Surface Chemistry

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Questions:

Generic 2

4 x 2 = 8

Write the reaction in which bismuthmolybdate is used as catalyst. Explain which characteristic of catalyst is shown by this reaction.

The action of enzyme is highly specific. Do you agree with this statement? Explain.

Define the following :
(i) Peptization
(ii) Reversible sols

What is meant by coagulation of a colloidal solution? Name any method by which coagulation of lyophobic sols can be carried out.

Generic 3

3 x 3 = 9

Classify colloids where the dispersion medium is water. State their characteristics and write an example of each of these classes.

(a) Which will adsorb more gas, a lump of charcoal or its powder and why?
(b) Describe the preparation of the following colloidal solutions. Name the method used in each case:
(i) Silver sol
(ii) Sulphur sol

(a) What are the two types of emulsions and how do they differ from one another? Give one example of each.
(b) Which one of the following electrolytes is most effective for the coagulation of Fe(OH)₃ sol and why?
NaCl, Na₂SO₄ , Na₃ PO₄

Multiple Choice Question

5 x 1 = 5

Which of the following relation is correct ?
(i) x/m = constant at high pressure
(ii) x/m = constant xp^1/n (at intermediate pressure)
(iii) x/m = constant xpⁿ (at lower pressure)

(a)

all are correct

(b)

all are wrong

(c)

(i) and (ii) are correct

(d)

(iii) is correct

Which of the following are the characteristics of chemisorption ?

1. High heat of adsorption

2. Irreversibility

3. Low activation energy

Select the correct answer using the code given below :

(a)

1 and 2 only

(b)

1 and 3 only

(c)

2 and 3 only

(d)

1,2 and 3

10.

The function of gum arabic in the preparation of Indian ink is

(a)

Coagulation

(b)

Peptization

(c)

Protective action

(d)

Adsorption.

11.

Which of the following is correct about lyophilic sols?

(a)

They are irreversible

(b)

They are formed by inorhanic substances.

(c)

They are readily coagulated by addition of electrolytes.

(d)

They are self-stabilized.

12.

The most adsorbed gas on activated charcoal is

(a)

N₂

(b)

H₂

(c)

CO₂

(d)

CH₄

Generic 5

4 x 5 = 20

13.

Reetu is purely vegetarian but her husband Rohit likes non-vegetarian food. He regularly buys nonvegetarian goods (fish, meet, eggs etc.) and store these in fridge. The intermixing of odours creates problem for Reetu. Their daughter Rihana is a student of science. One day Rihana suggested her mother to put some small pieces of charcoal in the fridge as she thinks that this will solve the problem to some extent.
(i) Do you think by putting some small pieces of charcoal in the fridge will solve the problem of intermixing of odours. Give reason for your answer.
(ii) What are the values associated with Rihana's suggestion?

14.

Radha and Meera are fast friends. They both study in class XII.Radha is a student of science while Meera follows commerce stream. One day, when they were playing, Radha got hurt and started bleeding. Meera took her to house and applied some alum on the cut. The bleeding stopped immediately.
(i)Why bleeding stopped when some alum is applied on the cut?
(ii) What are the values associated with Meera's decision to take Radha to her house and apply alum on her cut?

15.

In an adsorption experiment, a graph between log (x/m) versus log P was found to be linear with a slope of 45^o. The intercept on the log (x/m) axis was found to be 0.310. Calculate the amount of the gas adsorbed per gram of charcoal under a pressure of 0.5 atmosphere.

16.

20% of surface sites are occupied by N₂ molecules. The density of surface sites is \(6.023\times { 10 }^{ 14 }\) cm^-2 and total surface area is 1000 cm² . The catalyst is heated to 300 K while N₂ is completely disorbed into a pressure of 0.001 atm and volume 2.46 cm³. Find the active sites occupied by each N₂ molecule.

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Answers Key:

Generic 2

4 x 2 = 8

\(\underset { Propene }{ { CH }_{ 2 }=CH-{ CH }_{ 3 }+{ O }_{ 2 } } \xrightarrow{BiMoO_4}\underset { Acrolein }{ { CH }_{ 2 }=CH-{ CH }O } +H_2O\)
This reaction explains selectivity of the catalyst

Yes, the activity of enzymes is highly specific because they catalyse specific reaction and do not catalyse another. They can bind to specific substrate and not the other.

