surface chemistry Flashcards
(96 cards)
1
Q
what is adsorption
A
The accumulation of molecular species
at the surface rather than in the bulk of a solid or liquid is termed adsorption.
2
Q
what is adsorbate??
A
The molecular species or substance, which concentrates or
accumulates at the surface is termed adsorbate
3
Q
what is adsobent??
A
the material on the
surface of which the adsorption takes place is called adsorbent.
4
Q
Distinction between Adsorption and Absorption
A
in adsorption the concentration of the adsorbate increases only at the
surface of the adsorbent, while in absorption the concentration is
uniform throughout the bulk of the solid.
5
Q
what do you mean by sorption
A
when adsorption and absorption takes place simultaneously
6
Q
Mechanism of Adsorption
A
Adsorption arises due to the fact that the surface particles of the adsorbent
are not in the same environment as the particles inside the bulk.
Inside the adsorbent all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms
or molecules of their kind on all sides, and hence they possess unbalanced
or residual attractive forces. These forces of the adsorbent are responsible
for attracting the adsorbate particles on its surface.
7
Q
the extent of adsorption varies with??
A
The extent of
adsorption increases with the increase of surface area per unit mass of
the adsorbent at a given temperature and pressure.
8
Q
what is the heat of adsorption
A
During adsorption, there is always a decrease in residual
forces of the surface, i.e., there is decrease in surface energy which
appears as heat.
9
Q
is adsorption a exothermic process??
A
yes,, ∆H of adsorption is always negative.
10
Q
what happens when gas is adsorbed??
A
When a gas is adsorbed, the freedom of movement of its molecules become
restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., ∆S is negative.
11
Q
what is adsorption accompanied with
A
Adsorption is thus accompanied by
decrease in enthalpy as well as decrease in entropy of the system.
12
Q
condition for process to be spontaneous is
A
, the thermodynamic requirement is that,
at constant temperature and pressure, ∆G must be negative, i.e., there
is a decrease in Gibbs energy. On the basis of equation, ∆G = ∆H – T∆S
13
Q
is adsorbtion a spontaneous process
A
∆G can be negative if ∆H has sufficiently high negative value as – T∆S
is positive. Thus, in an adsorption process, which is spontaneous, a
combination of these two factors makes ∆G negative.
14
Q
what happen as the process proceeds
A
As the adsorption
proceeds, ∆H becomes less and less negative ultimately ∆H becomes
equal to T∆S and ∆G becomes zero. At this state equilibrium is attained.
15
Q
what is positive adsorbtion
A
If the concentration of an adsorbate at the surface of adsorbent is more than in the bulk of
the adjoining phases, it is called positive adsorption.
16
Q
what is negative adsorption
A
If concentration of an adsorbate at the surface of adsorbent is less than in the bulk of the
adjoining phases, it is called negative adsorption.
17
Q
what is desorption
A
The removal of the adsorbed substance from a surface is called desorption.
18
Q
types of adsorption
A
physical adsorption and chemical adsorption.
19
Q
what is physical adsorption
A
If accumulation of gas on the surface of a solid occurs on account of
weak van der Waals’ forces, the adsorption is termed as physical
adsorption or physisorption.
20
Q
what is chemical adsorption
A
When the gas molecules or atoms are
held to the solid surface by chemical bonds, the adsorption is termed
chemical adsorption or chemisorption
21
Q
what type of chemical bond are formed during chemisorption.
A
The chemical bonds may be
covalent or ionic in nature.
22
Q
energy involved during chemical adsorption
A
Chemisorption involves a high energy of
activation and is, therefore, often referred to as activated adsorption.
23
Q
can these physical adsorption and chemical adsorption processes occur simultaneously??
A
A physical adsorption at low
temperature may pass into chemisorption if the temperature is increased.
24
Q
characteristics of physisorption
A
1. It arises because of van der Waals’ forces. 2. It is not specific in nature. 3. It is reversible in nature. 4. It depends on the nature of gas. More easily liquefiable gases are adsorbed readily. 5. Enthalpy of adsorption is low (20-40 kJ mol–1 )in this case. 6. Low temperature is favourable for adsorption. It decreases with increase of temperature. 7. No appreciable activation energy is needed. 8. It depends on the surface area. It increases with an increase of surface area. 9. It results into multimolecular layers on adsorbent surface under high pressure.
