Download Unit XI Laboratory work № 11 Physical chemistry Questions: Surface

Unit XI
Laboratory work № 11
Physical chemistry
Questions:
1.
Surface phenomena. Surface tension and surface activity.
2.
Adsorption, its types.
3.
Monomolecular and polymolecular adsorption. Physical adsorption
and chemisorptions. Adsorption at solid surfaces. Adsorption isotherm. Frenflich
adsorption isotherm. Langmuir adsorption isotherm.
4.
Adsorption from solution. Ion exchange adsorption. Ionites.
Physical chemistry studies the dependence of conditions on interactions and
conversions.
We will focus on two parts of Physical Chemistry – Chemistry of Surface
Phenomena and Colloidal Chemistry.
§1. Chemistry of surface phenomena
A surface phenomenon is an event that occurs on the surface or interface of
phases. The measure of heterogeneity is the surface tension. This value determines
the sharpness of differences between phases. The great difference between them
means strong heterogeneity and the value of surface tension is increased.
σ= Din/cm; H/m; J/m2; erg/cm2
If σ = 0, a heterogeneity is absent.
Inside the substance all forces acting between the particles are mutually
balanced, but on the surface only the forces directed inwards and those in the
surface plane are balanced. The work of a new surface unit formation in the surface
tension. In other words, the surface tension is the work needed for a break of liquid
column with 0,5 cm2 cross-section.
Intermolecular interactions in the system increase the surface tension.
Gaseous
Liquid: slightly polar
increasing surface tension
polar
solid
The product of the surface tension and the surface area is called the surface
energy.
Gs = σ · S
Any system tends to decrease Gs by decreasing the contact surface S or
heterogeneity σ.
§2. Isotherm of surface tension. Surface activity.
If we add a substance in the system, it will change surface tension,
depending on value of own surface tension.
The surface-active substances – SAS – (proteins, soaps, and other organic
compounds) decrease surface tension. The surface inactive substances – SIS –
increases surface tension (nonorganic electrolytes). The surface-nonactive
substances – SNS – do not change the surface tension.
Graphical interpretation of the dependence of surface tension versus
concentration called the isotherm of surface tension.
The value, which characterizes the ability of substances to decrease the
surface tension and to be absorbed on the surface is the surface activity.
By researching the surface tension of aqueous solutions of organic
compounds, Traube established:
If carbon chain is made longer by -CH2-group, the surface activity will rise
in 3.0-3.5 times.
§3. Adsorption and its types
The changes of surface tension associate with adsorption process.
The take-out of gases, vapors and dissolved substances bu the surface layer
of the other substances is called adsorption.
The substance that adsorbs other is adsorbent, Substances, which are
adsorbed, are adsorbates.
There are two main classifications:
1.
according to the kinds of phases: L-L, G-L, G-S, L-S;
2.
according to the nature of adsorption: physical and chemical.
The adsorption value can be expressed by the following ways:
1.
Absolute adsorption is characterized by the number of moles (or
grams) of adsorbed substance per the unit of adsorbent mass.
A = X/m
Here, X is the number of moles (or grams) of adsorbed substance; m is the
adsorbent mass.
2.
Excess adsorption (Gibbs adsorption) shows the excess of adsorbate
on the surface layer from that of its concentration in a volume per unit surface area
I = A/S0
Here, S0 is the specific surface area; A is the absolute adsorption.
The adsorption value depends upon the nature of components, temperature,
concentration (or partial pressure) of adsorbate.
Adsorbed molecules oscillate in a definite way. Some of the molecules may
detach themselves and return to the surrounding medium, their place taken by other
molecules; as a result a definite state of adsorption equilibrium is established,
during which the number of molecules adsorbed by the surface is the same as that
leaving it.
The state of adsorption equilibrium depends both on the concentration of the
adsorbed substance in the adjacent to the adsorbed, and on the temperature. As the
concentration increase, the absolute quantity of adsorbed molecules grows,
although the percentage adsorbed is lower than at low concentration.
Shifts of equilibrium due to changes in temperature are governed by the fact
that adsorption is accompanies by the liberation of heat.
Therefore, according to Le Chatelier’s Principle, a rise in temperature shifts
adsorption equilibrium in the direction od desorption, i.e., liberation of the
adsorbed molecules. A drop in temperature, on the other hand, increases the
amount of substance adsorbed.
Since adsorption takes place only the surface of a substance, it is clear that
the greater the total surface area of the adsorbent, the more molecules it can
adsorb. Therefore, porous and powdered substance, as a rule, have a higher
adsorptive capacity than substances.
The principal differences between physical and chemical adsorption:
1.
Physical adsorption is reversible always. Chemical adsorption is
nonversible.
2.
The heat of chemo sorption is greater than the heat of physical
adsorption
3.
The physical adsorption does not need activation energy.
4.
Molecules of adsorbate are not changed during physical adsorption. In
case of chemical adsorption the transit of changes take place.
Liquid-gas and liquid-liquid adsorption.
The particles of gases or dissolved substances can not be adsorbed by
liquids. Adsorption accompanies the dissolution process.
