Electrochemistry 3 Flashcards
explain change in conductivity of a solution with change in concentration
Conductivity always decreases with
decrease in concentration both, for weak and strong electrolytes.
This can be explained by the fact that the number of ions per unit
volume that carry the current in a solution decreases on dilution.
The conductivity of a solution at any given concentration is the
conductance of one unit volume of solution kept between two platinum electrodes with unit area of cross section and at a distance
of unit length.
G=kA/l=k
what is molar conductivity
Molar conductivity of a solution at a given concentration is the
conductance of the volume V of solution containing one mole of
electrolyte kept between two electrodes with area of cross section A and
distance of unit length.
Λ=κA/l
Λ=κV
V- volume of soln containing 1 mol of electrolyte
how does molar conductivity change with concentration
-Molar conductivity increases with
decrease in concentration. This is
because the total volume, V, of solution
containing one mole of electrolyte also
increases. It has been found that decrease
in k on dilution of a solution is more
than compensated by increase in its
volume.
-Physically, it means that at a
given concentration, Lm
can be defined
as the conductance of the electrolytic
solution kept between the electrodes of a
conductivity cell at unit distance but
having area of cross section large enough
to accommodate sufficient volume of
solution that contains one mole of the
electrolyte.
-When concentration
approaches zero, the molar conductivity
is known as limiting molar
conductivity
variation of molar conductivity with concentration for strong electrolytes can be explained?
For strong electrolytes, Lm
increases slowly with dilution and can be
represented by the equation:
Lm = L°m – A c ½
This is called Debye-Huckel-Onsager equation.
It can be seen that if we plot (Fig. 2.6) Lm against
c1/2, we obtain a straight line with intercept equal to L°m and slope equal to ‘–A’.
The value of the constant ‘A’ for a given solvent and temperature depends on the type of electrolyte i.e., the charges on the
cation and anion produced on the dissociation of the electrolyte in the
solution.
types of strong electrolytes
Thus, NaCl, CaCl2
, MgSO4 are known as 1-1, 2-1 and 2-2
electrolytes respectively. All electrolytes of a particular type have the
same value for ‘A’.
state kohlrausch’s law of independent migration of ions
Kohlrausch
law of independent migration of ions. The law states that limiting
molar conductivity of an electrolyte can be represented as the sum of the
individual contributions of the anion and cation of the electrolyte.
In general, if an electrolyte on dissociation gives n+
cations and n–
anions then its limiting molar conductivity is given by:
L°m = n+ l°+ + n– l°–
why does the molar conductivity of strong electrolytes increase with dilution
- a strong electrolyte is completely dissociated in solution so the no of ions in the solution remain a constant
- at high concentration, there are stronger interionic attractions which slow down the movement of ions and hence conductance decreases
With the decrease in concentration, the interionic attractions decreases and conductance increases and approaches max limiting value at infinite dilution
why does the molar conductivity for weak electrolytes increase with dilution
weak electrolytes dissociate to a much smaller extent and hence its conductance is smaller than that of strong eelctrolytes,
There is an increase in conductance especially near infinite diltuion. This is because as the concentration of weak electrolyte is reduced, more of it ionises.
Thus conductance increases w/ dilution due to the increase in no of ions in the solution.
Lm increases steeply (Fig. 2.6) on
dilution, especially near lower concentrations. AT infinte dilution, alpha=1
can the limiting molar conductivity of weak electrolytes be obtained by extrapolation of the graph
- we can get the limiting molar conductivity of the strong electrolytes by extrapolating the graph to reach the y axis
-In such cases Lm increases steeply (Fig. 2.6) on
dilution, especially near lower concentrations. Therefore, L
°
m cannot be
obtained by extrapolation of Lm to zero concentration. At infinite dilution
(i.e., concentration c ® zero) electrolyte dissociates completely (a =1),
but at such low concentration the conductivity of the solution is so low
that it cannot be measured accurately.
this prblm is solved by kohlrausch’s law
explain the main parts of copper refining process
- it is done by the process of electrolysis- electrical energy to bring about chemical reactions
-cathode-pure cu strip- negative-reduction
Cu2+(aq) + 2e– —-> Cu (s)
-anode- impure copper block-positive-oxidation
Cu(s) —-> Cu2+(s) + 2e–
-electrolyte: aq CuSO4 in the presence of few drops of conc H2SO4
Thus copper is dissolved (oxidised) at anode and deposited
(reduced) at cathode. This is the basis for an industrial process in
which impure copper is converted into copper of high purity
what is anode mud in cu refining
it has a lot of precious metals like Se,Pt, Au, Ag
the insoluble impurities settle down at the bottom of the anode and are known as anode mud.
how is na/mg/al obtained
Sodium and magnesium metals are produced by the electrolysis of
their fused chlorides.
aluminium is produced by electrolysis of
aluminium oxide(alumina) in presence of cryolite.( hall heroult process).
There were no constant current sources available during Faraday’s
times. The general practice was to put a coulometer (a standard electrolytic
cell) for determining the quantity of electricity passed from the amount
of metal (generally silver or copper) deposited or consumed.( just know)
faraday’s laws of electrolysis
(i) First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).
(ii) Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights.
(Atomic Mass of Metal ÷ Number of electrons required to reduce the cation).
Q = It
what is 1 faraday
the charge on one mole of electrons is called one faraday.
what does the products of electrolysis depends upon?
i) The products of electrolysis depends upon the nature of the material being electorlysed.
ii) It depends on the nature of the electrodes.If the electrode is inert (e.g., platinum or gold), it does not participate in the chemical reaction and acts only as source or sink for electrons. On the other hand, if the electrode is reactive, it participates in the electrode reaction.
iii)The products of electrolysis depend on the different oxidising
and reducing species present in the electrolytic cell and their standard
electrode potentials
