ sols are lyophobic.

Answer: (d) Both the amount and the sign of the charge

Explanation:
1. Hardy-Schulze rule: the ion carrying charge opposite to that of the sol coagulates it, and the greater the charge the greater the coagulating power.
2. Answer: (d)

Teacher's Note:
1. Sign decides which ion is effective; magnitude decides how effective.
2. Flocculation value is inversely related to coagulating power.

Answer: (c) Low osmotic pressure

Explanation:
1. Colloidal particles are large, so the number of particles per unit volume is small and colligative properties are low.
2. Answer: (c)

Teacher's Note:
1. Colligative properties depend on particle number.
2. Macromolecular colloids can still be used for molar-mass determination by osmotic pressure.

Answer: (b) Fe³⁺ ions

Explanation:
1. The particle adsorbs the ion common to its own lattice, here Fe³⁺.
2. So the sol is positively charged.
3. Answer: (b)

Teacher's Note:
1. Common-ion adsorption rule.
2. If NaOH were used the OH⁻ would be adsorbed instead.

Answer: (a) Sol particles move towards the opposite electrode

Explanation:
1. Charged colloidal particles migrate under an electric field towards the electrode of opposite charge.
2. The dispersion medium stays put in electrophoresis (it moves in electro-osmosis).
3. Answer: (a)

Teacher's Note:
1. Electrophoresis moves particles; electro-osmosis moves medium.
2. It proves that sol particles carry charge.

Answer: (d) All of the above

Explanation:
1. An emulsifier stabilises an emulsion by forming a film around droplets; soaps, detergents and lyophilic sols such as gelatin all do this.
2. Answer: (d)

Teacher's Note:
1. Emulsifier means emulsifying agent.
2. Different emulsifiers favour O/W or W/O types.

Answer: (b) Lower colligative properties

Explanation:
1. Many solute molecules aggregate into one micelle, so the number of independent particles falls.
2. Colligative properties depend on particle number, so they are lower.
3. Answer: (b)

Teacher's Note:
1. Association reduces particle number.
2. Micelles form above the CMC and Kraft temperature.

Answer: (d) [Fe(CN)₆]⁴⁻

Explanation:
1. Minimum flocculation value means maximum coagulating power.
2. For a positive sol, the anion with the highest charge, [Fe(CN)₆]⁴⁻, is most effective.
3. Answer: (d)

Teacher's Note:
1. Flocculation value is the minimum concentration (mmol per litre) that coagulates a sol in a fixed time.
2. Lower value, stronger coagulant.

Answer: (a) Aluminium chloride

Explanation:
1. A negative sol is coagulated by cations; Al³⁺ has the greatest charge, so least amount is needed.
2. Answer: (a)

Teacher's Note:
1. Hardy-Schulze rule: Al³⁺ > Ba²⁺ > Na⁺.
2. Used in delta formation and water purification with alum.

Answer: (b) Blood

Explanation:
1. Blood is a natural colloid (a sol of proteins and cells).
2. NaCl and sugar form true solutions; RCOONa (soap) forms an associated colloid, but it is man-made.
3. Answer: (b)

Teacher's Note:
1. Other natural colloids: milk, egg white, clouds.
2. Soap is an associated colloid.

Answer: (d) All of these

Explanation:
1. Gelatin is a lyophilic colloid; its particles coat lyophobic sols and prevent their coagulation by electrolytes.
2. It protects gold, As₂S₃ and Fe(OH)₃ sols alike.
3. Answer: (d)

Teacher's Note:
1. Protective colloids are measured by gold number.
2. Used in photographic emulsions.

Answer: (b) Sols are coagulated by effective ions whose charge is opposite to that of the sol, and ions of higher charge are much more effective than those of lower charge

Explanation:
1. The coagulating ion carries a charge opposite to the sol, and coagulating power rises steeply with that charge.
2. Answer: (b)

Teacher's Note:
1. Opposite sign neutralises the sol.
2. Higher valence means a lower flocculation value.

Answer: (d) Alcohol

Explanation:
1. Gelatin is a lyophilic sol stabilised by a solvation layer as well as charge.
2. Small amounts of electrolytes do not coagulate it; dehydrating agents such as alcohol remove the solvent layer.
3. Answer: (d)

Teacher's Note:
1. Lyophilic sols need a dehydrating agent plus an electrolyte.
2. Lyophobic sols are coagulated easily by electrolytes.

Answer: (d) None of the above

Explanation:
1. Gold number is the number of milligrams of a protective colloid that just prevents coagulation of 10 mL of a standard gold sol on adding 1 mL of 10% NaCl.
2. It measures the protective colloid, not gold, so none of (a) to (c) is right.
3. Answer: (d)

Teacher's Note:
1. It is a property of the protective colloid.
2. Do not confuse it with the amount of gold.

Answer: (a) I, II, III

Explanation:
1. Air (gas in gas), sea water (salts in water) and glucose solution are homogeneous.
2. Gem stones are coloured solid sols, pearl is a solid-in-liquid or gel-like colloid and blood is a sol, so IV, V and VI are colloids.
3. Answer: (a)

Teacher's Note:
1. Gem stones are solid sols.
2. Air counts as a gaseous solution.

Answer: (c) Butter : O/W

Explanation:
1. Butter is water dispersed in fat, a W/O emulsion, so the O/W label is wrong.
2. Milk (fat in water), cold cream (water in oil) and vanishing cream (oil in water) are correctly matched.
3. Answer: (c)

Teacher's Note:
1. O/W has oil as dispersed phase.
2. Butter and cold cream are W/O.

Answer: (d) A catalyst alters the speed of a reaction

Explanation:
1. A catalyst may increase the rate (positive catalyst) or decrease it (negative catalyst or inhibitor), i.e. it alters the speed.
2. So the broad statement (d) is the most correct.
3. Answer: (d)

Teacher's Note:
1. Both positive and negative catalysis exist.
2. A catalyst never changes the equilibrium position.

Answer: (b) Number of free valencies

Explanation:
1. Adsorption of reactants on the catalyst surface uses free valencies, so more free valencies mean greater activity (the basis of transition metal catalysts).
2. Answer: (b)

Teacher's Note:
1. Free valencies are what adsorb reactants.
2. Finely divided catalysts have more surface and more valencies.

Answer: (a) Transition metals

Explanation:
1. Transition metals have vacant d-orbitals and variable oxidation states, which allow them to bind reactants and form intermediates.
2. Answer: (a)

Teacher's Note:
1. Fe, Ni, Pt, V₂O₅ are well-known catalysts.
2. The d-electrons help in adsorption.

Answer: (b) Fe acts as the catalyst and Mo as the promoter

Explanation:
1. Finely divided iron is the catalyst; molybdenum increases its activity and is the promoter.
2. Answer: (b)

Teacher's Note:
1. Promoters enhance catalyst activity.
2. Poisons, in contrast, reduce it.

Answer: (d) All of the above

Explanation:
1. All three statements describe zeolites correctly.
2. Answer: (d)

Teacher's Note:
1. Zeolites such as ZSM-5 are shape selective catalysts.
2. Used to convert alcohols to gasoline.

Answer: (c) The size of the pores of the catalyst, which can trap only selected molecules

Explanation:
1. In a shape selective catalyst the pore structure lets only molecules of suitable size and shape enter or leave.
2. Answer: (c)

Teacher's Note:
1. Zeolites are the best-known examples.
2. ZSM-5 converts alcohols to hydrocarbons.