L-3
ELECTROCHEMISTRY
POINTS TO BE REMEMBERED:-
Electrochemistry may be defined as the branch of chemistry which deals with
the relationships between electrical energy and chemical changes taking place
in redox reactions.
A
cell
is of two types:-
I. Electrochemical cells (Galvanic Cell)
II.
Electrolytic
cell.
In Galvanic cell the chemical
energy of a spontaneous redox reaction is converted into electrical work.
In
Electrolytic cell electrical energy is used to carry out a non-spontaneous
redox reaction.
Electrode
Potential is the tendency of an element to loose or gain the electrons when it
is in equilibrium with its ions.
M(s)
M n+ (aq) + ne-
Standard Electrode Potential (E0)
is the electrode potential at standard state (when P=1 bar, T=298K & concentration=1M)
E0cell
= E0cathode-E0anode
The standard potential of the cells
are related of standard Gibbs energy.
∆rG°= -nFE0cell
The standard potential of the cells is related to
equilibrium constant.
∆rG°=
-RT lnk
∆rG°= -2.303RTlogK
Concentration
dependence of the potentials of the electrodes and the cells are given by
Nernst equation.
aA + bB→cC + dD
Nernst
equation can be written as Ecell = E°cell – (RT/nF) ln
[][
]
·
Substance which dissociate into ions in aq.
Solution & conduct electricity is called Electrolyte.
·
Degree of Dissociation (α) is the ratio of no. of
ions produced to the total no. of molecules of electrolyte.
·
Conductance(c) is the reciprocal of resistance.
C=1/R
·
Electrolytic conductance increases with increase in
temp.
·
The reciprocal of resistivity is called
Conductivity or sp. Conductance.
Κ=1/ρ =1/R ×
l/a
It depends on
no. of ions per unit volume.
·
Molar Conductance, λm =1000× Κ/C(in mole/l)
·
Molar Conductivity decreases with increase in
concentration but it increases with dilution.
·
Molar Conductivity at zero concentration of an
electrolyte is called the conductivity at infinite dilution (λm∞).
·
The
conductivity, K of an electrolytic solution depends on the concentration of the
electrolyte, nature of solvent and temperature.
·
Molar
Conductivity, Λm is defined by K/C where ‘C’ is the concentration.
Conductivity decreases but molar conductivity increases with decrease in
concentration. It increases slowly with decrease in concentration for strong
electrolyte while the increase is very sharp in dilute solutions.
Kohlrausch’s Law states that the molar conductivity of an
electrolyte (strong or weak) at infinite dilution is the sum of the
contribution of the conductivities of its cation & anion.
λ∞m (AxBy)
= x λ∞m (Ay+) + y λ∞m (Bx-)
Faraday’s laws of
Electrolysis
Faraday’s I Law of Electrolysis,
The amount
of chemical substance deposited at any electrode during
Electrolysis is proportional to the
quantity of electricity passed through the electrolyte.
W=Z × i× t
Where: W = wt. Of substance deposited
Z=Electrochemical Eq.
Z=E/96500
E = equivalent weight
i= Current t=time (sec.)
Faraday’s IInd
Law of Electrolysis:
The amount of different
substances liberated by the same quantity of
Electricity
passed in same time through the electrolytic solution is proportional to their
chemical equivalent weights.
W1/W2=E1/E2
W = wt. Of substance deposited E = equivalent weight
Batteries and fuel cells are very useful forms of galvanic
cells. There are mainly two types of batteries.
·
A Primary Cells the cell which can’t be recharged
while Secondary Cell can be recharged again & again.
·
Fuel Cell is the cell in which energy is produced
from the combination of fuel gases like H2, CO,CH4
directly.
·
Efficiency of a fuel cell(η)= (ΔG/ΔH) × 100
·
Corrosion is a process of eating away of a useful
metal by reaction with O2 & moisture. It is an electrochemical
phenomenon.
·
Galvanisation is a better process to prevent the
rusting of iron.
In
corrosion metal is oxidized by loss of electrons to oxygen and form oxides.
Anode
(Oxidation): 2Fe(s) → 2Fe2+ + 4e-
Cathode
(Reduction): O2(g) + 4H+(aq) + 4e- →
2H2O
Atmospheric
Oxidation:
2Fe2+(aq)
+ 2H2O(l) + 1/2O2(g) → Fe2O3(s) +
4H+(aq)
SHORT ANSWER
QUESTIONS (1 MARKS)
1.