25
when are Vander Waal's bond stronger
vander Waals’ forces are stronger near the critical temperatures.
26
what is Le–Chateliers’s principle
More of gas is adsorbed when pressure is increased as the
volume of the gas decreases ,,Since the adsorption
process is exothermic, the physical adsorption occurs readily at
low temperature and decreases with increasing temperature
27
characteristics of chemical adsorbtion
```
1. It is caused by chemical bond
formation.
2. It is highly specific in nature.
3. It is irreversible.
4. It also depends on the nature
of gas. Gases which can react
with the adsorbent show
chemisorption.
5. Enthalpy of adsorption is high
(80-240 kJ mol–1) in this case.
6. High temperature is favourable
for adsorption. It increases with
the increase of temperature.
7. High activation energy is
sometimes needed.
8. It also depends on the surface
area. It too increases with an
increase of surface area.
9. It results into unimolecular
layer.
```
28
what is adsorption theorm
The variation in the amount of gas adsorbed by the adsorbent with
pressure at constant temperature can be expressed by means of a
curve termed as adsorption isotherm.
29
what does Freundlich adsorption isotherm do
Freundlich, in 1909, gave an
empirical relationship between the quantity of gas adsorbed by unit
mass of solid adsorbent and pressure at a particular temperature.
30
how will be the relationship can be explain(Freundlich adsorption isotherm)
The
relationship can be expressed by the following equation:
x/m=k.p^1/n (n>1)
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particulartemperature.
31
how is the relationship explained (Freundlich adsorption isotherm)
The relationship is generally represented in the
form of a curve where mass of the gas adsorbed per gram of
the adsorbent is plotted against pressure (Fig. 5.1). These curves
indicate that at a fixed pressure, there is a decrease in physical
adsorption with increase in temperature. These curves always
seem to approach saturation at high pressure.
32
who do we verify Freundlich adsorption theorm
The validity of Freundlich isotherm can be
verified by plotting log x/m on y-axis (ordinate)
and log p on x-axis (abscissa). If it comes to be
a straight line, the Freundlich isotherm is valid,
otherwise not
33
the slope of straight line in Freundlich adsorption isotherm is given by
1/n ,. The intercept
on the y-axis gives the value of log k.
Freundlich isotherm explains the behaviour
of adsorption in an approximate manner. The
factor 1/n can have values between 0 and 1
34
when adsorption independent of pressure
when 1/n=0
35
when does adsorption varies directly with pressure
when 1/n=1
36
when does Freundlich adsorption isotherm fail??
The experimental isotherms always seem to approach saturation at
high pressure. This cannot be explained by Freundlich isotherm. Thus,
it fails at high pressure.
37
for which process Freundlich adsorption isotherm valid
physical adsorption.
38
what do we do to overcome Freundlich adsorption isotherm's limitations
To overcome the limitation of Freundlich adsorption isotherm,
Langmiur developed a new isotherm called as Langmuir Adsorption Isotherm
39
how is Langmuir Adsorption Isotherm represented
x/m=ap/1+bp (a and b are constants)
at very high pressure ,(bp>>>1) x/m=a/b
At very low pressure,(bp<<<<1) x/m=ap
● In case of chemisorption, x m/ initially increases with
temperature and then decreases.
● At low temperature, x/m is small. As temperature is
increased, the molecules of the adsorbate gain energy and
become equal to activation energy. Therefore, initially, x/m
increases with rise in temperature.
The graph between extent of adsorption ( / ) x m and
temperature T is called adsorption isobar.
40
what is adsorption from solution phase
Solids can adsorb solutes from solutions also.
41
characteristics of adsorption from solution phase
(i) The extent of adsorption decreases with an increase in temperature.
(ii) The extent of adsorption increases with an increase of surface area
of the adsorbent.
(iii) The extent of adsorption depends on the concentration of the solute
in solution.
(iv) The extent of adsorption depends on the nature of the adsorbent
and the adsorbate.
42
mechanism of adsorption from solution phase
The precise mechanism of adsorption from solution is not known.
