Surface Chemistry — Page 8 | NEET Chemistry

Q71

A particular adsorption process has the following characteristics : (i) It arises due to van der Waals forces and (ii) it is reversible. Identify the correct statement that describes the above adsorption process :

A Enthalpy of adsorption is greater than 100 kJ mol−1.

B Energy of activation is low.

C Adsorption is monolayer.

D Adsorption increases with increase in temperature.

Correct Answer

Option B

Solution

From the given characteristics, the adsorption is an example of physical adsorption. (i) Physical adsorption is multimolecular layer. (ii) It decreases with increase in temperature. (iii) Enthalpy of adsorption is low 20–40 kJ mol−1 for physical adsorption. (iv) Energy of activation is low because in this type of adsorption no chemical reaction takes place.

Q72

Which of the following statements about colloids is False ?

A Freezing point of colloidal solution is lower than true solution at same concentration of a solute.

B Colloidal particles can pass through ordinary filter paper.

C When silver nitrate solution is added to potassium iodide solution, a negatively charged colloidal solution is formed.

D When excess of electolyte is added to colloidal solution, colloidal particle will be precipitated.

Correct Answer

Option A

Solution

Freezing point of colloidal solution is same as true solution at same concentration of a solute.

Q73

The enthalpy change for the adsorption process and micelle formation respectively are

A

\mathrm{\Delta H_{ads} > 0~and~\Delta H_{mic} < 0}

B

\mathrm{\Delta H_{ads} > 0~and~\Delta H_{mic} > 0}

C

\mathrm{\Delta H_{ads} < 0~and~\Delta H_{mic} < 0}

D

\mathrm{\Delta H_{ads} 0}

Correct Answer

Option D

Solution

Adsorption is the process of a substance (adsorbate) being attracted to a surface (adsorbent).

The enthalpy change for adsorption, $\mathrm{\Delta H_{ads}}$ , is negative because the adsorbent and adsorbate are brought closer together, which releases energy.

Micelle formation is the process of surfactant molecules clustering together in water to form micelles.

The micelles have a hydrophobic (water-repelling) core and a hydrophilic (water-loving) surface.

The enthalpy change for micelle formation, $\mathrm{\Delta H_{mic}}$ , is positive because the surfactant molecules are moving from a state of high entropy (dispersed in water) to a state of low entropy (clustered together).

Here is a table summarizing the enthalpy changes for adsorption and micelle formation: .tg .tg Process Enthalpy change Adsorption $\mathrm{\Delta H_{ads} < 0}$ Micelle formation $\mathrm{\Delta H_{mic} > 0}$

Q74

Adsorption of a gas on a surface follows Freundlich adsorption isotherm. Plot of log

{x \over m}

versus log p gives a straight line with slope equal to 0.5, then: (

{x \over m}

is the mass of the gas adsorbed per gram of adsorbent)

A Adsorption is independent of pressure.

B Adsorption is proportional to the pressure.

C Adsorption is proportional to the square root of pressure.

D Adsorption is proportional to the square of pressure.

Correct Answer

Option C

Solution

According to Freundlich adsorption isotherm, in the median range of pressure

{x \over m} \propto {P^{{1 \over n}}}

$\Rightarrow$

{x \over m}

= kP

^{{1 \over n}}

taking log both sides, we get, log

{x \over m}

= logk +

{1 \over n}

logP Here in graph between log

{x \over m}

and logP, slope is

{1 \over n}

and intercepts = log k. Given that, slope = 0.5

\therefore\,\,\,

{1 \over n}

= 0.5 $\Rightarrow$ n = 2

\therefore\,\,\,

{x \over m}

= kP

^{{1 \over 2}}

= k

\sqrt P

$\therefore$ Adsorption is proportional to the square root of pressure.

