Electrochemistry shamsiye

ELECTROCHEMISTRY
Electrode potential
Mr Saad Miraji

Measurements of electrode potential
• We can always measure the electrode
potential for an electrode in combination with
standard electrode which is standard
hydrogen electrode (SHE)
• It was conventionally agreed that electrode
potential for hydrogen is zero.

Cont..
• When electrode potential of any element is
measured against hydrogen electrode at ,1
atm and 1M concentration is called standard
electrode potential.
• That is, if the electrode potential is measured
under standard conditions (1M solution, 1atm
pressure, ) relative to standard hydrogen
electrode it is a standard electrode potential.

Diagram of setup of measure of
standard electrode potential…

• Salt Bridge Is an inverted U-tube that contains
electrolyte (e.g. ) which connects the
solution of two half cells to balance the charge
and to complete the circuit.
• NOTE :
The standard redox potential is actually
reduction potential. All elements below
hydrogen have negative reduction potential
and positive oxidation potential hence strong
reducing agents.
• All elements above hydrogen have positive
reduction potential and negative oxidation
potential hence strong oxidizing agents.

• Consider the following arrangements for
determine electrode potential for:
I. Zinc.
=
The zinc electrode is negative.
• Cell reaction:

II. Copper
The copper electrode is positive. (It can easily
be reduced)
• Cell reaction:
• Overall:

Application of standard electrode potential
I. Construction of electrochemical cells.
II. Prediction of occurrence of chemical reaction
III. Determination of pH a solution without pH
meter
IV. Replacement of elements in the
electrochemical series.
V. Determination of equilibrium constant.
VI. Determination of concentration of ions.

CONSTRUCTION OF ELECTROCHEMICAL
CELLS
• Electrochemical cells are devices that use chemical
reactions to produce electrical power.
• These are sometimes known as Galvanic or Voltaic
cells. e.g. dry cells, car batteries.
• It contains two half cells connected together by
external circuits.
• Half cell is an arrangement which consists of an
electrode dipped into a solution containing its ions.
• When the two half cells are connected, the resulting
component is called electrochemical cell. A good
example is Daniel cell. It is constructed from Zinc and
copper electrode.

Procedure for construction of
electrochemical cell
1. Identify between the electrodes, which electrode is
supplying or gaining electrons by studying their electrode
potential.
II. An electrode with negative standard electrode potential is
more reactive (supplies electrodes) than the positive one.
• If both are negative the one which has more negative
electrode potential is more reactive.
E.g. Sn = -0.16
Mg = -0.25 (more reactive)
• If both electrodes are positive, the one which has less
positive value is more reactive.
E.g. 0.07v (more reactive)
0.2v

• The electrode which supplies electrons should
be placed on the left and electrode which
gains electrons should be on the right hand
side.
• E.g. construct a Daniel cell and shows the
direction of flow of electrons and current
given that
•

• The electrode which supplies electrons oxidation takes
place and the electrode is oxidized.
• This electrode is called ANODE.
• The electrode which receives electrons reduction takes
place and the electrode is REDUCED.
• It is called CATHODE.
Zn – anode
Cu – cathode
• Overall reaction is obtained by adding the two half
reactions and is called cell reaction.
Anodic reaction:
Cathodic reaction:
Cell reaction:

• Electrochemical cell can also be represented in
an abbreviation way known as cell notation.
Anode cathode
I.e. for Daniel cell

Examples …
• Given the overall reactions, write their
corresponding cell notations
a)
• From the cell notation give the cell reactions
a)

ELECTROMOTIVE (EMF) FORCE OF A CELL
• The difference between electrode potential of
the two electrodes constituting an
electrochemical cell is known as electromotive
force (emf) or cell potential.
• This acts as a driving force for a cell reaction
and it is expressed in volts.
• The emf of a cell is calculated by subtracting
the standard electrode potential of the left
electrode from that of the right electrode.

• For Daniel cell :
= 0.34 – (-0.76)
= 1.1v
Note: in applying this formulae of calculation of
cell potential, make sure only reduction
potentials are used and not otherwise.

