Physical and Colloidal Chemistry

33 Pages·2018·1.359 MB·Russian

by  Khorunzhij V. V.

LIBRARY OF THE PEDIATRIC UNIVERSITY
V.V. KHORUNZHIJ
E.M. GOLINETS
PHYSOCAL
AND COLLOIDAL
CHESiflSTRY
Saint-Petersburg
ВОЗВРАТИТЕ КНИГУ Г
[Е ПОЗЖЕ
обозначенного здесь
срока
Ministry
health
Russian Federation
E.M. GOLINETS
PHYSICAL
Saint-Petersburq
State a
Pediatric AND COLLOIDAL
Medical
University
CHEMISTRY
Manual
for students
Saint-Petersburg
УДК 541.1+541.18
ББК 24.5
24.6
Х82
iKhorunzhij, V.V.
Х82 Physical and Colloidal Chemistry. Manual for students. / [V.V. Khorunzhij
E.M. Golinets. - SPb.: SPbGPMU, 2018. - 32 p
ISBN 978-5-907065-57-4
The manual includes important topics of physical and colloidal chemistry.
The first part of the manual gives the description of the surface phenomena and
adsorption. The second part concerns such subject of colloidal chemistry as definition
and classification of colloidal systems, formation of colloids, the main properties,
purification, stability and destruction of colloids. Typical examples of physical and
colloidal chemistry problems with given solutions and also a number of problems for
self-solution are presented.
The manual will help the students to learn general chemistry better.
Editor-in-chief: Professor [V.V. Khorunzhij
Reviewer: Professor L.A. Danilova, reader L.B. Prokhorova
УДК 541.1+541.18
ББК 24.5
24.6
Утверждено учебно-методическим советом Государственного бюджетного
образовательного учреждения высшего профессионального образования
«Санкт-Петербургский государственный педиатрический медицинский
университет» Министерства здравоохранения Российской Федерации
ISBN 978-5-907065-57-4 © СПбГПМУ, 2018
9. Surface phenomena. Adsorption
1.1 Introduction
In 1773, Scheele observed that the charcoal had the property to take up the gases. Further,
it was observed that the metals such as platinum, nickel, palladium, etc. could be used as catalyst
in the hydrogenation of organic compounds. These metals have the same property as that of
charcoal.
In olden times and even today, charcoal powder is used to remove the brown color of raw
sugar to make it white, All these phenomena can be attributed to the surface property of the
solids, called adsorption.
The term adsorption was first used by H. Kayser in 1881, and it refers strictly to the
existence of higher concentration of any particular component at the surface of solid and liquid
substances.
1.2 Adsorption as surface phenomenon.
Adsorption is a surface phenomenon. The molecules present on the surface of the liquid
and solid substances behave in different ways than those in the bulk. The inside molecules of a
liquid or solid substances are attracted equally from all the sides, while the surface ones are
attracted only by the interior or inward forces
Unbalanced forces
(Fig. 1.1). As a result of these unbalanced
Surface molecules
forces, solid and liquid substances tend to к
minimize their imbalanced surface train by VN> 4rN) кГh7 (V.f 7s* Molecules of
attracting and retaining the other molecules on .__adsorbent m
their surface. Vv*>r Vr■T4 S■7\ ■ lf 7" Molecthuele b autltkracted
from all the sides
j:> 4.г 4 7*-----e7-
Thus, the surface of the liquids or solids
has a tendency to attract the available X ) V>J tt JN t^>T
molecules towards itself and hold them by
forming a bond between them and the surface Fig- 1 -1 ■ Adsorption as surface phenomenon
molecules. Thus, there is an accumulation of one substance on the surface of the other substance
forming a higher concentration layer. This phenomenon is termed as adsorption.
Definition of adsorption and absorption
Adsorption: It may be defined as the phenomenon of accumulation of one substance on the
surface of some other substance forming a higher concentration layer.
It may be also defined as the change in concentration at the interfacial layer between the
two phases of a system due to a surface or residual forces. (Fig. 1.2)
Absorption: It may be defined as a phenomenon, in which a substance passes through the
surface of another substance and becomes uniformly distributed in its volume. Thus, absorption
3
is a bulk phenomenon (Fig. 1.2). A simple example is given by an absorption of ink by a blotting
paper.
Difference between Adsorption and Absorption.
Adsorption Absorption
(1) It is a surface phenomenon. It is a bulk phenomenon.
A higher concentrated layer of a A substance is uniformly distributed
substance is formed on the surface throughout the body of some other
of some other substance substance.
