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DEPARTMENT OF CHEMISTRY
UNIT I
ELECTROCHEMISTRY
Two marks:
1. Define the terms (i) Single electrode potential (ii) Electrochemical
cells.
Single electrode potential (E): It is the measure of tendency of a
metallic electrode to lose or gain electrons, when it is in contact with a
solution of its own salt.
Electrochemicall Cell: Electrochemical cell is the one, in which chemical
energy is converted into electrical energy. The oxidation and reduction
reactions of an electrochemical cell produce chemical energy which is
converted into electrical energy
Example : Daniel cell
2.What are the factors that affect electrode potential?
(i)
The nature of the metal.
(ii) The temperature.
(iii) The concentration of metal ions in solution
3. Zinc reacts with dil. H2SO4 to give hydrogen but Ag does not.
Explain.
E0 ( Ag+, Ag ) = 80 V and E0 (Zn2+, Zn) = - 0.76 V
Since Zinc possesses negative reduction potential and also it is
placed above hydrogen in emf series, Zn liberates hydrogen.
But silver possesses positive reduction potential and also it is
placed below hydrogen in emf series , Ag does not liberate hydrogen.
4. Suggest a method to determine the electrode potential of Zinc.
EMF Measurement :
Zn electrode is coupled with saturated calomel electrode and the
emf of the cell is measured. Here Zn acts as anode and calomel electrode
acts as cathode.
Ecell = E0cal - E0Zn
5. What is electrode potential ?
It is the measure of tendency of a metallic electrode to lose or gain
electrons, when it is in contact with a solution of its own salt.
6. Write Nernst equation for electrode potential. Mention the
applications of Nernst equation.
E = Eo + 2.303RT/nF log [Mn+]
i) Nernst equation is used to calculate electrode potential of unknown
metal
ii) Corrosion tendency of metals can be predicted.
7. Define a reference electrode. Give one example.
Reference electrode is the electrode whose potential is known (or)
arbitrarily fixed as Zero. It is used to measure the electrode potential of
another unknown electrode by combining with it.
Example : Standard Hydrogen Electrode ; Calomel Electrode
8. Why can’t glass electrode be used for determination of pH a solution
of high alkalinity?
At above 12 pH (high alkalinity), cations of the solution affect the
glass and make the electrode useless.
9.What is electromotive(electrochemical) series? What is its
significance?
When various metals are arranged in the order of their increasing
values of standard reduction potential on the hydrogen scale, then the
arrangement is called electrochemical series.
Significance :
1. Standard EMF of the cell can be calculated.
2. Equilibrium constant can be calculated.
3. It predicts the spontaneity of the reactions.
4. It gives the relative ease of oxidation or reduction of elements.
10. What are reversible and irreversible cells?
A cell is said to be reversible if it obeys the following three
conditions of thermodynamic reversibility, (e.g. Daniel cell)
i)
If the external emf is equal to the actual emf of the cell no current
will flow.
ii)
If the external emf is smaller than the actual emf of the cell current
will flow.
iii)
If the external emf is greater than the actual emf of the cell current
will flow in the opposite direction.
If the cell do not obey the above condition, that cell is called a
irreversible cell.
11. Mention some important advantages of potentiometric titrations.
1. Apparatus required is cheap and easily available.
2. This method can be used for coloured solutions.
3. Fixing up end point is easier when compared to the titrations in
which indicators are used to fix up end points.
4. Very dilute solutions can be titrated with accuracy.
5. Several components may be titrated in the same solution.
12. Mention some advantages and disadvantages of hydrogen electrode.
Advantages :
1. It is a standard reference electrode with which the potentials of
other electrodes are calculated
2. The results are highly accurate
3. The error due to electrical leakage is negligible
Disadvantages :
1. It requires hydrogen gas and is difficult to set up and transport
2. It requires considerable volume of test solution.
3. The solution may affect the surface of the platinum electrode.
4. The potential of the electrode is altered by changes in pressure.
13. Distinguish between reversible and irreversible cells giving one
example for each.
REVERSIBLE CELL
IRREVERSIBLE CELL
Cell reaction is reversed when The cell reaction is not completely
external potential greater than cell reversed.
potential, is applied.
Ex: Zn/Zn2+//Cu2+/Cu
Ex:Zn/H2SO4/Cu
.
14.Differences between electrolytic cells and electrochemical cells
S.
Electrolytic cell
Electrochemical cell
NO
1.
Electrical energy is converted into Chemical energy is converted into
chemical energy
electrical energy
2.
The anode carries positive charge. The anode carries negative
charge.
3.
The cathode carries negative The cathode carries positive
charge.
charge.
4.
Here the electrons are supplied to But, the electrons are drawn from
the cell from the external battery. the cell.
Twisted two marks questions:
1.Why is it not possible to measure the reduction potential of an isolated
half cell /electrode ?
A half cell does not work independently and we can only determine
the relative value of the electrode potential with respect to the other
reference electrodes.
2. Cell is made from Aluminium and Silver half cells.Which electrode is
the anode and which is the cathode?
Aluminium has more negative electrode potential (E0 = - 1.70 V)
than silver (E0 = + 0.80 V). So at Aluminium electrode, Al lose electrons
and oxidation takes place. Hence Al is anode and Ag is cathode and the cell
is
Al / Al3+// Ag+/Ag
3.In a cell made from Mg and Pb half cells , how do electrons move?
The Magnesium half cell is more negative (E0 = - 2.40 V) with
respect to lead half cell (E0 = - 0.13 V).So at Mg electrode , Mg atoms lose
electrons.Hence the electrons travel from Mg electrode to Pb electrode.
4.Can we use nickel spatula to stir a solution of copper sulphate?
No, since the reduction potential of Ni2+/Ni electrode is less of
Cu2+/Cu electrode(+0.34V).Nickel will otherwise displace copper from
copper sulphate solution as below.
Ni(s) +Cu2+ (aq)  Cu(s)  +Ni2+ (aq)
5.Why is a salt bridge used in the construction of a cell?
1. To prevent liquid junction potential
2. To provide electrical continuity
6.Electrode potential of zinc is assigned a negative value (-0.76V)
whereas that of copper a positive value (+0.34v).Give reason.
Zinc electrode is anodic with respect to standard hydrogen
electrode.Hence its electrode potential is assigned a negative sign.On the
other hand copper electrode is cathodic with respect to standard hydrogen
electrode, so it is assigned a positive sign.