Short Question Answer

Surface Chemistry

Short Answer

1. Give two important characteristics of physiscorption

1. It is instantaneous and non specific

2. When pressure increases, physisorption also increases

3. Multilayer of adsorbate formed

2. Differentiate physisorption and chemisorption

Physical adsorption or van der waals adsorption or Physisorption

1. It is instantaneous

2. It is non-specific

3. Pressure increases the amount of adsorption increases.

4. Decreases with increase in temperature.

5. Heat of adsorption is low

6. Multilayer

7. Occurs on all sides.

Chemical adsorption or Chemisorption or Activated adsorption

1. It is very slow

2. It is specific

3. Pressure can not alter.

4. When temperature is raised first increases and then decreases.

5. Heat of adsorption is high

6. Monolayer

7. Occurs at fixed sites

3. In case of chemisorption, why adsorption first increases and then decreases with temperature?

• Increase in temperature will provide the molecule necessary activation energy for chemical bond formation hence the rate of adsorption increases.

• At a certain temperature all bonds are formed and now further increase in temperature will favours desorption.

4. Which will be adsorbed more readily on the surface of charcoal and why? NH3 or O2 ?

• The critical temperature of ammonia gas is quite higher than carbon dioxide.

• Therefore it is easily combines with the surface of charcoal.

• Therefore NH₃ will be more readily adsorbed on the surface of the charcoal than CO₂.

5. Heat of adsorption is greater for chemisorptions than physisorption. Why?

• In adsorbed state the adsorbate is held on the surface of the adsorbent by strong chemical bond and it is much stronger than physisorption.

• Hence heat of adsorption is greater.

6. In a coagulation experiment 10 mL of a colloid (X) is mixed with distilled water and 0.01M solution of an electrolyte AB so that the volume is 20 mL. It was found that all solutions containing more than 6.6 mL of AB coagulate with in 5 minutes. What is the flocculation values of AB for sol (X)?

Answer:

A minimum of 6.6mL of AB is required to coagulate the sol. The moles of AB in the sol is

(6.6 × 0.01) / 20 = 0.0033 moles

This means that a minimum of 0.0033 moles or 0.0033 × 1000 = 3.3 milli moles are required for coagulating 1 litre of sol.

Flocculation value of AB for X = 3.3

7. Peptising agent is added to convert precipitate into colloidal solution. Explain with an example.

• Addition of suitable electrolytes to the precipitated particles give colloidal solution.

• The process is termed as peptisation and the electrolyte added is called peptising agent.

Example:

 AgCl       ^___HCl→      AgCl

Precipitate                 Colloid

AgCl precipitate is converted to AgCl colloid by adding an peptising agent HCl.

8. What happens when a colloidal sol of Fe(OH)3 and As2S3 are mixed?

• Fe(OH)₃ is positively charged colloid whereas AS₂O₃ is negatively charged colloid.

• Coagulation takes place when the positively charged Fe(OH)₃ is mixed with negatively charged AS₂O₃.

9. What is the difference between a sol and a gel?

Sol: Liquid in solid is called sol

Example: Inks, colloidal gold

Gel: Solid in liquid is called gel

Example: Butter, cheese

10. Why are lyophillic colloidal sols are more stable than lyophobic colloidal sol.

• Due to solvation the interaction between the dispersed phase and dispersed medium is strong in the lyophillic colloidal sols.

• In lyophobic sol the stability is due to charge. The interaction between the dispersed phase and dispersion medium is very little. Hence lyophillic colloidal sols are more stable than lyophobic colloidal sol.

11. Addition of Alum purifies water. Why?

Addition of alum coagulate the suspended impurities in water and the drinking water is purified.

12. What are the factors which influence the adsorption of a gas on a solid?

i) Nature of adsorbent

ii) Nature of adsorbate

iii) Pressure

iv) Temperature

i. Nature of adsorbent

• Higher the surface area of adsorbent, higher is the amount adsorbed.

ii. Nature of adsorbate

• Due to greater van der waal’s force of attraction easily liquefiable gases are adsorbed readily. Eg: CO₂, NH₃

• Permanent gases are having low critical temperature, can not be liquefied easily and adsorbed slowly. Eg. H₂, N₂ and O₂

iii. Effect of Pressure

• In Physisorption, when pressure increases the amount of adsorption increases.

• Pressure can not alter the amount of chemisorption.

iv. Effect of Temperature

• Physisorption decreases with increase in temperature.

