Transcript Document 7298641

Electrical conductivity of
electrolyte’s solutions
Plan
1. Weak electrolytes.
2. Strong electrolytes.
3. Electric
conductance
electrolytes solutions.
4. Conductometry.
Assistant Kozachok S.S. prepared
of
Molecules of certain substances dissociate in a solvent
to give two or more particles. For example:
Consequently, the total number of particles increases in
solution and, therefore, the colligative properties of
such solutions will be large.
Van’t Hoff factor ‘i’ to express the extent of association
or dissociation of solutes in solution. It is the ratio of
the normal and observed molar masses of the
solute…
In case of association, observed molar mass being more than
the normal, the factor ‘i’ has a value less than 1. But in
case of dissociation, the Van’t Hoff factor is more than 1
because the observed molar mass has a lesser value.
In case of solutes which do not undergo any association
or dissociation in a solvent. Van’t Hoff factor ‘i’ will be
equal to 1 because the observed and normal molar
masses will be same.
Since the molar mass are inversely proportional to the
colligative property, Van’t Hoff factor may also be
expressed as:
i = Observed value of colligative property/Normal
value of colligative propert
Inclusion of Van’t Hoff (i) modifies the equation for
colligative properties as follows:
The Arrhenius theory
ACID A substance that provides H+ ions in
water
BASE A substance that provides OH- ions in
water
The symbol H+ does not really represent the structure of
the ion present in aqueous solution. As a bare
hydrogen nucleus (proton) with no electron nearby, H+
is much too reactive to exist by itself. Rather, the
attaches to a water molecule, giving the more stable
hydronium ion, H3O+ .We’ll sometimes write H+ for
convenience, particularly when balancing equations.
The main principles Arrhenius’s theory of electrolytic
dissociation:
1. Compounds dissociate into ions when dissolved in water.
2. This process is reverse.
3. The ions don’t interact among themselves.
Degree of dissociation
It is defined as the fraction of total substance that
undergoes dissociated into ions
Cdissociated . N dissociated .


С general
N gen eral
where m is the number of particles in solution, i is
Van’t Hoff factor
For example: the electrolytes of the type AB, such as KCl,
NaCl, etc., the the number of particles in solution m = 2
Degree of dissociation depends into:
1) Concentration
2) Temperature
3) Nature of the substance
•
•
•
•
According to the Degree of dissociation (α)
electrolytes can be classified into the following:
strong electrolytes are compounds that dissociate to
a large extent ( α> 30%) into ions when dissolved in
water. For example, HCl, H2SO4, HNO3, HI, NaOH,
KOH, KCl.
medium strong electrolytes α = 2 - 30%. H3PO4,
H3PO3.
weak electrolytes are compounds that dissociate to
only a small extent α<2%. For example, NH4OH, H2S,
HCN, H2CO3.
nonelectrolytes α = 0 are compounds that don’t
dissociate when dissolved in water.
Debye-Hückel theory of strong
electrolytes:
1. Ions of electrolytes interact with themselves
according to the electrostatic’s law.
2. The nature of solvent influences on the interaction
between ions (inductivity).The dielectric permeability
of a solvent shows the difference between the ion’s
attraction in a solvent and in a vacuum.
3. A central ion is surrounded by the ion’s atmosphere.
4. The size of the central ion is like a point charge.
5. Decreasing of the active concentration of the strong
electrolyte’s solution in the comparing with its
general analytic concentration. a < c
The thickness of the ion’s atmosphere decreases
with the increasing of the charge value and
ion’s concentration and the ionic strength of the
solution.
The general interaction of the ions increasing with
the increasing of solution’s concentration
according to the reducing of the average
distance between the ions. Increasing of the
ion’s interaction coursing the reducing of the
ionic activity.
The model of the hydrates
sphere and an ion’s
atmosphere
The properties of the strong electrolytes solution.
Activity
When the concentration of a solute is greater than about
0.1 mol/m-3, or we have strong electrolytes solution
(NaCl, HCl, etc.), interactions between the solute
molecules or ions are significant, and the effective and
real concentrations are no longer equal. It becomes
necessary to define a new quantity called the activity,
which is a measure of concentration but takes into
account the interactions between the solution species.
The relative activity, ai, of a component i is
dimensionless and is defined by equation 6.6 where μi
is the chemical potential of component i or ionic
strength (I) , μi0 is the standard chemical potential of i,
R is the molar gas constant, and T is the temperature
or
μi = μi0 + R T ln ci + R T ln fi
For the ideal solution:
μi = μi0 + R T ln ci
The relative activity of a solute is related to its molarity
by the following equation where fi is the activity
coefficient of the solute, and CM is the molarity.
ai = fi CM
Thermodynamics dissociation constant of strong
electrolyte’s solution is calculated by:
Ka 
aM z  aA zaMA

