Surface Chemistry - MCQs with Explanations

1. Which of the following gas molecules have maximum value of enthalpy of physisorption in a gas mask?

• (a) \(CH_4\)
• (b) \(N_2\)
• (c) \(SO_2\)
• (d) \(H_2\)

Correct Answer: (c)

Explanation: Enthalpy of physisorption is related to the critical temperature or van der Waals' forces of the gas. The order of critical temperatures for these gases is \(SO_2 > CH_4 > N_2 > H_2\), making \(SO_2\) the most easily adsorbed.

2. Which of the following electrolyte will have maximum flocculation value for \(Fe(OH)_3\) sol?

• (a) NaCl
• (b) \(Na_2S\)
• (c) \((NH_4)_3PO_4\)
• (d) \(K_2SO_4\)

Correct Answer: (a)

Explanation: \(Fe(OH)_3\) is a positively charged sol. According to the Hardy-Schulze rule, the ion with the lowest charge (\(Cl^-\) in NaCl) will have the minimum coagulating power and thus the maximum flocculation value.

3. Micelles are formed only:

• (a) below the CMC and the Kraft temperature
• (b) above the CMC and below the Kraft temperature
• (c) above the CMC and above the Kraft temperature
• (d) below the CMC and above the Kraft temperature.

Correct Answer: (c)

Explanation: Micelle formation occurs only when the surfactant concentration is above the Critical Micelle Concentration (CMC) and the temperature is above the Kraft temperature.

4. Bredig's arc method cannot be used to prepare colloidal solution of:

• (a) Pt
• (b) Fe
• (c) Ag
• (d) Au

Correct Answer: (b)

Explanation: Bredig's arc method is unsuitable for iron because, at high temperatures, iron reacts with water (steam) instead of forming a stable colloidal sol.

5. A colloidal solution is subjected to an electric field. The particles move towards anode. The coagulation of same sol is studied using NaCl, \(BaCl_2\) and \(AlCl_3\) solutions. Their coagulating power should be:

• (a) \(NaCl > BaCl_2 > AlCl_3\)
• (b) \(BaCl_2 > AlCl_3 > NaCl\)
• (c) \(AlCl_3 > BaCl_2 > NaCl\)
• (d) \(BaCl_2 > NaCl > AlCl_3\)

Correct Answer: (c)

Explanation: Since particles move to the anode, the sol is negatively charged. According to the Hardy-Schulze rule, coagulating power increases with the valency of the cation: \(Al^{3+} > Ba^{2+} > Na^+\).

6. Among the following, the surfactant that will form micelles in aqueous solution at the lowest molar concentration at ambient conditions is:

• (a) \(CH_3(CH_2)_{15}N^+(CH_3)_3Br^-\)
• (b) \(CH_3(CH_2)_{11}OSO_3^-Na^+\)
• (c) \(CH_3(CH_2)_6COO^-Na^+\)
• (d) \(CH_3(CH_2)_{11}N^+(CH_3)_3Br^-\)

Correct Answer: (a)

Explanation: The CMC decreases as the length of the hydrocarbon chain increases because larger molecules are less soluble and associate more easily. Option (a) has the longest chain.

7. Zeolites are very important oxide catalysts used in:

• (a) decomposition of nitramide
• (b) mutarotation of glucose
• (c) petroleum industries in cracking
• (d) hydrolysis of ester.

Correct Answer: (c)

Explanation: Zeolites are shape-selective catalysts widely used in the petroleum industry for hydrocarbon cracking and isomerisation.

8. Which relation represents Freundlich adsorption isotherm?

• (a) \(x/m = kP\) at low pressure
• (b) \(x/m = k\) at high pressure
• (c) \(log\ x/m = log\ k + 1/n\ log\ P\)
• (d) All of the above.

Correct Answer: (d)

Explanation: All these equations describe the Freundlich isotherm under different pressure conditions (low, high, and logarithmic form for moderate pressure).

9. Which of the following shows the maximum hydrophobic behaviour?

• (a) Glycine
• (b) Stearic acid
• (c) Glucose
• (d) Adenine.

Correct Answer: (b)

Explanation: Stearic acid has a very long non-polar hydrocarbon tail, which makes it highly hydrophobic compared to the other polar or smaller molecules listed.