(2) It depends upon the surface area It is independent of the surface area
(3) It depends on temperature and It is independent of the temperature
pressure and pressure
(4) It is accompanied by evolution of It is not accompanied by evolution or
heat (it is an exothermic process) absorption of heat
(5) It is a fast process It is a comparatively slow process
(6) It is a reversible process It is an irreversible process
Definition of adsorbent and adsorbate
Adsorbent: The substance which adsorbs the other substance (adsorbate) is called the
adsorbent. In other words, adsorbent is a substance on the surface of which adsorption takes
place.
Adsorbate: The substance which gets adsorbed on the surface of an adsorbent is called the
adsorbate.
Examples:
(1) When animal charcoal is added to a solution of acetic acid, then after some time,
some of the molecules of acetic acid get adsorbed by animal charcoal and the
concentration of acetic acid solution is bound to be decreased.
(2) When a solution of ink in water is mixed with animal charcoal, the colouring
matter in ink is adsorbed by the charcoal and the colour of the ink solution
becomes faint
In the above examples, the animal charcoal functions as an adsorbent. But in the first
example, acetic acid and in the second, the colouring matter of ink, function as an adsorbate.
In a simple diagrammatic way, the adsorbent and the adsorbate can be shown in Fig. 1.2a.
Forces between adsorbent and adsorbate: In the process of adsoiption, the molecules of
an adsorbate are held by an adsorbate either by Van der Waal’s forces, or by a residual chemical
forces (chemical forces), which forms a chemical bond. Thus, the forces, operating between the
adsorbent and the adsorbate are of the two types: (1) relatively weak and long-ranged Van der
Waal forces and (2) relatively strong and short-ranged chemical forces forming a chemical bond.
Types of adsorption: According to the nature of the forces which a responsible for
adsorption, it is studied as a physical adsorption and chemical adsorption.
(1) Physical adsorption: When the molecules of an adsorbate are held on the surface
of an adsorbent with the help of weak Van der Waal’s forces, the adsorption is
termed as physical or Van der Waal’s adsorption or an activated adsorption. An
example is given by the adsoiption of H2 or O2 gases on the surface of charcoal.
(2) Chemical adsorption: When the molecules of an adsorbate are held on the
surface of an adsorbent with the help of strong chemical bonds, the adsorption is
termed as chemical adsorption or chemisorption. For example, the adsorption of
oxygen on the surface of tungsten oxide, WO3.
Distinction between Physical and Chemical Adsorption:
Physical adsorption Chemical adsorption
0 ) The molecules of adsorbate and The molecules of adsorbate and
adsorbent are held together by weak adsorbent are held together by means
Van der Waals forces of strong chemical bond(moiecules of
4
adsorbate form chemical bond with
molecules of adsorbent)
(2) It is characterized by low heats of It is characterized by high heats of
adsorption (in the range of adsorption (in the range of
20-40 kJ mol'1) 200-400 kJ mol*1)
(3) Multimolecular layers of adsorbate Monomolecular layers of adsorbate are
are formed on the surface of the formed on the surface of the adsorbent.
adsorbent
(4) It is a reversible process It is an irreversible process
(5) It is common (not specific) It is highly specific
Л6) It occurs at low temperature It occurs at relatively high temperatures
Factors affecting Adsorption:
The various factors affecting the process of adsorption of a gas over a solid surface are
given as follows: (1) Temperature, (2) Pressure, (3) Surface area of an adsorbent, (4)
Nature of adsorbent, (5) Nature of adsorbate, (6)Concentration of an adsorbent.
(1) Temperature: The process of adsorption is exothermic in nature. Hence, the rate of
adsorption increases with lowering of temperature and decreases with the increase in
temperature. Graphically, in can be shown
according to Fig. 1.3. Thus, adsorption varies
•£
inversely with temperature. The amount of heat о
energy evolved in adsorption is called heat of л
adsorption. и
еп ь-
ТЭ О
м
(2) Pressure: The increases in the pressure of a gas
о Е
(adsorbate) increases the extent (rate) of adsorption. о 3
However, this increase in adsorption is not directly ее. i
proportional to the pressure. The effect of pressure
is applicable at low temperatures. When the Temperature
pressure of gas increases, the rate of adsorption Fig. 1.3. Effects of temperature
increases, but only up to a certain level. At high on adsorption
pressures, the amount of gas adsorbed remains
constant and the rate of adsorption becomes
independent of the pressure. The adsorption of
nitrogen gas on charcoal at different temperatures
can be shown according to figure 1.4. It is clearly
seen from the graph that, at lower temperature, the
rate of adsorption is more and it increases with
increase in pressure, but only up to a certain extent.