What
is the effect of temperature on molar conductivity? Molar conductivity of
an electrolyte increases with increase in temperature.
2.
Why
is it not possible to measure single electrode potential?
It is not possible to measure
single electrode potential because the half-cell containing single electrode
cannot exist independently, as charge cannot flow on its own in a single
electrode.
3.
Name
the factor on which emf of a cell depends:-
emf of a cell depends on following factor-
a.
Nature
of reactants.
b.
Concentration
of solution in two half cells.
c.
Temperature
d.
Pressure
of gas.
4.
What
are the units of molar conductivity?
ohm-1cm2mol-1
or Scm2mol-1
5.
Write
Nernst equation – For the general cell
reaction
ECell
= E0Cell – (RT/nF) ln
6.
What
is the emf of the cell when the cell reaction attains equilibrium?
Ans:
Zero
7. What is the electrolyte used in a
dry cell?
Ans: A paste of NH4Cl, MnO2 and C
8.
How
is cell constant calculated from conductance values?
Cell constant = Conductivity X
Resistance.
9.
What
flows in the internal circuit of a galvanic cell?
Ans: Ions
10.
Define
electrochemical series.
The
arrangement of various electrodes in the decreasing or increasing order of
their standard reduction potentials is called electrochemical series.
SHORT
ANSWER QUESTIONS (2 MARKS)
- How can you increase the
reduction potential of an electrode For the reaction
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Mn+(aq)
+ ne-- → M(s)
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Nernst equation is:
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E (Mn+/M) = E0Mn+
/M – 2.303 RT / nF log 1 / [Mn+]
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E (Mn+/M) can be increased by
a.
increase
in concentration of Mn+ ions in solution
b.
by
increasing the temperature.
2. Calculate emf of the following cell at 298K:
Zn/Zn2+ (10-4 M)
|| Cu2+ (10-2M)/Cu
Given:E0Zn2+/Zn=-0.76V E0Cu2+/Cu=+0.34V
Cell reaction is as follows.
Zn(s) +
Cu2+(aq) → Zn2+(aq)
+ Cu(s)
n=2 T=298K
Ecell =(E0Cu2+/Cu-E0Zn2+/Zn)-0.0591/2V
log [Zn2+(aq)]
[Cu+(aq)]
= 0.34V-(-0.76)-0.02955V log 10-4
10-2
=
1.10V-0.02955V
log 10-2
=
1.10V
+ 2 X 0.02955 V
=
1.10V
+ 0.0591 V
=
1.1591V
Oxidation
takes place at anode. Now lower the reduction Potential, easier to oxidize.
Reduction Potential of Br-, H2O, F- are in the
following order.
Br -< H2O < F-
Therefore in aq. Solution of KBr. Br- ions are
oxidized to Br2 in preference to H2O. On the other hand,
in aq. Solution of KF, H2O is oxidized in preference to F-.
Thus in this case oxidation of H2O at anode gives O2 and
no F2 is produced.
- What happens when a piece of copper is added to:
(a) an aq solution of FeSO4 (b) an Aq solution of FeCl3?
- Nothing will happen when the piece of copper is added
to FeSO4 because reduction potential E0 Cu2+/Cu(+0.34)
is more than the reduction potential E0(Fe2+/Fe)
(-0.44V).
- Copper will dissolve in an aq solution of FeCl3
because reduction potential E0Fe3+/Fe2+(+0.77V)
is more than the reduction potential of E0Cu2+/Cu(+0.34)
Cu(s)+ 2FeCl3 (aq) → Cu+2(aq) + 2FeCl2(aq)
- Define corrosion. Write chemical formula of rust.
Corrosion
is a process of destruction of metal as a result of its reaction with air and
water, surrounding it. It is due to formulation of sulphides, oxides,
carbonates, hydroxides, etc. Formula of rust- Fe2O3.xH2O
1.
Write any three differences between potential difference and e.m.f.
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E.M.F
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POTENTIAL DIFFERENCE
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1.It is difference between
electrode potential
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1.it is difference of potential
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of two electrodes when no current
is flowing
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between electrode in a closed
circuit.
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through circuit.