Freundlich’s equation approximately describes the behaviour of
adsorption from solution with a difference that instead of pressure,
concentration of the solution is taken into account,
43
is Freundlich adsorption isotherm and Langmuir Adsorption Isotherm applicable in adsorption from solution phase
Freundlich adsorption isotherm and Langmuir adsorption
isotherm are also applicable to adsorption from solutions.
Freundlich adsorption isotherm, x/m=kc^1/n
Langmuir Adsorption Isotherm, x/m=aC/(1+bC)
(C is the equilibrium concentration, i.e., when adsorption is complete)
44
application of adsorption
(i) Production of high vacuum
(ii) Gas masks
(iii) Control of humidity
(iv) Removal of colouring matter from solutions:
(v) Heterogeneous catalysis
45
what are catalysts??
Substances, which accelerate the rate of a chemical reaction and
themselves remain chemically and quantitatively unchanged after the
reaction, are known as catalysts, and the phenomenon is known as
catalysis
46
what are promoters and poisons
Promoters are substances that enhance the activity of a catalyst while
poisons decrease the activity of a catalyst
47
catalyst can be broadly divided into 2 groups namely
homogeneous and hetrogeneous
48
what are homogeneous catalyst
When the reactants products and the catalyst are in the same phase
(i.e., liquid or gas), the process is said to be homogeneous catalysis.
49
what are heterogenous catalyst
The catalytic process in which the reactants and the catalyst are in
different phases is known as heterogeneous catalysis.
50
what is Adsorption Theory of Heterogeneous Catalysis
This theory explains the mechanism of heterogeneous catalysis.
The old theory, known as adsorption theory of catalysis, was
that the reactants in gaseous state or in solutions, are adsorbed
on the surface of the solid catalyst.
51
what is modern Adsorption Theory of Heterogeneous Catalysis
The modern adsorption theory is the combination of intermediate
compound formation theory and the old adsorption theory.
52
mechanism of Adsorption Theory of Heterogeneous Catalysis
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst’s surface through
formation of an intermediate (Fig. 5.3).
(iv) Desorption of reaction products from the catalyst surface, and thereby,
making the surface available again for more reaction to occur.
(v) Diffusion of reaction products away from the catalyst’s surface.
The surface of the catalyst unlike the inner part of the bulk, has
free valencies which provide the seat for chemical forces of
attraction. When a gas comes in contact with such a surface, its
molecules are held up there due to loose chemical combination. If
different molecules are adsorbed side by side, they may react with
each other resulting in the formation of new molecules. Thus,
formed molecules may evaporate leaving the surface for the fresh
reactant molecules.
This theory explains why the catalyst remains unchanged in mass
and chemical composition at the end of the reaction and is effective
It however, does not explain the action of
catalytic promoters and catalytic poisons.
53
what are the two important features of solid catalyst
activity and selectivity
54
activity of solid catalyst
The activity of a catalyst depends upon the strength of chemisorption
to a large extent. The reactants must get adsorbed reasonably
strongly on to the catalyst to become active
However, they must
not get adsorbed so strongly that they are immobilised and other
reactants are left with no space on the catalyst’s surface for
adsorption.
55
selectivity of solid catalyst
2. Selectivity of a catalyst is its ability to direct a reaction to
yield a particular product.
Thus, it can be inferred that the action of a catalyst is highly selective
in nature. As a result a substance which acts as a catalyst in one
reaction may fail to catalyse another reaction.
56
characteristics of catalysts
● They become temporarily involved in a reaction providing
an alternative reaction path of lower activation energy than
that for the uncatalysed reaction.
● They catalyse both forward and backward reactions to the
same extent and thus have no effect on the equilibrium
constant.
● The catalyst remains unchanged in amount and chemical
composition at the end of the reaction. It may undergo some
physical change.
● In certain reactions, the rate of the reaction is dependent on
the concentration of the catalyst, e.g. rate of inversion of
cane sugar is dependent on the concentration of H
+ used as
catalyst.
● Rate of the reaction in certain heterogeneous reaction varies
with surface area of the catalyst. Hence, finely divided
metals are preferred in the form of catalyst.