Prediction of the occurrence of
chemical reaction….
• If the emf of the cell calculated is negative the
reaction is non spontaneous i.e. the reaction
does not occur in the way it is written unless
external forces apply but the reaction is
spontaneous in the opposite direction.
• If the emf of the cell is positive the reaction is
spontaneous

• Example: given the following reaction
• Predict the direction of the reaction given that
reduction potential of nickel (Ni2+/Ni) Eo = -0.25v
of mercury Hg+/Hg = 0.14v
Solution:
Cell notation:
• The reaction is non spontaneous since emf is
negative hence backward reaction is favored.

Example 2…
• Briefly explain what happen when
I.Fe is dipped in CuSO4 solution
II.Cu is dipped in FeSO4 solution

(i) since Fe has a negative reduction potential
then it has a positive oxidation potential and
hence will displace copper metal from CuSO4
solution
(ii)Since Cu has positive reduction potential,
then it has a negative oxidation potential and
hence it will NOTE displace Fe from
FeSO4solution therefore no reaction will be
possible.

EFFECTS OF CONCENTRATION AND
TEMPERATURE ON CELL POTENTIALS
• We have been considering electrode potential
under standard conditions of molar solution,
pressure of 1 atom and 298K.
• When the conditions are altered the values of
electrode potential changes thus we have to
define the potential of the cell under non-
standard conditions.
• The Nernst equation shows the relationship
between emf of the cell at standard conditions
and emf under non-standard conditions.

• R – Universal gas constant (8.314 Jmol-1K-1)
• n – Number of moles electrons being
transferred
• F – Faraday’s constant (96500c)
• At standard temperature (298K)

• OR
• The above equation can be applied to half
reactions and overall reactions
NOTE
Always write a balanced cell reaction to obtain
‘n’ and position of ions, either reactants or
products together with their stoichiometry.

Example…
• Calculate the emf of the given cell at standard
temperature.
• Given:

Solution.…
= -0.13 – (-2.56)
= 2.43v

Concentration cell….
• If two plates of the same metal are dipped
separately into two solutions of the same
electrolytes at different concentration and the
solution are connected with a salt bridge, the
whole arrangement is found to act as a
galvanic (electrochemical) cell.. Why??
• This is because at different concentration of
the electrolyte, the two half cells have
different electrode potential.

• The plate with more dilute solution has a
lower electrode potential and hence it acts as
an anode (the –ve electrode)
• The plate with more concentrated solution
has a higher electrode potential and hence it
acts as a cathode (the +ve electrode)

Example…
• Calculate the emf of the following
concentration cell
Solution:

Cont…..
Using Nernst equation;
= 0.0591v

Measurement of ph of a solution using
standard electrode potential.
• pH is the degree of alkalinity, or acidity of a
substance. It is obtained by using the hydrogen
ion concentration pH = -
• The pH of a solution is determined by using any
electrode provided its standard electrode
potential is known and the concentration of ions
in that electrode should be 1M. This will be one
of the half cells another half cell is made up
hydrogen electrode dipped into the solution
whose pH is to be determined

• Reaction:
H2(g) 2 H+
(aq) +2e- E 0 = 0.00v
2Ag+
(aq)+ 2e- Ag(s) E 0 = 0.8v
H2(g) + 2Ag+
(aq) 2H+
(aq)+ Ag(s) E0 = 0.8V

Applying Nernst equation at standard
temperature
E cell = cell –
= 0.8 –
E cell= 0.8 + 2 (
E cell = 0.8 + 0.0591
pH= =

Question…….
Calculate the of the following cell and
hydrogen ion concentration.
Zn/Zn2+ // H+ /H2, pt
Zn2+ /Zn = -0.76v
E cell = 0.115v

EQUILIBRIUM CONSTANT OF
GALVANIC CELL
• Consider to what happens to a Daniel’s cell if we use it
to do some electrical work i.e. it can be connected to a
small electric motor.
• After sometimes the motor will stop. The cell will run
down.
• When this happens then there is no overall transfer of
electricity from one half cells to the other. When there
is no overall change taking place in a chemical reaction
the equilibrium has been established.
• At equilibrium, electron density of both electrodes is
equal and there is no transfer of electricity between
the two half cells.