Gibbs established what the distribution of dissolved substance in liquid take
place to decrease surface tension to the maximum.
If the surface tension is decreased during process the adsorption value will
rise. The substance is accumulated. Positively adsorbed substances, such as fats,
carboxylic acids, alcohols, cholesterol, are surface-active substances.
If the substance reduces the surface tension, the adsorption value is negative
and the particles of substance are pushed out the surface layer. They are surfaceinactive substances (SIS – nonorganic salts, carbohydrates).
Substances that do not change the surface tension are called surfacenonactive substances (SNA).
The first portion of acid are distributed on the surface layer and decrease the
surface tension sharply. Then the concentration of acid rises and it diffuses inside
H2O. The surface tension drops to the value of 27.0 erg/cm2 and stays constant.
The limit (scope) of positive adsorption has been reached.
If water is added to butyric acid, the adsorption process will flow in the
opposite order.
§4. ADSORPTION THEORIES
1. Monomolecular adsorption
This theory was created on the base of adsorption of gases on solids. But it
be used for liquid-solid adsorption.
Authors are Lovits and Longmuir.
The main statements:
1.
Adsorption proceeds on the active centers to the physical interactions
(defects, unsaturated bonds). The surface is homogeneous and the active centers
number is constant for specific sorbent.
2.
Each center keeps one molecule only.
3.
We may ignore the interaction of adsorbate molecules in the layer.
This theory is equitable for any concentration.
I.
С → 0 (the concentration is small)
II.
C → ∞(the saturation of layer)
III. adsorption is constant
2. Polymolecular adsorption.
Bunauer, Emmet and Teller Theory
This theory is used in cases of low temperature and high concentrations of
adsorbate. The adsorption occurs on solids. A few layers of molecules are formed.
OA – monomolecular layer;
AB – the area is close to saturation.
Adsorption rises sharply and is finished by the concentration of vapor is
narrow pours of adsorbate. This process is called the capillary condensation. Ps –
saturated pressure in the surface layer.
In a real life these process occur together and they cannot be separated.
The main statements:
1.
The surface of adsorbent has active centers, each of them can hold one
molecule. Molecules of the first layer are the active centers for the second layer,
and so on.
2.
Molecules are kept at adsorption layers physical forces.
3.
Molecules do not react in the layers.
§5. EQUATIONS AND ISOTHERM OF ADSORPTION
The graph of dependence of adsorption versus concentration (pressure) of
adsorbate is the adsorption isotherm.
The series of empiric formulas are offered for the description of isotherm.
1.
Friendlich equation
2.
Friendlich equation does not describe the limit value of adsorption,
when adsorption does not depend on the concentration.
This limiting value is determined by the Longmuir equation:
ADSORPTION OF ELECTROLYTES THEIR SOLUTIONS.
The adsorption of the dissolved substances is more complicated than the
adsorption of gases.
There are some difficulties:
1.
The presence of the third component – solvent. Its molecules may
complete with adsorbate molecules for place at surface of adsorbent.
2.
Interaction between adsorbate and solvent.
3.
Electrostatic interaction between adsorbent ions (if the adsorbate is an
electrolyte).
The dissolved strong electrolyte in the solution is ionized form, completely.
Ions adsorb on polar surface only.
The main factors, which make conditional specificity of strong electrolyte
adsorption, are the sign of charge of adsorbent surface, the value and sign of ion
charge, its radius and the degree of solvatation.
Adsorption ability of ions (especially, cations) on the surface rises due to
increasing radius.
Cs+ >Rb+> NH4+> K+ >Na+ >Li+
NO3- >I- >Br- >Cl- > FThere are three types of strong electrolyte adsorption:
1.
Equivalent adsorption is characterized by the equivalent amount of
cations and anions adsorbed on the surface.
2.
Selective adsorption (Fajans Rule): ions, which can complete lattice of
the adsorbent. are adsosrbed on the surface, predominantly.
3.
Exchanged adsorption is the most wide-spread type of strong
electrolyte adsorption. It is a process during which an adsorbent and solution
exchange their same-charged ions in equivalent amounts.
RM1 + M2+ ↔RM2 + M1+
Or
RA1 + A2- ↔RA2 + A1Here
RM1 and RM2 are adsorbents contained cations M1+ and anions A1- and they
can exchange. These adsorbents are named ionites. Ionites are solid natural or
synthetic compounds that are slightly soluble in water and organic compounds.
Ionites have net structure (skeleton, stencil). Atomic groups R+ are fixed in
the lattice. Their charge is compensated by M+ or A- (counter-ions, compensating
ions). Fixed ions with movable counter-ions (R-M+ or R+A-) are called ionogenic
groups.
According to types of ionogenic groups, all ionites sre classified by
cationites R-M+ (acidic ionites. They exchange by cations) and anionites R+A(basic ionites. They exchange by anions).
Ionic exchange is a reversible process. Hence, ionites can be regenerated. By
treating with acid, the regeneration of cationites is carried out. By base flushing,
anionites activity is reduced.