● The catalyst does not initiate the reaction and are specific in
their action, e.g. starting from H2
and CO, three different
products are possible using different catalysts
57
what are enzyme catalysis
Enzymes are biochemical catalysts. They are proteins and
| extremely specific in nature.
58
mechanism of enzyme catalysis
```
There are a number of cavities present on the surface of colloidal particles
of enzymes. These cavities are of characteristic shape and possess active
groups such as -NH2
, -COOH, -SH, -OH, etc. These are actually the activecentres on the surface of
enzyme particles. The
molecules of the reactant
(substrate), which have
complementary shape, fit
into these cavities just like
a key fits into a lock. On
account of the presence of
active groups, an activated
complex is formed which
then decomposes to yield
the products
```
59
steps of mechanism of enzyme catalysis
step 1:
| E+ S----->,E+P (slow, rate determining)
60
what is auto catalysis
Autocatalysis is the phenomenon in which one of the
| products formed during the reaction acts as catalyst
61
what is induced catalysis
is the type of catalysis, one reaction
influences the rate of other reaction which does not
occur under ordinary condition. For example, the
reduction of HgCl2 by oxalic acid is slow but becomes
faster if reduction is made in mixture of KMnO4
and
HgCl2
, where both are reduced. Reduction of KMnO4
thus, induces the reduction of HgCl2
62
what are colloids
A colloid is a heterogeneous system in which one substance is
dispersed (dispersed phase) as very fine particles in another substance
called dispersion medium.
63
Distinction Among True Solutions,
| Colloids and Suspension
Depending upon the size of dispersed particles In true solutions particles are present as single molecule or ion. The size of the dispersed
particles is less than 1 nm. True solutions are homogeneous.
In colloidal solution, colloidal particles have an anormous surface area per unit mass
as a result of their small size. It is heterogeneous.
64
classification of colloids (on the basis of appperance)
The colloidal systems, in which the dispersion medium is
a liquid or gas, are called sols. They are called hydrosol or
aquasol, if the dispersion medium is water.
vWhen the
dispersion medium is alcohol or benzene, they are
accordingly called alcosol or benzosol
The colloidal systems, in which the dispersion medium is
gas are called aerosols.
65
classification of colloids (on the basis of affinity of phase)
Lyophilic colloids and Lyophobic colloids
66
Lyophilic colloids
Lyophilic colloids represent such colloidal systems in
which the particles of dispersed phase have great affinity
for the dispersion medium.
These are reversible colloids
If water is the dispersion medium, the terms used are
hydrophilic and hydrophobic colloids.
67
lyophobic colloids
Lyophobic colloids represent such colloidal systems in
which particles of the dispersed phase have no affinity for
the dispersion medium.
These are irreversible
If water is the dispersion medium, the terms used are
hydrophilic and hydrophobic colloids.
Lyophobic colloids are less stable due to the presence of
electric charge on their particles.
68
classification of colloids (on the basis of molecular size)
Multimolecular colloids, Macromolecular colloids &Associated colloids or micelles
69
Multimolecular colloids
Multimolecular colloids are the colloids in which
colloidal particles consist of aggregate of atoms or small
molecules with diameter less than 10−9 m or 1 nm,
70
Macromolecular colloids
Macromolecular colloids are the colloids in
which colloidal particles themselves are large molecules, Associated colloids or micelles are the substances which
behave as normal electrolytes at low concentration but as
colloids at higher concentration. This is because at higher
concentration, they form associated particles called
micelles
The concentration above which micelle formation occurs is called CMC (Critical Micelle Concentration) and the temperature above which micelle formation occurs is called Kraft temperature.
71
preparation of colloids
Lyophilic colloids may be prepared by simply warming the
solid with the liquid dispersion medium. On the other hand,
lyophobic colloids have to be prepared by special methods.
72
dispersion methods
These methods involve the breaking of bigger particles to the
size of colloidal particles.
Electro disintegration method and Peptisation
73
Electro disintegration method
his
process involves dispersion as well as condensation. By
this method, colloidal solutions of metals like gold, silver
etc., are obtained.
74
Peptisation
involves the conversion of freshly prepared
precipitate into colloidal solution by adding suitable
electrolyte. The suitable electrolyte is known as
peptising agent.