10. Zeta potential (or electrokinetic potential) is the:

• (a) potential required to bring about coagulation of a colloidal sol
• (b) potential required to give the particles a speed of 1 cm/s in the sol
• (c) potential difference between fixed charged layer and the diffused layer having opposite charge
• (d) potential energy of the colloidal particles.

Correct Answer: (c)

Explanation: It is defined as the potential difference existing between the stationary fixed layer and the mobile diffused layer of opposite charges in a colloidal system.

11. For coagulation of \(As_2S_3\) which is most effective?

• (a) \(Na_3PO_4\)
• (b) \(MgCl_2\)
• (c) \(CaCl_2\)
• (d) \(AlCl_3\)

Correct Answer: (d)

Explanation: \(As_2S_3\) is a negatively charged sol. \(AlCl_3\) is the most effective coagulant because the \(Al^{3+}\) ion has the highest positive charge among the options.

12. During adsorption, which of the following is less than zero?

• (a) \(\Delta G\)
• (b) \(\Delta S\)
• (c) \(\Delta H\)
• (d) All of these.

Correct Answer: (d)

Explanation: Adsorption is spontaneous (\(\Delta G < 0\)), exothermic (\(\Delta H < 0\)), and leads to a decrease in the randomness of the gas molecules (\(\Delta S < 0\)).

13. The separation of colloidal particles from those of molecular dimensions is known as:

• (a) photolysis
• (b) dialysis
• (c) pyrolysis
• (d) peptisation.

Correct Answer: (b)

Explanation: Dialysis uses a semipermeable membrane that allows small molecules and ions to pass through while retaining larger colloidal particles.

14. Which of the following represents physical adsorption? (Refer to x/m vs Temperature graphs)

• (a) Constant decrease
• (b) Increase then decrease
• (c) Constant horizontal line
• (d) Constant increase.

Correct Answer: (a)

Explanation: In physical adsorption, the extent of adsorption (\(x/m\)) decreases continuously as the temperature increases.

15. Freshly obtained \(SnO_2\) is added to water containing a little HCl. The sol obtained would be due to preferential adsorption of ________ ions.

• (a) positively charged, \(H^+\)
• (b) negatively charged, \(SnO_3^{2-}\)
• (c) positively charged, \(Sn^{4+}\)
• (d) positively charged, \(Sn^{2+}\)

Correct Answer: (c)

Explanation: \(SnO_2\) reacts with HCl to provide \(Sn^{4+}\) ions, which are then preferentially adsorbed on the surface of the \(SnO_2\) particles, forming a positive sol.

16. Which of the following is not an emulsion?

• (a) Butter
• (b) Ice-cream
• (c) Milk
• (d) Cloud

Correct Answer: (d)

Explanation: Cloud is an aerosol (liquid in gas), whereas the others are emulsions (liquid in liquid or liquid in solid).

17. In oxidation of oxalic acid by \(KMnO_4\), the colour of \(KMnO_4\) disappears slowly at the start of reaction but disappears very fast afterwards. This is an example of:

• (a) induced catalysis
• (b) auto-catalysis
• (c) negative catalysis
• (d) enzyme catalysis.

Correct Answer: (b)

Explanation: This is auto-catalysis because the \(Mn^{2+}\) ions produced during the reaction act as a catalyst for the subsequent reaction steps.

18. An oil-soluble dye is mixed with an emulsion and emulsion remains colourless. Then, it is:

• (a) O-in-W
• (b) W-in-O
• (c) W-in-W
• (d) O-in-O

Correct Answer: (a)

Explanation: In an oil-in-water (O/W) emulsion, the dye only colours the dispersed oil droplets, leaving the continuous water background colourless.

19. For adsorption of gas on solid surface, the plots of \(log\ x/m\) vs \(log\ P\) is linear with slope equal to:

• (a) \(1/n\)
• (b) \(1/k\)
• (c) \(log\ k\)
• (d) \(log\ P\)

Correct Answer: (a)

Explanation: Based on the Freundlich equation \(log(x/m) = log\ k + (1/n) log\ P\), the slope is \(1/n\).

20. The curve showing the variation of adsorption with pressure at constant temperature is called:

• (a) adsorption isobar
• (b) adsorption isotherm
• (c) adsorption isostere
• (d) all of these.

Correct Answer: (b)

Explanation: An adsorption isotherm is a graph of the extent of adsorption against pressure at a single, constant temperature.

21. Freundlich adsorption isotherm better explains:

• (a) physisorption
• (b) chemisorption
• (c) both (a) and (b)
• (d) neither (a) nor (b).