Peptisation. The process of converting freshly prepared precipitates into colloidal form by the addition of a suitable electrolyte is called peptization. The electrolytes used for the purpose are called peptizing agents. For example, on treating precipitate of iron (III) oxide with a small amount of FeCl3 solution, gives a reddish brown colored colloidal solution. During peptization, the suitable ions from the added electrolyte are adsorbed by the particles of the precipitate. The charged particles repel one another and form a colloidal solution. In this case, Fe3+ ions from ferric chloride are adsorbed by Fe(OH)3 precipitate:
\(Fe{ \left( OH \right) }_{ 3 }+{ Fe }^{ 3+ }\rightarrow Fe{ \left( OH \right) }_{ 3 }.{ Fe }^{ 3+ }\)
Similarly, a precipitate of silver chloride can be peptised-by shaking with a dilute solution of silver nitrate to give a colloidal solution of silver chloride.
(ii) The reversible sols are those in which if the dispersion medium is separated from the dispersed phase (say by evaporation), the sol can again be formed by simple remixing it with the dispersion medium. Lyophilic sols are reversible in nature.

The phenomenon of precipitation of a colloidal sol by the addition pf excess of an electrolyte is called coagulation of cell ideal sol. Coagulation of lyophobic sols can be carried out by the following methods:
(i) By addition of an electrolyte: When an electrolyte is added to the sol, colloidal particles take up ions carrying opposite charge from the electrolyte and get neutralized resulting in coagulation

Generic 3

3 x 3 = 9

(i) Sol: When solid is dispersed in water it is called sol, e.g. starch sol and gold sol.
(ii) Emulsion: When liquid is dispersed in water, it is called emulsion, e.g. milk.
(iii) Foam: When gas is dispersed in water, it is called foam or froth, e.g. lemonade froth.

(a) The charcoal powder will adsorb more gas because it has more surface area.
(b) (i) Bredig's Arc method: Take silver electrodes in form of electric arc in water and pass high voltage through it. A lot of heat will be generated which will vaporize silver metal into vapors which on condensing form silver sol.
(ii) Oxidation: Pass H2S gas through an aqueous solution of nitric acid. A colloidal solution of sulfur will be obtained.
\({ H }_{ 2 }S+[O]\xrightarrow [ Conc.H{ NO }_{ 3 } ]{ } { H }_{ 2 }O+S\)

(a) (i) Oil in water: When oil is dispersed in water, it is called oil in water, e.g. milk.
(ii) Water in oil: When water is dispersed in oil, it is called water in oil, e.g. butter in a liquid state.
(b) Fe(OH)3 is a positively charged colloid, therefore, Na3 P04 will be most effective for coagulation. It is because PO!- ions are negatively charged and have the highest charge among these electrolytes.

Multiple Choice Question

5 x 1 = 5

(a)
1 and 2 only

10.

11.

(d)
They are self-stabilized.

12.

Generic 5

4 x 5 = 20

13.

(i) Yes, putting some small pieces of charcoal in the fridge can solve the problem of intermixing of odours to some extent. This is because charcoal is a good adsorbent of odours or vapours. Further breaking a big lump of charcoal into small pieces will increase the surface area of adsorbent, more is the extent of adsorption.
(ii) Values associated with Rihana's suggestion are:
(i) Love for her parents
(ii) Keen observation
(iii) Use of chemistry in daily life
(iv) A will to experiment and explore
(v) Presence of mind.

14.

(i) Blood is a colloidal solution of albuminoid substance. The styptic action (blood stopping action) of alum is due to coagulation of blood forming a clot which stops further bleeding.
(ii) Values associated with Meera's decision are:
(a) Friendship
(b) Keen observation
(c) Applying knowledge of chemistry to daily life
(d) Responsibility
(e) Love and care.

15.

At P = 0.5 atm, \(\frac { x }{ m } kP1/n\ =\ 2\times { (0.5) }^{ 1 }=1.0\)

16.

\(\frac { No.of\quad sites\quad occupied }{ No.of\quad { N }_{ 2 }\quad molecules } =\frac { 1.2046\times { 10 }^{ 17 } }{ 6.023\times { 10 }^{ 16 } } =2\)