lg f  0.51z z

I
lg f = -0.5 Z i2√ μi
μi (I) = 0.5 (CM1 Z12 + CM2 Z22+..... CMi Zi2)
The activity coefficient of the electrolytes
depends only upon the ionic strength of the
solution and in dilution solutions of strong
electrolytes has the same value if this solutions
have equal ionic strength.
Electrochemistry is the branch of science which
deals with the relationship between electrical
energy and chemical energy and
interconversion to one from into another.
Electrolysis is the changes in which electrical
energy causes chemical reaction to occur.
The changes in which electrical energy is
produced as a result of chemical change. The
devices used to produce electrical energy from
chemical reactions are called electrical cells,
galvanic or voltic cells.
Conductors are the substances which allow the
passage of electric current.
N.B. The best conductors are metals such as copper,
silver, tin.
Non-conductors or insulators are the substances
which don’t allow the passage of electrical current
through them. Examples are rubber, wax, wood.
Types of conductors
1. Metallic conductors. There are metallic substances
which allow the electricity to pass trough them
without undergoing any chemical change.
2. Electrolytes. There are substances which allow the
electricity to pass through them in their molten states
or in the form of their aqueous solutions and
undergo chemical decomposition.
Metallic conduction
Electrolytic conduction
Metallic conduction is carried by
the movement of electrons
No change in the chemical
properties of the conductor
Electrolytic conduction is
carried by the movement of ions
It involves the decomposition of
the electrolyte as a result of the
chemical reaction
It does not involve the transfer
of any matter
It involves the transfer of matter
as ions
Metallic conduction decreases
which increase in temperature
Electrolytic conduction
increases which increase in
temperature
When the voltage is applied to the electrodes dipped
into an electrolytic solution, ions of the electrolyte
move and, therefore, electric current flows through
the electrolytic solution. The power that the
electrolytes to conduct electric current is termed
conductance or conductivity.
Ohm’s law. The current flowing through a
conductor is directly proportional to the potential
difference across it.
Or the strength of current flowing through a
conductor is directly proportional to the potential
difference applied across the conductor and
inversely proportional to the resistance .
I=V/R,
where I is the current strength (in amperes) and V is
the potential difference applied across the conductor
(in volts), R is the resistance of the conductor ( in
Scheme of electrolysis of sodium
chloride melt
The conductometry cell
l/S – the constant of the cell
Resistance It measures the obstruction to the flow of current.
The resistance of a conductor is proportional to the length (l)
and inversely proportional to the area of cross-section.
where ρ (rho) is the constant of proportionality and is called
specific resistance or resistivity. The resistance depends upon
the nature of the material. Its units are ohm (Ω )
R= ρ , if l =1 cm, a=1
In other words, specific resistance is the resistance between
opposite faces of one centimetre cube of the conductor.
Conductance. It is a measure of the ease with
which current flows through a conductor.
Specific conductance or conductivity. It may
be defined as the conductance of a solution of 1
cm length and having 1 sq. cm as the area of
cross-section. In other words, specific
conductance is the conductance of one
centimetre cube of a solution of an
electrolyte.
It’s generally denoted by κ (kappa)
Units. The units of specific conductance are
In SI units,
Κ = C l/a,
where С is electrical conductance, a-area
Specific conductance is defined by the number of the
ions and the their velocity. The more ion’s
concentration and more their velocity the more will be
conductance.
Therefore there are some factors that influence on the
value of κ: the nature of solvent and solute, the
concentration of electrolyte’s solution, temperature.
Dependence of the specific conductance from the upper
factors is expressed by the following equation:
Κ= (u+ + u- ) F c α
= (u+ + u- ) F c α
where u+,u- are the mobility of the cation and the anion
(at the V is the potential difference applied across the
conductor = 1 V, and the length = 1 m).
C is a molar concentration, α is the degree of
Equivalent conductance or molar conductance
It is defined as the conducting power of all the ions
produced by dissolving one gram equivalent of an
electrolyte in solution.
It is denoted by the symbol (lambda).
where C is the concentration of the solution in equivalent per
litre or is the molar concentration of the solution
Units of the equivalent conductance are:
Molar conductance. It’s defined as the conducting power of
all the ions produced by dissolving one gram mole of an
electrolyte in solution.
where M is the concentration in moles per litre
Factors for variation of molar conductance:
1. Nature of electrolyte
2. Concentration of the solution
3. Temperature
1. Nature of electrolyte. The conductance of an
electrolyte depends upon the number of ions present
in the solution. Therefore, the greater the number of
ions in the solution, the greater is the conductance.
The number of ions produced by an electrolyte
2. Concentration of the solution
The molar conductance of electrolytic solution varies
with the concentration of the electrolyte. In general,
the molar conductance of an electrolyte increases with
decreases in concentration or increases in dilution.
Variation of Molar Conductance with Concentration for
Strong Electrolytes
In case of strong electrolytes, there is the tendency for molar
conductance to approach a certain limiting value when the
concentration approaches zero, when the dilution is
infinite. The molar conductance when the
concentration approaches zero (infinite dilution) is
called molar conductance at infinite dilution.
It has been observed that the variation of molar conductance
with concentration may be given by the expression
where b is a constant depending upon ion charge, viscosity
of solvent, temperature, dielectric permeability (of a
solvent, and
is called molar conductivity at infinite
dilution.
For strong electrolyte at the absence of infinite dilution
   b 