Correct Answer: (a)

Explanation: The Freundlich model is generally more applicable to physical adsorption than to chemical adsorption.

22. A colloidal system in which solid is dispersed in a liquid is called:

• (a) precipitate
• (b) sol
• (c) emulsion
• (d) gel.

Correct Answer: (b)

Explanation: A system consisting of a solid dispersed in a liquid medium is known as a sol.

23. Which of the following has maximum value of flocculating power?

• (a) \(Pb^{2+}\)
• (b) \(Pb^{4+}\)
• (c) \(Sr^{2+}\)
• (d) \(Na^+\)

Correct Answer: (b)

Explanation: Flocculating (coagulating) power increases significantly as the charge (valency) of the ion increases.

24. Which one of the following is a property of physisorption?

• (a) Non-specific nature
• (b) High specificity
• (c) Irreversibility
• (d) None of these

Correct Answer: (a)

Explanation: Physical adsorption is non-specific; any gas can be adsorbed on any solid to some extent by van der Waals' forces.

25. The simplest way to check whether a system is colloidal is by:

• (a) Tyndall effect
• (b) Brownian movement
• (c) electrodialysis
• (d) measuring particle size

Correct Answer: (a)

Explanation: The Tyndall effect (scattering of light) is a quick diagnostic test to distinguish a colloid from a true solution.

26. If \(x/m\) is the mass of adsorbate adsorbed per unit mass of adsorbent, \(p\) is the pressure... which represents "Langmuir adsorption isotherm"?

• (a) \(log(x/m) = log(a/b) + (1/a) log\ p\)
• (b) \(x/m = b/a + 1/ap\)
• (c) \(x/m = (1+bp)/ap\)
• (d) \(1/(x/m) = b/a + 1/ap\)

Correct Answer: (d)

Explanation: Starting from the standard Langmuir equation \(x/m = ap/(1+bp)\), the reciprocal is \(1/(x/m) = 1/ap + b/a\).

27. Gold numbers of protective colloids A (0.50), B (0.01), C (0.10) and D (0.005). The correct order of their protective powers is:

• (a) \(D < A < C < B\)
• (b) \(C < B < D < A\)
• (c) \(A < C < B < D\)
• (d) \(B < D < A < C\)

Correct Answer: (c)

Explanation: Protective power is inversely proportional to the gold number. Therefore, D (lowest gold number) has the highest power, and A (highest gold number) has the lowest.

28. Butter is a colloid formed when:

• (a) fat is dispersed in water
• (b) fat globules are dispersed in water
• (c) water is dispersed in fat
• (d) none of the above.

Correct Answer: (c)

Explanation: Butter is a water-in-oil emulsion where water is dispersed in a fat medium.

29. A liquid which markedly scatters light... but leaves no residue when passed through filter paper is:

• (a) a suspension
• (b) a true solution
• (c) lyophobic sol
• (d) lyophilic sol.

Correct Answer: (c)

Explanation: Scattering light (Tyndall effect) while passing through standard filter paper is a property of stable lyophobic sols.

30. Dyeing of fibre involves the process of:

• (a) adsorption
• (b) absorption
• (c) sorption
• (d) all of these.

Correct Answer: (d)

Explanation: Dyeing involves sorption, which is a combination of both adsorption and absorption occurring simultaneously.

31. Match (Rain cloud, Milk of magnesia, Soap suds, Butter):

• (a) 3 2 1 4
• (b) 2 1 4 3
• (c) 3 2 4 1
• (d) 2 3 1 4

Correct Answer: (b)

Explanation: Cloud is an aerosol (2); Milk of magnesia is a sol (1); Soap suds is foam (4); Butter is a gel (3).

32. Colloidal particles exhibit Tyndall effect due to:

• (a) polarisation of light
• (b) scattering of light
• (c) reflection of light
• (d) refraction of light

Correct Answer: (b)

Explanation: The visibility of the beam's path in a colloid is caused by the scattering of light by the particles.

33. The dialysis of colloidal solution depends upon:

• (a) nature of the membrane used
• (b) difference in the temperature of the two liquids
• (c) nature of particles present in the solution
• (d) all of these.

Correct Answer: (d)

Explanation: All these physical and chemical factors influence the efficiency and rate of the dialysis process.

34. Column chromatography is based upon:

• (a) preferential adsorption
• (b) preferential desorption
• (c) eluent to be used
• (d) size of the adsorbent.

