Exams › JEE Main › Chemistry › Surface Chemistry
97 questions with worked solutions.
Answer: log x/m = log k + 1/n log p
The correct option expresses the Freundlich adsorption isotherm in a logarithmic form, where the amount adsorbed per unit mass of adsorbent (x/m) is related to the pressure (p) and constants (k and n). This relationship indicates that as pressure increases, the amount adsorbed also increases, following a specific non-linear pattern.
Answer: All of these
Irreversible (lyophobic) colloids cannot be reformed simply by adding the dispersion medium once coagulated. Sols of clay, platinum (and other metals), and Fe(OH)3 are all lyophobic/irreversible in water, so 'All of these' is correct.
Q3. Which of the following forms cationic micelles when its concentration exceeds a certain limit?
Answer: Cetyl trimethyl ammonium bromide
Cetyl trimethyl ammonium bromide has a positively charged head group, so above its CMC it forms cationic micelles. (SDS is anionic.)
Q4. In a micelle, how are the polar and non-polar regions arranged?
Answer: The polar part lies on the outside, while the non-polar part is on the inside
In a micelle, the polar regions face outward toward the surrounding water, allowing for interaction with the aqueous environment, while the non-polar regions are tucked away inside, minimizing their exposure to water and stabilizing the structure.
Answer: AlCl3 > BaCl2 > NaCl
Migration to the anode means the sol is negatively charged, so cations cause coagulation. By the Hardy-Schulze rule, higher cation charge means greater coagulating power: Al3+ > Ba2+ > Na+, i.e. AlCl3 > BaCl2 > NaCl.
Q6. Which of the following electrolytes is the most effective for coagulating ferric hydroxide sol?
Answer: K3[Fe(CN)6]
Ferric hydroxide sol is positively charged, so the most effective coagulant has the most highly charged anion. K3[Fe(CN)6] supplies [Fe(CN)6]^3-, a trivalent anion, making it the most effective coagulant by the Hardy-Schulze rule.
Q7. How does alum aid in the purification of water?
Answer: by aluminium, which causes mud particles to coagulate
Alum supplies Al3+ ions which neutralize the negative charge on suspended clay/mud particles, causing them to coagulate and settle.
Q8. Which statement is incorrect about the adsorption of a gas on the surface of a solid?
Answer: Adsorption keeps increasing continuously as temperature rises
Physical/chemical adsorption is exothermic, so adsorption decreases (does not increase) with rising temperature; that statement is incorrect. The other statements are true: dH and dS are both negative, adsorption is specific (some substances adsorb more), and the process is reversible.
Q9. What is the usual range of adsorption enthalpy for physisorption, expressed in kJ/mol?
Answer: 10 - 40
Physisorption involves weak van der Waals forces with adsorption enthalpy roughly 20-40 kJ/mol, best matched by the 10-40 range. The 40-100 range corresponds to chemisorption.
Q10. Which of the following is an example of an intrinsic colloid?
Answer: Glue
Intrinsic colloids are lyophilic substances that form colloids spontaneously, such as glue, starch, and gelatin. As2S3, sulphur, and metals like iron are extrinsic (lyophobic) colloids. The example of an intrinsic colloid is glue.
Answer: 1/n
Freundlich isotherm log(x/m) = log k + (1/n) log P, so a plot of log(x/m) vs log P has slope = 1/n.
Q12. Identify the statement that is not correct.
Answer: Tannin used in the leather industry consists of positively charged colloidal particles.
Tannin used in leather tanning forms negatively charged colloidal particles, not positive. The other three statements (colloidal gold injection, latex for rubber, AgBr-gelatin photographic emulsion) are correct, so the tannin statement is the incorrect one.
Answer: Combining the two sols produces no change.
Combining colloidal iron(III) hydroxide and colloidal gold does not result in any observable change because the two sols have opposite charges, which typically leads to stability rather than interaction or coagulation.
Q14. Which of the following is expected to exhibit the Tyndall effect?
Answer: Aqueous soap solution above the critical micelle concentration.
Above the critical micelle concentration soap molecules aggregate into micelles, a colloidal (associated colloid) system that scatters light and shows the Tyndall effect. Below CMC, and NaCl/sugar solutions, are true solutions that do not.
Answer: x/m = p^T
Valid adsorption relations are x/m = f(p) at constant T (isotherm), x/m = f(T) at constant p (isobar), and p = f(T) at constant x/m (isostere). The expression x/m = p^T is meaningless and does not describe adsorption.
Q16. According to Langmuir’s adsorption model for a gas on a solid surface, which statement is true?
Answer: The amount of gas hitting a fixed surface area is directly proportional to the gas pressure.
In Langmuir's model the rate at which gas molecules strike a fixed area of surface is directly proportional to the gas pressure. (Adsorption/desorption rates DO depend on fractional coverage, and each site holds only one molecule, so those statements are false.)