Dependence of the specific
electric conduction from an
electrolyte’s concentration
С, mol/м3
The variation of molar conductance with concentration can be
studied by plotting the values of
against square root of
concentration
Variation of Molar Conductance with Concentration
of Weak Electrolytes
The weak electrolytes dissociate to a much lesser extent
as compared to strong electrolytes. Therefore, the
molar conductance is low as compared to that of
strong electrolytes.
Conductance behaviour of weak electrolytes.
The variation of with dilution can be explained on the
basis of number of ions in solution. The number of
ions furnished by an electrolyte in solution depends
upon the degree of dissociation with dilution. With the
increases in dilution, the degree of dissociation
increases and as a result molar conductance
increases.
Conduction behavior of strong electrolytes.
For strong electrolytes, there is no increase in the
number of ions with dilution because strong
electrolytes are completely ionised in solution at all
concentrations (by definition). However, in
concentrated solutions of strong electrolytes there are
strong forces of attraction between the ions of
opposite charges called inter-ionic forces. As a result,
the molar conductance increases with dilution.
3. Temperature
The conductance of an electrolyte depends upon the
temperature with increase n temperature, the
conductance of an electrolyte increases.
KOHLRAUSCH’S LAW
Thus, it may be concluded that each ion makes definite
contribution to the molar conductance at infinite
dilution irrespective of the other ions.
Kohlrausch’s law of independent migration of ions
states that:
At infinite dilution when the dissociation is
complete, each ion makes a definite contribution
towards molar conductance of the electrolyte
irrespective of the nature of the other ion with
which it’s associated.
If molar conductivity _of the cation
is denoted by
and that of anion by then the law of independent
migration of ions is:
Application of Kohlrausch’s law
1. Calculation of Molar Conductance at Infinite Dilution
for Weak Electrolytes
2. Calculation of Degree of Dissociation of Weak
Electrolytes.
Molar conductance of a weak electrolyte depends upon
its degree of dissociation.
x
C
(     )
1000
   (   ).
Velocity of the ion’s mobility and the number of the
ion transfer.
As a rule the ion’s mobility is from 4*10 -8 till 8*10 -8 in
aqueous infinite dilution solution, except ion of hydroxonium
ion (u=36,3 *10-8 m2V-1c-1) and hydroxyl ion (u =20,5*10-8 m2V-1c-1).
It’s explained by the special serial transmission mechanism of
their conductance.
Velocity of a cation and an anion in each solution in general
doesn’t equal, therefore there is not equal quantity of
electricity that is transferred by ions.
The mechanism of electric
conduction for the
+
hydrogen ion Н
The number of the ion transfer is the relation of an
amount of electricity is carried by the one type ions to
the general quantity of electricity, which passed
through electrolyte:
ti = Qi/Q
where Qi is the quantity of electricity which is carried by
the ions of i type through the cross-section of the
electrolyte’s solution, which is calculated from the next
3
formula:
2
Qi=ziFciuiaτ
where, zi is valence, ci is concentration, mol/m, ui is the
ion’s mobility, a is the cross section, m , τ is time, s.
t+ = u+/u- + u+,
t- = u-/u- + u+, t+ + t- = 1
At infinite dilution,
λ∞+ + λ∞- = λ∞,
Therefore, λ∞+ = λ∞ t+
and
λ∞- = λ∞ tConductometric titration (conductometry)
The main concept of this method is the changing of
the electrical conductance of an electrolyte’s solution
during a titration. The change of the electrical
conductance is grounded on the displacing of an
one ions by the another, which have other
mobility.
Equivalent point is defined accurate by the graph.
λ=
1/
R
The curves of
conductometry titration
1.
The titration of
НCl NaOH
2. The titration of
CH3COOH NaOH
3. The titration of
the mixture of
(НCl (а) і CH3COOH
(б) NaOH
V titrant
Usage of direct conductometry:
• For definition of individual electrolytes in solution
• For analysis of medicines: the determination of weak acid and
the substances with weak-acid
property: phenobarbital,
sulfadimine, thymol. Weak base - caffeine
• For definition of electrolytes in mix when impurities
concentration don’t change
• For continuous control of manufactures
• For control of water treatment process
• For sewage pollution assessment
• For definition of general content of salts in mineral, ocean and
fluvial water
• For control of operations filter washing and ion-exchange
material regeneration
• For definition of cleanliness slightly soluble precipitate or
organic drugs
• For definition of dampness of organic solvent, gases, crystal
salts, paper
• For detecting in chromatography