Q75

3 g of activated charcoal was added to 50 mL of acetic acid solution (0.06N) in a flask. After an hour it was filtered and the strength of the filtrate was found to be 0.042 N. The amount of acetic acid adsorbed (per gram of charcoal) is:

A 36 mg

B 42 mg

C 54 mg

D 18 mg

Correct Answer

Option D

Solution

Let the weight of acetic acid initially be

{w_1}

in

50

ml

of

0.060

N

solution. Let the

N = {{{w_1} \times 1000} \over {M.wt. \times 50}}\,\,\,

(Normality

=0.06

N

)

0.06 = {{{w_1} \times 1000} \over {60 \times 50}}

\Rightarrow \,\,\,\,\,{w_1} = {{0.06 \times 60 \times 50} \over {1000}}

= 0.18\,g = 180\,mg.

After an hour, the strength of acetic acid

=0.042

N

so, let the weight of acetic acid be

{w_2}

N = {{{w_2} \times 1000} \over {60 \times 50}};0.042 = {{{w_2} \times 1000} \over {3000}}

\Rightarrow \,\,\,\,\,\,\,\,\,\,{w_2} = 0.126g = 126mg

So amount of acetic acid adsorbed per

3g

= 180 - 126\,mg = 54\,mg

Amount of acetic acid adsorbed per

g

= {{54} \over 3} = 18\,mg

Q76

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R Assertion A: Amongst

\mathrm{He}, \mathrm{Ne}, \mathrm{Ar}

and

\mathrm{Kr}

; 1

\mathrm{g}

of activated charcoal adsorbs more of

\mathrm{Kr}

. Reason R: The critical volume

\mathrm{V}_{\mathrm{c}}\left(\mathrm{cm}^{3} \mathrm{~mol}^{-1}\right)

and critical pressure

\mathrm{P}_{\mathrm{c}}

(atm) is highest for Krypton but the compressibility factor at critical point

\mathrm{Z}_{\mathrm{c}}

is lowest for Krypton. In the light of the above statements, choose the correct answer from the options given below

A Both A and R are true and R is the correct explanation of A

B A is false but R is true

C Both A and R are true but R is NOT the correct explanation of A

D A is true but R is false

Correct Answer

Option D

Solution

MW order, $\mathrm{Kr}>\mathrm{Ar}>\mathrm{Ne}>\mathrm{He}$ $Z$ (at critical point) $=\dfrac{3}{8}$

Q77

The coagulating power of electrolytes having ions Na+, Al3+ and Ba2+ for arsenic sulphide sol increases in the order:

A Na+ < Ba2+ < Al3+

B Al3+ < Ba2+ < Na+

C Ba2+ < Na+ < Al3+

D Al3+ < Na+ < Ba2+

Correct Answer

Option A

Solution

According to Hardly Schulze rule, greater the charge or cation, greater is its coagulating power for negatively charged solution.

Hence the correct order of coagulating power :

N{a^ + } < B{a^{2 + }} < A{l^{3 + }}

Q78

For coagulation of arsenious sulphate sol, which one of the following salt solution will be most effective ?

A BaCl2

B AlCl3

C NaCl

D Na3PO4

Correct Answer

Option B

Solution

Sulphide is $-$ ve charged colloid.

As coagulation is directly proportional to the effective charge, so cation with maximum charge will be most effective for coagulation.

Al3+ > Ba2+ > Na+ coagulating power.

Q79

Among the colloids cheese (C), milk (M), and smoke (S), the correct combination of the dispersed phase and dispersion medium, respectively is :

A C : liquid in solid ; M : liquid in liquid ; S : solid in gas

B C : liquid in solid ; M : liquid in solid ; S : solid in gas

C C : solid in liquid ; M : liquid in liquid ; S : gas in solid

D C : solid in liqid ; M : solid in liquid ; S : solid in gas

Correct Answer

Option A

Solution

Cheese (C) is liquid dispersed in solid. Milk (M) is liquid dispersed in liquid. Smoke (S) is solid dispersed in gas.

Q80

The Tyndall effect is observed only when following conditions are satisfied: (a) The diameter of the dispersed particles is much smaller than the wavelength of the light used. (b) The diameter of the dispersed particle is not much smaller than the wavelength of the light used. (c) The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude. (d) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.

A (b) and (d)

B (a) and (c)

C (b) and (c)

D (a) and (d)

Correct Answer

Option A

Solution

Tyndall effect is observed only when (i) The diameter of the dispersed particle is not much smaller than the wavelength of the light used. (ii) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.

So, (b) and (d) are correct.