Further, high increase in pressure at a given
temperature will not affect the rate of adsorption
and the curve becomes almost parallel to the
pressure axis.
Fig. 1.4. Variation of adsorption
(3) Surface are of adsorbent: Adsorption is a surface with pressure at different temperatures
phenomenon and hence in depends upon the
surface area of the adsorbent. Larger the surface area of adsorbent, greater is the extent
of adsorption. Thus, more finely divided particles or the rough surface area of the
adsorbent or the adsorbent in the colloidal state shows greater extent of adsorption due
to their larger surface area.
(4) Nature of adsorbent: The extent of adsorption depends upon the nature of the
adsorbent. It is found to be more on rough surfaces, porous solids, on more finely
5
divided powders and colloidal solutions, etc. In some cases, adsorption becomes
selective. For example, charcoal adsorbs only ethylene from a mixture of ethylene and
coal gas and finely divided nickel adsorbs hydrogen to a greater extent then nitrogen.
This also depends upon the nature of the adsorbent and this type of adsorption is
termed as preferential adsorption.
(5) Nature of adsorbate: Generally, almost all the gases are adsorbed on the surface of a
solid adsorbent, but the extent of adsorption differs from gas to gas.
It is found that the gases which can be easily liquefied and are highly soluble in water,
are adsorbed to a greater extent, because the greater Van der Waal’s forces are
produced between the adsorbent and these gases.
(6) Concentration of adsorbate: The extent of adsorption also depends upon the
concentration of the adsorbate, which is used in the form of a solution. If the concentration
of an adsorbate increases, the adsorption also increases. Finally, it reaches to a maximum
value, when the equilibrium is reached. After that, the further increase in concentration of
the adsorbate has no effect on the rate of an adsorption. For example, 1 M solution of acetic
acid is adsorbed faster then 0.1 M acetic acid solution on the activated charcoal.
Definitions of Gibbs adsorption and absolute adsorption
Gibbs adsorption: An excess of adsorbate in a surface layer per unit area Г is called as
Gibbs adsorption and is measured in [mole * m2J, [kg * in2], etc.
Absolute adsorption: An amount of substance adsorbed by a unit mass of adsorbent is
called an absolute adsorption A [mole*g'', kmole*kg*,,etc]. In general case, A is greater that
Г, however, for surface-active substances bulk concentration can be neglected and А~Г.
1.3 Surface energy. Surface tension.
Molecules and atoms, located at the phase interface in a near-surface layer of a liquid or a
solid phase, have an excess of energy as compared to the particles located in the bulk medium.
Surface tension: An excess of energy in a near-surface layer per unit area of the phase
interface, is called a surface tension cr [N m~' ] or [J m"2]. Total free surface energy fESUrr):
Ewrr=oS[J], (1.1)
where S is the area of phase interface.
According to the second law of thermodynamics, processes proceed spontaneously if the
following conditions are satisfied:
(a) p, T=const, AG=AH-TAS<0, i.e. when Gibbs energy decreases
(b) V, T=const, AF=AU-TAS<0, i.e. when Helmholtz energy decreases.
Thus, the thermodynamical system tends to minimize its energy. As applied to a free surface
energy it can be done by decreasing the area of phase interface (S) or decreasing the surface
tension (o).
Definitions of surface-active and surface inactive substances.
Substances, which are accumulated in the near-surface layer during the solution and
lower the surface tension, are called the surface-active substances (SAS). SAS molecules consist
of polar and unpolar parts. Such groups as -COOH, -OH, -CHO, -NH2, -NO2, have polar
properties, so they have aquation ability, and are called hydrophilic. Unpolar molecules, such as
carbon chains or cycles have no aquation ability and are called hydrophobic. An ability of SAS
to lower the surface tension characterizes their surface activity.
Substances, which increase the surface tension during the solution, are called surface-
inactive substances (SIS).
6
1.4 Adsorption at the interfaces liquid-gas and liquid-liquid.
Such cases were studied by Gibbs, who obtained the dependence of an excess interfacial
concentration on the initial concentration and surface tension:
C dcr
Г = — (1.2)
RT dc
where
Г is an adsorption of substance in a near-surface layer, [kmole-m'2],
C is the concentration of the substance in the solution at the adsorption moment, [kmole-m'3],
R is the universal gas constant, 8.31 [J-mole''-К'1]
T is the absolute (thermodynamical) temperature [K],
da/dc is the surface activity, [J-m-kmole*1].