Applying the Nernst equation to a
Daniel’s cell
Since the reaction is at equilibrium, the ratio
• KC = 1.67 x 1037

• The large value tells us that, the equilibrium
lies almost entirely in favour of copper metal
and zinc ions.
• Generally, the cell at equilibrium at standard
temperature is given by the following
expression;

Examples …
• Calculate KC for the following voltaic cell

ELECTROCHEMICAL SERIES
• Is the series of standard electrode potential
with respect to their elements from more
negative standard electrode potential to the
more positive standard electrode potential.
• Is the arrangement of electrodes of elements
in order of reducing power

Uses….
1. It is a good guide for predicting reaction that
takes place in solution especially
displacement reaction.
• Displacement reaction
– Is a types of reaction in which an atom or element
displace another element or atom in a compound
• Therefore, the electrochemical series can be
used to determine which element will displace
which element in the displacement reaction.

Uses…
2. Displacement of hydrogen from mineral
acids
Metals which are higher in the electrochemical
series than hydrogen reacts with acids and
replaces hydrogen but metals below hydrogen
have no action with mineral acids.
Complete the following reaction:-
i. Zn(s) + 2HCl (aq) ZnCl2(s) + H2(g)
ii. Cu(s) + HCl(aq) No reaction

Uses…
3. The knowledge of electrochemical series helps as
on choosing method for extraction of metals.
Example
Higher most metal can’t be extracted from the oxides by
chemical reduction process. This is because they are
strong reducing agent hence they can be reduced
easily from their oxide by electrolysis.

• Question.
For electrolysis, fused or molten metal should be
used and not aqueous solution. Why?
Answer:-
In aqueous solution, there are H+ ions. Hence
metals prefer to react with element in lower
electrochemical series than the metal itself,
hence for electrolysis we use fused or molten
metal and not aqueous solution.

CORROSION AND ITS PREVENTION
• Corrosion is the deterioration of the metals
due to the chemical reactions taking place on
the surface. Usually, the process is due to the
loss of metal to a solution in some forms by a
redox reaction (unwanted redox reactions)

• For corrosion to occur on the surface of a metal
there must be anodic area where a metal can be
oxidized to metal ions as electrons are produced.
Anode area:-
M (s) Mn+ + ne+
And cathodes area where electrons are
consumed by any of all of several half reactions.
Cathodic reaction:-
2H+
(aq) + 2e- H2 (g)
2H2 O(l) + 2e- 2OH-
(aq)+ H 2(g)
4e- + O2(g) +H2 O(l) 4OH-
(aq)
• Anodic reactions occur at cracks or around the
area with some impurities.

RUSTING OF IRON
• The most common corrosion process is rusting of iron
• Rust is hydrated iron (III) Oxide (Fe2O3. XH2O) which
appears as a reddish brown substance on the surface of
the iron bar.
• Both water and air (oxygen) are required for rusting
to occur.
• The presence of dissolved salts and acid in water
increases its conductivity and speed up the process of
rusting.
• If the iron object has free access to oxygen and H2O as
in flowing water, a reddish brown ion (III) oxide will be
formed which is a rust.

• For iron:
Anode: Fe(s) Fe2+
(aq)+ 2e-
Cathode:O2(g) + 2H2O(l) + 4e- 4OH-
(aq)
2Fe(s) + O2(g) + 2H2 O(l) 2Fe2+
(aq)+ 40H-
(aq)
(rust)
4Fe(OH)2(s) + O2(g) 2H2O(l) + 2Fe2 O3.XH2 O

• If oxygen is not freely available: The farther
oxidation of iron (II) hydroxide is limited to
formation of magnetic iron oxide.
•
• 6Fe(OH)2(s) + O2(g) 2Fe3 O3.H2O + 4H2O(l)
• Fe3O4 + H2O(l) Fe3O4 + 4H2O
Black magnetite

Electrolytes…
• These are substances which allow electricity to pass through them
in their molten state or in form of their aqueous solution, and
undergo chemical decomposition e.g., acids, bases and salts.
Classification of electrolytes
All electrolytes do not ionize by the same extent in the solution.
According to this we have strong and weak electrolytes
Strong electrolytes are those that ionize completely into ions in the
solution
e.g. Salts, mineral acids, some bases.
Weak electrolytes are these that ionize partially into ions in the solution
e.g. in organic acids. HCN, Na4OH.

Electrolytic conduction,
• When a voltage is applied to the electrodes dipped into
an electrolytic solution, ions of the electrolyte move
towards their respective electrodes and therefore
electric current flows through the electrolytic cell. The
process of the electrolyte to conduct electric current is
termed as conductance or conductivity.
• Like metallic conductors, electrolytic solution also obey
ohm's law which states that “the strength of the
current flowing through a conductor is directly
proportional to the potential difference applied across
the conductor and inversely proportional to the
resistance of the conductor
i.e. V = RI.