Q17. Among the following gases, which one shows the highest enthalpy of physisorption?
Answer: C2H6
Enthalpy of physisorption increases with the ease of liquefaction (higher critical temperature) of the gas. Among the choices C2H6 is most easily liquefied, so it shows the highest enthalpy of physisorption.
Answer: Al2(SO4)3
Sb2S3 sol is negatively charged, so the cation determines coagulation. By the Hardy-Schulze rule the highest cationic charge is most effective: Al3+ (from Al2(SO4)3) is the most efficient coagulant.
Q19. Which of the following does not promote physical adsorption?
Answer: High temperature
Physical adsorption is favoured by high pressure, low temperature, a high critical temperature of the adsorbate, and an exothermic (negative deltaH) process. High temperature decreases physical adsorption, so 'high temperature' is the factor that does not promote it.
Answer: Zero-order
In zero-order adsorption, the rate of adsorption is constant and independent of the concentration of the gas, which occurs when the surface is fully covered by a single molecular layer at high pressure.
Q21. At the critical micelle concentration (CMC), surfactant molecules
Answer: come together
At the critical micelle concentration the surfactant molecules associate and come together to form micelles.
Answer: 18 mg
Acetic acid adsorbed = (0.06 - 0.042) N * 0.050 L = 0.0009 mol = 0.0009*60 = 0.054 g = 54 mg total. Per gram of charcoal = 54/3 = 18 mg.
Answer: (ii) and (iv)
The Tyndall effect occurs when light is scattered by particles in a colloid or a fine suspension, which requires the particle size to be comparable to the wavelength of light (condition ii). Additionally, a significant difference in refractive indices (condition iv) enhances the scattering effect, making the Tyndall effect observable.
Q24. The aerosol is a kind of colloid in which:
Answer: solid is dispersed in gas
An aerosol consists of tiny solid particles or liquid droplets suspended in a gas, which allows it to remain dispersed in the air, making option A the correct choice.
Answer: 18 mg
The amount of acetic acid adsorbed is calculated by determining the difference in normality before and after adsorption, which indicates how much was removed by the charcoal. In this case, the decrease from 0.06 N to 0.042 N shows that 0.018 N of acetic acid was adsorbed, and when converted to mass per gram of charcoal, it results in 18 mg per gram.
Answer: Only 1/n appears as the slope
In the Freundlich adsorption isotherm, the relationship between log(x/m) and log(p) is linear, where the slope is determined solely by the constant 1/n, indicating the degree of adsorption. The constant k influences the intercept but does not contribute to the slope.
Q27. Which one of the following is not a property of physical adsorption ? [JEE-Main On line-2018]
Answer: Unilayer adsorption occurs
Unilayer adsorption is characteristic of chemisorption, where a single layer of adsorbate forms on the adsorbent surface, while physical adsorption can involve multilayer formation. Therefore, stating that unilayer adsorption occurs is incorrect in the context of physical adsorption.
Answer: 1/n
The linear relationship between log(x/m) and log P indicates that the adsorption follows a power law, where the slope of the plot is related to the exponent of the pressure term in the adsorption isotherm. Since n > 1, the slope being 1/n reflects the inverse relationship between the amount adsorbed and the pressure, confirming that as pressure increases, the amount adsorbed increases at a decreasing rate.
Q29. Which of the following statements about colloids is false?
Answer: Freezing point of colloidal solution is lower than true solution at same concentration of a solute
At the same mass concentration a colloid contains far fewer (much larger) particles than a true solution, so its freezing-point depression is smaller and its freezing point is higher, not lower. Hence the freezing-point statement is the false one.
Answer: C: liquid in solid; M: liquid in liquid; S: solid in gas
Cheese is a gel (liquid dispersed in solid), milk is an emulsion (liquid fat in liquid water), and smoke is an aerosol (solid particles in gas). Hence: C liquid in solid; M liquid in liquid; S solid in gas.
Q31. Haemoglobin and gold sol are examples of:
Answer: Positively and negatively charged sols, respectively
Haemoglobin forms a positively charged sol, while gold sol is negatively charged. Therefore they are positively and negatively charged sols, respectively.
Answer: AlCl3
AlCl3 is effective for coagulating arsenious sulphide sol because it provides aluminum ions, which have a high charge density and can neutralize the negative charges on the colloidal particles, leading to their aggregation and precipitation.
Answer: (A) → (R); (B) → (Q); (C) → (P)
In adsorption chromatography the sample benzaldehyde is the adsorbate (R), alumina is the adsorbent/stationary phase (Q), and acetonitrile is the mobile phase (P). So A->R, B->Q, C->P.
Answer: The osmotic pressure of a colloidal solution is of higher order than the true solution at the same concentration.