75
condensation or chemical methods
These methods involve the growing of size of the dispersed
phase or the size of colloidal particles.
(i) By double decomposition
(ii) By hydrolysis
(iii) By oxidation
76
(i) By double decomposition
As2O3+3H2S-------->As3S3+3H2O
77
(ii) By hydrolysis
FeCl3+3H2O------->Fe(OH)3+3Hcl
78
(iii) By oxidation
H2S+2HN03------->2H20+2NO2+S
79
purification of colloid solution
(i) Dialysis
(ii) Electrodialysis
(iii) Ultrafiltration
(iv) Ultra-centrifugation
80
dialysis
It is the process of removing small molecules or
ions from a sol by diffusion through a semipermeable
membrane. In this process, impure colloidal solution is
placed in a bag of semipermeable membrane, dipping in
water, the ions diffuse through membrane. Ferric
hydroxide sol can be purified by this method.
81
(ii) Electrodialysis
If dialysis is carried out under the
influence of electric field, it is called electrodialysis. This
speeds up the migration of ions to the opposite
electrodes.
82
(iii) Ultrafiltration
Separation of sol particles from the liquid
medium and electrolytes by filtration through an ultrafilter is
called ultrafiltration.
83
(iv) Ultra-centrifugation
In ultra-centrifuge, the colloidal
particles settle down at the bottom and impurities remain in
the solution.
84
properties of colloidal solutions
(i) Brownian Movement
(ii) Tyndall Effect
iii) Electrophoresis
(iv) Coagulation or Flocculation
(v) Hardy-Schulze Rule
85
(i) Brownian Movement
colloidal particles are always in a state
of rapid random motion (zig-zag movement) which is termed
as Brownian movement.
86
(ii) Tyndall Effect
When a strong and converging beam of light is
passed through a colloidal solution, its path becomes visible
due to scattering of light by particles. It is called Tyndall effect
87
iii) Electrophoresis
The phenomenon, involving the migration of
colloidal particles under the influence of electric field
towards the oppositely charged electrode, is called
electrophoresis.
● Sedimentation potential or Dome effect is the reverse of
electrophoresis. It is set up when a particle is forced to
move in a resting liquid.
● Electrophoretic mobility of colloidal particles is the
distance travelled by particles in one second under a
potential gradient in one volt per cm. Since, different
colloids have different mobilities, so this method is used
for separation of proteins, nucleic acids, polysaccharides
etc
88
(iv) Coagulation or Flocculation
The precipitation of particles of
the dispersed phase in a sol is known as coagulation. The
minimum amount of an electrolyte required to
cause precipitation of one litre of a colloidal solution is called
coagulation value or flocculation value. The reciprocal
of coagulation value is regarded as the coagulating power.
89
(v) Hardy-Schulze Rule
Higher the valency of the active ion, the
| greater will be its power to precipitate the sol.
90
Protective Action of Lyophilic Colloids
As lyophobic sols are unstable (e.g. Au, Ag) and get easily
precipitated, the addition of lyophilic colloids like gums,
soaps etc., makes it difficult to precipitate. The process is
known as protection and the lyophilic colloids are termed
as protective colloids.
91
what are emulsions
These are the colloidal solutions of two immiscible liquids
or partially immiscible liquid in which the liquids act as
the dispersed phase as well as the dispersion medium.
there are two types ;oil in water type and water in oil type
92
oil in water type
Oil in water type, e.g. milk in which tiny droplets of
| liquid fat are dispersed in water.
93
water in oil type
Water in oil type, e.g. stiff greases, in which water
| being dispersed in lubricating oil.
94
what are emulsifying agents
During the preparation of emulsion, a small amount of
some substances such as soap, gum, agar and protein etc.,
are added to stabilise the emulsion. These substances are
known as emulsifying agents.
95
characteristics of emulsion
● Emulsions can be broken into constituent liquids by
heating, freezing, centrifuging or chemical destruction
of emulsifying agent.
● They show Brownian movement and Tyndall effect.
● These can be diluted with any amount of the dispersion
medium.
96
what are surfactants
• Surfactants are the substances which get preferentially
absorbed at the air-water, oil-water and solid-water
interfaces.