• Resistance of any conductor is directly
proportional to the length L and inversely
proportional to the area of cross – section.
• R =
• R = ρ x
• ρ=Resistivity
• Resistivity is the resistance of a conductor
having unit length and unit area of cross-
section. SI unit Ohm-meter (á½©m).

• Conductance is the measure of the ease with
which the current flows through a conductor a
(λ orΛ)
Conductance is the reciprocal of electric
resistance. ( )
• From the above expression high resistance
means low conductance and vice versa.
• Conductivity is the reciprocal of resistivity and
is also called specific resistance (kappa)
• = (

• For an electrolytic cell, l is the distance apart
between the two electrodes and A is the total
area of cross-section of the two electrodes.
Therefore, for a given cell, l and A are
constant. If the dimensions of the cell are not
altered, the ration is referred as cell
constant (K)

= K
R = ρ =
= , = =
=
• Molar conductance (λm)
Is the conductivity of volume of a solution
which contains 1 mole of the solute.
Or
It is the conducting power of the ions produced
by dissolving 1 mole of an electrolyte in a
solution.

• Molar conductance is given by where
v = volume containing 1 mole of a solute and
is called dilution.
Concentration of an electrolyte depends on
the volume i.e.
V =
=
• Where C is the concentration in mol/dm3
and is m-1

VARIATION OF MOLAR CONDUCTIVITY WITH
CONCENTRATION
• The intensity of electricity that can pass through the
solution depends on
i.The number of concentration of free ions present
in the solution.
ii. Speed with which ions move to their respective
electrodes.
• An increase in concentration gives an increase in total
number of solute particles in a given volume of
solution and this might well be expected to give an
increased conductivity.
• Conduction in strong electrolytes.

• The molar conductivity is high since strong
electrolytes ionize completely into free ions and
the molar conductivity increases slightly in
dilution. WHY?
• In strong electrolytes, there are vast numbers of
ions which are close to each other. These ions
tend to interfere with each other as they move
towards their respective electrodes. The positive
ion are held back by the negative ions and vice
versa which in turn interrupts their movement to
the electrodes (reduce the speed with which they
move)
• Dilution ions get separated from each other and
at an average distance they can move freely or
easily.

• NOTE:-
• As the dilution increases, there comes a time
for amount of interference become small so
that further dilution to has no effect.
• At this point the molar conductivity remains
constant and it is known as molar conductivity
at zero concentration .

Conduction in weak electrolyte..
• The molar conductivity is less because there is
less number of particles as the minority of
particles are dissociated into ions. On dilution
the molar conductivity increases as the
molecules dissociate more into ions which
increases the number of free ions. Therefore
for weak electrolytes the molar conductivity
depends on the degree of dissociation of
molecules into ions.

MOLAR CONDUCTIVITY AND DEGREE
OF DISSOCIATION
• The molar conductivity of weak electrolyte is proportional to the
degree of dissociation.
i.e.
= K ................ (i)
• At infinity dilution, the weak electrolyte is completely ionized.
= 1 or 100%
= K (Constant)........... (ii)
Taking ratio of equation (i) and (ii)

Ostwald's dilution law
• It states that
• “For a weak electrolyte the degree of
dissociation is proportional to the square root
of reciprocal of concentration”
• Consider a weak binary electrolyte AB (i.e.
ethanoic acid) in solution with concentration C
(mol/ ).

AB A+
(aq) + B-
(aq)
1mole 0 0 start
At equilibrium
But V =
AB A+
(aq) + B-
(aq)
K =
But AB = CH3 COOH
=

• For a weak electrolyte, degree of dissociation
is very small thus the expression
1- 1
Ka=
K a = C

Example…
• 1. At 250c the solution of O.1M of ethanoic acid
has a conductivity of 5.0791x10-2 m-1mol-1.
calculate the pH of the solution and dissociation
constant Ka (Answer Ka=1.69 10-2M)
• 2. A 0.001 moldm-3 solution of ethanoic acid was
found to have molar conductivity of
14.35Sm2mol-1. Use this value together with
molar conductivity at infinity dilution of ethanoic
acid (C ) is 390.7Ð… to calculate :-
i. Calculate degree of dissociation of acid
ii. Equilibrium constant

Electrochemistry shamsiye

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