This is an equation of fundamental importance and theoretically it is applicable to any system. If
we replace derivatives by finite differences, we obtain
_ С Д a C cr, — a,
Г = - ■ = - • -2 , (1.3)
RT ДС RTCj-C,
where the quantities C|, cri, C2,02correspond to the initial and final state, respectively.
Let us analyze this equation. If the concentration is increased then C2 is always greater than
Ci, and three cases are possible:
(a) Сг>С] (ДОО) and 0г>0| (До>0), consequently, Г<0, i.e. adsorption is negative,
which means that the concentration at the surface is smaller than that in the bulk.
This case is typical for SIS (salts, alkali, inorganic acids).
(b) Сг>С| (ДОО) and <J2=cti (До=0), Г=0, i.e. adsorption doesn’t take place, hence,
the concentration in die bulk and at the surface is the same (sucrose).
(c) Сг>С| (ДОО) and a2<C| (Да<0), consequently, Г>0, i.e. adsorption is positive,
which means that the concentration at the surface is larger than that in the bulk.
This case is typical for SAS (carbon acids, spirits, amines).
It turns out for the SAS, that an increasing of the length of the radical by one homological
difference (CH2-) increases surface activity in diluted solutions by a factor of 3.2 (Duclot -
Traube rule).
1.5 Adsorption at the surface of solids
Solid adsorbents can be divided by a type of the surface at smooth and porous. For
adsorbents with smooth surface (glass, mica, monocrystals) Langmuir (1915) suggested a theory
of a monomolecular adsorption of gases and derived and equation describing an adsorption,
which was found later to be applicable for another interfaces as well:
Г , where (1.4)
C is the equilibrium concentration of adsorbent, [kmole-m'3],
Г is the adsorption per unit surface area, [kmole-m'2],
Г» is the saturated adsorption, [kmole-m'2],
К is the constant of the adsorption equilibrium.
Let us analyze this equation:
(a) For very small concentrations C«K,
Г = ПоС/К; amount of adsorbated substance
is proportional to it's concentration (see Fig.
1.5, part I);
(b) For large concentrations C»K, Г = Г®; a
saturated independent of concentration
amount of adsorbated substance is reached
7
(see Fig. 1.5, part II);
(c) If C=K we have Г= Г J2. This provides a way for graphical determination of the
constant of the adsorption equilibrium, i.e. at the adsorption equal to the half of
it’s saturated values die value of equilibrium constant is equal to the equilibrium
concentration of an adsorbate.
Graphical dependence of adsorption on the concentration is called Langmuir adsorption
isotherm, shown in Fig. 1.5
1.6 Freundiich adsorption isotherm
Freundlich, studied the adsorption of gases on the surface of the solid adsorbent and put
forward an empirical equation showing the variation of adsorption with pressure at some
constant temperature, known as Freundlich Adsorption Isotherm.
Statement: (for Freundlich Adsorption Isotherm)
It is an empirical equation which shows the variation in the adsorption of a gas on the
surface of a solid with its pressure at a constant temperature over a limited range of a pressure.
Since it was given by Freundlich, is it called as Freundlich Adsorption Isotherm.
Mathematical expression: Mathematically, Freundlich Adsorption Isotherm can be
expressed as follows:
— - P,/n or — -KP,/n, where (1.5)
m m
x is the mass of the gas (adsorbate) adsorbed,
m is the mass of adsorbent. Hence, x/m is an amount of adsorbate adsorbed by unit mass of the
adsorbent:
P is equilibrium pressure,
К and n are the constants, depending on the nature of adsorbate and adsorbent, for the given
system at a particular temperature.
1/n stands for a fractional power between unity and zero.
This isotherm holds true only for moderate pressure and at low temperatures.
In case of the adsorption of a solution by an adsorbent, the above equation can be written in
the following form:
—=KC,/U, where (1.6)
m
C is an equilibrium concentration of the adsorbate,
x is the mass of the adsorbate, and
К and n are the two constants for the given system at a particular temperature.
Graphical representation of Freundlich adsorption isotherm:
From the Freundlich adsorption isotherm equation, x/m=KP,/0. If the logarithms are taken
both the sides, we get
x/m
(c)
log,0—=log10K+-logl0P,
m n x/m=const
which has the same form as a linear (b) S ' (at high pressure)
dependence
x/m-P’“ (at moderate pressure)
y=mx+c.
Now, two graphs can be plotted for the above
equations:
(1 > If x/m is plotted against P, a smooth x/m-P (ai low pressure)
curved is obtained, which looks like a
parabolic curve shown in Fig. 1.6. (a)
The graph indicates that at a very low Fig. 1.6. Freundlich adsorption isotherm
pressure x/m is directly proportional
8