• When temperature is raised chemisorption first increases and then decreases

13. What are enzymes? Write a brief note on the mechanism of enzyme catalysis.

Enzymes are complex protein molecules with three dimensional structures. It catalyse the chemical reaction in living organism.

The following mechanism is proposed for the enzyme catalysis

E + S ⇌ ES → P + E

Where E is the enzyme

S is the substrate (reactant)

ES is activated complex

P is the product.

14. What do you mean by activity and selectivity of catalyst?

Activity: The reactants must get adsorbed strongly on the catalyst to become active. The activity depends on the strength of chemisorption.

Selectivity: it is ability of a catalyst to direct a reaction to yield a particular product selectively.

15. Describe some feature of catalysis by Zeolites.

• Zeolites are microporous, crystalline, hydrated, alumino silicates

• Extra cations are present in the framework, to balance the extra negative charge.

• It is shape selective.

• Reactant shape selectivity: When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal

• Transition state selectivity: If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed.

• Product selectivity: It is encountered when certain product molecules one too big to diffuse out of the zeolite pores

• Zeolites carrying protons are used in the petrochemical industry

Zeolites carring Na⁺ ions are used as catalysis

16. Give three uses of emulsions.

1. Food stuffs like milk cream, butter, etc are present in colloidal form

2. The cleansing action of soap is due to the formation of emulsion of soap molecules with dirt and grease

3. Latex is the emulsion of natural rubber with negative particles. By heating rubber with sulphur, vulcanized rubbers are produced for tyres, tubes

17. Why does bleeding stop by rubbing moist alum

Blood is a colloidal form. When alum is rubbed to the bleeding part of the body the coagulation of blood takes place and the bleeding is arrested.

18. Why is desorption important for a substance to act as good catalyst?

• The formed product should leave from the surface of the catalyst to create free surface for other reactant molecule.

• If the desorption does not occur then the other reactants are left with no space on the surface of the catalyst and the yield will be poor.

• Hence desorption is important for a substance to act as good catalyst.

19. Comment on the statement: Colloid is not a substance but it is a state of substance.

The size of the colloidal particle is between 1 - 200 nm. Any particle is present between this size are behaves as colloid. Hence it is a state of substance and not a substance.

20. Explain any one method for coagulation

By mixing two oppositively charged sol:

When colloidal sols with opposite charges are mixed mutual coagulation takes place. It is due to migration of ions from the surface of the particles

21. Write a note on electro osmosis

The movement of dispersion medium under the influence of electric potential is called electro osmosis.

A sol is electrically neutral. The medium carries an equal but opposite charge to that of dispersed particles.

Under the influence of electric field the medium moves in a direction opposite to that of the sol particles.

22. Write a note on catalytic poison

• The substances which decreases or completely destroys the activity of catalyst in a catalysed reaction are known as catalytic poisons.

• In the Haber's process, the Fe catalyst is poisoned by H₂S.

23. Explain intermediate compound formation theory of catalysis with an example

i) In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate

ii) Activation energy for the intermediate formation is lower than product formation

iii) The intermediate reacts with other reactant or decompose to give products and the catalyst is regenerated.

Consider the general reaction:

A+B → AB                   (1)

A+C → AC (intermediate)            (2)

C is the catalyst

AC+B → AB+C          (3)

The formation and decomposition of the intermediate accelerate the rate of the reaction.

Example

Thermal decomposition of KClO₃ in presence of MnO₂ proceeds as follows.

2KClO₃ → 2KCl+3O₂

Mechanism

2KClO₃ + 6MnO₂ → 6MnO₃ + 2KCl

MnO₃ is an intermediate. 

6MnO₃ → 6MnCO₂ +3O₂

Catalyst MnO₂ is regenerated

24. What is the difference between homogenous and hetrogenous catalysis?

Homogenous catalysis

The reactants, products and catalyst are present in the same phase

Eg: C₁₂H₂₂O₁₁ + H₂O  ^___H2SO4__→ C₆H₁₂O₆ + C₆H₁₂O₆

Heterogeneous catalysis

Catalyst is not present in the same phase as that of reactants or products.

Eg:  N₂ + 3H₂ ←^__Fe_→  2NH₃

25. Describe adsorption theory of catalysis.

Adsorption theory also called as contact catalysis.

Langmuir explained the various steps involved in a heterogeneous catalysed reaction.

1. Reactant molecules diffuse from bulk to the catalyst surface.

2. The reactant molecules are adsorbed on the surface of the catalyst.

3. The adsorbed reactant molecules are activated and form activated complex which is decomposed to form the products

4. The product molecules are desorbed.

5. The product diffuse away from the surface of the catalyst.