The statement about the osmotic pressure of a colloidal solution being higher than that of a true solution at the same concentration is incorrect because colloidal particles do not contribute to osmotic pressure in the same way that solute molecules do, leading to lower osmotic pressure compared to true solutions.
Q35. Among the following the false statement is -
Answer: Latex is a colloidal solution of rubber particles which are positively charged
Latex is actually a colloidal solution of rubber particles that are negatively charged, not positively charged, making this statement false.
Answer: (2) (a) and (b)
(a) Adsorption enthalpy becomes less negative as the surface saturates - true. (b) NH3 (more easily liquefiable, higher critical temp) is adsorbed more than N2 - true. (c) residual surface forces DECREASE on adsorption, and (d) adsorption is exothermic so adsorbate amount DECREASES with temperature - both false. Answer: (a) and (b).
Answer: (i)-(e), (ii)-(c), (iii)-(a), (iv)-(f)
Foam (gas in liquid) = froth (e); Gel (liquid in solid) = jellies (c); Aerosol (solid/liquid in gas) = smoke (a); Emulsion (liquid in liquid) = milk (f). So (i)-(e), (ii)-(c), (iii)-(a), (iv)-(f).
Q38. Kraft temperature is the temperature -
Answer: Above which the formation of micelles takes place
Kraft temperature is the threshold above which surfactants, such as detergents, can effectively form micelles, which are essential for their cleaning action. Below this temperature, micelle formation is limited, reducing the effectiveness of the detergent.
Answer: 48.00
Intercept log k = 0.4771 gives k = 3 and slope 1/n = 2, so x/m = 3*(0.04)^2 = 0.0048 g = 48.00 x 10^-4 g.
Answer: (2)
Ordinary filter paper cannot stop colloidal particles (its pores are larger than colloidal size); only ultrafilters can. Statements about colligative properties, Hardy-Schulze flocculation (Al3+>Na+) and Brownian motion are all correct, so the incorrect one is (2).
Answer: a-ii, b-iv, c-i, d-iii
Deacon's process uses CuCl2 (ii); Contact process uses V2O5 (iv); cracking of hydrocarbons uses zeolite ZSM-5 (i); hydrogenation of vegetable oils uses finely divided Ni (iii). So a-ii, b-iv, c-i, d-iii.
Answer: 1/n
In the Freundlich adsorption isotherm, the parameter x represents the degree of non-linearity in the adsorption process, and it is defined as the reciprocal of the Freundlich constant 'n'. Therefore, at moderate pressures, the extent of adsorption is proportional to P raised to the power of 1/n, indicating that the adsorption capacity increases with pressure but at a diminishing rate.
Answer: Their viscosity is of the order of that of H2O.
Hydrophilic sols typically have a higher viscosity than water due to the presence of larger particles and interactions between them, making the statement about their viscosity being similar to that of water false.
Q44. The nature of charge on resulting colloidal particles when FeCl3 is added to excess of hot water is:
Answer: Positive
When FeCl3 is dissolved in hot water, it dissociates and the iron ions (Fe3+) can hydrolyze to form positively charged colloidal particles. This results in a net positive charge on the colloidal particles.
Answer: KFe[Fe(CN)6]
The correct option, KFe[Fe(CN)6], represents potassium ferricyanide, which forms a colloidal complex with ferric ions in acidic conditions, resulting in the characteristic Prussian blue color.
Q46. Tyndall effect is more effectively shown by:
Answer: lyophobic colloid
The Tyndall effect is most pronounced for lyophobic colloids, whose large, weakly-solvated particles have a refractive index very different from the medium and scatter light strongly. Lyophilic sols scatter weakly; true solutions and suspensions do not show it the same way.
Q47. Lyophilic sols are more stable than lyophobic sols because:
Answer: the colloidal particles are solvated.
Lyophilic (solvent-loving) sols have a strong affinity for the medium, so the particles are heavily solvated with a protective solvent layer. This solvation is the main reason they are far more stable than lyophobic sols, which rely mainly on charge.
Answer: 2.00
Initial moles = 0.2 L x 0.2 M = 0.04 mol; final = 0.2 L x 0.1 M = 0.02 mol, so 0.02 mol adsorbed = 0.02 x 60 = 1.2 g. Per gram of charcoal = 1.2/0.6 = 2.0 g.
Answer: b and c only
Micelle formation is driven by the hydrophobic effect: ordered water around the hydrocarbon chains is released, so the entropy change is positive (c) and the process is endothermic (b). Hence b and c only.
Answer: (2)
The second statement is incorrect because the Tyndall effect occurs when the diameter of the dispersed particles is comparable to or larger than the wavelength of light, not smaller. This scattering of light is what allows us to distinguish colloidal solutions from true solutions.