Correct Answer: (a)

Explanation: Chromatography relies on the selective or preferential adsorption of different components onto a solid phase.

35. In multimolecular colloidal sols, atoms or molecules are held together by:

• (a) H-bonding
• (b) van der Waals forces
• (c) ionic bonding
• (d) polar covalent bonding.

Correct Answer: (b)

Explanation: In multimolecular colloids (like sulphur sol), small particles are held in larger aggregates by weak van der Waals forces.

36. Hardy-Schulze rule: coagulation power of cations follow the order:

• (a) \(Ba^{2+} > Na^+ > Al^{3+}\)
• (b) \(Al^{3+} > Na^+ > Ba^{2+}\)
• (c) \(Al^{3+} > Ba^{2+} > Na^+\)
• (d) \(Ba^{2+} > Al^{3+} > Na^+\)

Correct Answer: (c)

Explanation: Coagulating power increases with higher valency: \(+3 > +2 > +1\).

37. Which of the following reaction gives a colloidal sol?

• (a) \(MgCO_3 \rightarrow MgO + CO_2\)
• (b) \(2Na + 2H_2O \rightarrow 2NaOH + H_2\)
• (c) \(HNO_3 + 3H_2S \rightarrow 3S + 4H_2O + 2NO\)
• (d) \(Cu + CuCl_2 \rightarrow Cu_2Cl_2\)

Correct Answer: (c)

Explanation: The oxidation of \(H_2S\) by \(HNO_3\) results in the formation of colloidal sulphur.

38. Colloidal solutions of gold... are of different colours because of:

• (a) different diameters of colloidal gold particles
• (b) variable valency of gold
• (c) different concentration of gold particles
• (d) impurities produced by different methods.

Correct Answer: (a)

Explanation: The colour of a gold sol depends on the size (diameter) and shape of the particles.

39. Chromatography is based on the general principle of:

• (a) phase rule
• (b) phase distribution
• (c) interphase separation
• (d) phase operation.

Correct Answer: (b)

Explanation: Chromatography is based on the differential distribution (partition) of components between a mobile phase and a stationary phase.

40. During adsorption the magnitude of -ve \(\Delta H\) value:

• (a) goes on increasing
• (b) goes on decreasing
• (c) remains same
• (d) first increases then decreases.

Correct Answer: (b)

Explanation: As adsorption progresses, the number of available active sites decreases, leading to a decrease in exothermicity (\(\Delta H\)).

41. In which condition, adsorption will not take place?

• (a) \(\Delta H\) is negative
• (b) when \(\Delta H\) is positive and \(\Delta S\) is positive
• (c) \(\Delta H\) is positive and \(\Delta S\) is negative
• (d) \(\Delta G\) is negative.

Correct Answer: (c)

Explanation: If \(\Delta H > 0\) and \(\Delta S < 0\), then \(\Delta G\) must be positive, making the process non-spontaneous and impossible.

42. Adsorption is an exothermic process, because during adsorption:

• (a) surface energy increase
• (b) surface energy decrease
• (c) surface energy remain same
• (d) residual forces of attraction increase

Correct Answer: (b)

Explanation: Adsorption reduces the residual forces on the surface, causing a decrease in surface energy, which is released as heat.

43. Colloid (1.0 x 10^-5 g) prevents 20 mL of gold sol precipitation. Gold number in \(x\times10^{-3}\) mg, value of x is:

• (a) 1
• (b) 2
• (c) 3
• (d) 4

Correct Answer: (a)

Explanation: Based on the definition of Gold Number, the amount needed for 10 mL is 0.005 mg. Standard calculation leads to \(x=1\).

44. The formation of a gas at the surface of tungsten is due to adsorption, the order of reaction is:

• (a) first
• (b) second
• (c) zero
• (d) third

Correct Answer: (c)

Explanation: Decomposition of gases on metal surfaces at high pressure typically follows zero-order kinetics.

45. Adsorption is accompanied by:

• (a) decrease in enthalpy of the system
• (b) increase in entropy of the system
• (c) increase in free energy of the system
• (d) \(T\Delta S\) for the process is positive.

Correct Answer: (a)

Explanation: Spontaneous adsorption requires \(\Delta G < 0\). Since \(\Delta S\) is negative, the enthalpy (\(\Delta H\)) must also be negative for the process to occur.