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2. it is the maximum voltage
obtained From a
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2.it is less than maximum voltage
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cell.
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Obtained from a cell.
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3. it is responsible for steady
flow of Current.
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3.it is not responsible for
steady
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Flow of current.
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2.
Why
an electrochemical cell stops working after sometime?
The
reduction potential of an electrode depends upon the concentration of solution
with which it is in contact. As the cell works, the concentration of reactants
decrease. Then according to Le chatelier’s principle the direction of the
reaction will shift in backward direction. On the other hand if the
concentration is more on the reactant side then it will shift the equilibrium
in forward direction. When cell works
concentration
in anodic compartment increases E0 anode will increase and in cathodic compartment concentration decreases
and hence E0 cathode will decrease. Now EMF of cell is E0cell=
E0cathode –E0anode
A decrease
in E0cathode and a corresponding
increase in E0anode
I tmeans that EMF of the cell will
decrease and will ultimately become zero i.e., cell stops working after some
time.
3.for the standard cell : Cu(s)/Cu2+(aq)||
Ag+(aq)/Ag(s)
E0cell Cu2+/Cu
=+0.34 V
E0 Ag+/Ag
=+0.80 V
i.
Identify
the cathode and the anode as the current is drawn from the cell.
ii.
Write
the reaction taking place at anode.
iii.
Calculate
the standard cell potential.
1.
From the cell representation Ag/Ag+ electrode is
cathode and Cu/Cu2+ electrode is anode .
2. At anode : Cu(s)→ Cu2+ (
aq )+2e
E0cell
= E0cathode –E0anode
=
+.80 V –(+0.34V)
=
+0.80V-0.34V
=
0.46V
4. Can we
store copper sulphate in (i) Zinc vessel (ii) Silver vessel? Give reason.
Given E0 Cu2+/Cu = +0.34V, E0Zn2+/Zn=
-0.76V, E0Ag+/Ag = +0.80V
A metal
having lower reduction potential can displace a metal having higher reduction
potential from solution of its salt.
I. Since
standard reduction potential of Zn2+ (E0Zn2+/Zn
= -0.76V) is less than the standard reduction potential of Cu2+ (E0Cu2+/Cu=+0.34V),
Zn can displace copper from copper sulphate solution. Thus, CuSO4
solution can be stored in silver vessel.
5. How many grams of chlorine can be
produced by the electrolysis of molten NaCl with a current of 1.02 A for 15
min?
2NaCl(l)→ 2Na+(l) +2Cl-
2
Cl- →Cl2(g)+ 2e-
2mole 1mol
Q =nf
Q= 2 x 96500 C/mol= 1.93 x 105C
Quantity of electricity used = It
=
1.02
A X (15 X 60)sec
=
900C
Molar mass
of Cl2 = 2 X 35.5 = 71 gmol-1
1.93X 105
C of charge produce chlorine = 71g
1.93 X 105
C of charge produce chlorine = 71gm
900 C of
charge produce chlorine 71 X
900
1.93
X
105
=
0.331
gm
6.
The electrical resistance of a column of
0.05 M NaOH solution of diameter 1 cm and length 50 cm is 5.55 × 103
ohm. Calculate its resistivity, conductivity and molar conductivity.
7.
Three conductivity cells A, B and C
containing solutions of zinc sulphate, silver nitrate and copper sulphate
respectively are connected in series. A steady current of 1.5 amperes is passed
through them until 1.45 g of silver is deposited at the cathode of cell B. How
long did the current flow? What mass of copper and what mass of zinc got
deposited in their respective cells? (Atomic mass : Zn = 65.4 u, Ag =108 u, Cu
= 63.5 u)
1.
Explain
the term electrolysis. Discuss briefly the electrolysis of (i) molten NaCl (ii)
aqueous sodium chloride solution (iii) molten lead bromide (iv) water, (v) dil
H2SO4
2.
State
and explain Faraday’s Ist and Second Law for electrolysis.
3.
What
do you understand by ‘electrolytic conduction? How does conductivity and molar
conductivity depend on concentration of the electrolytes? What is the effect of
temperature on electrolytic conduction?
4.
How
is electrolytic conductance measured experimentally?
HOT
QUESTIONS
1 Mark questions:-
1. Why in a concentrated solution, a
strong electrolyte shows deviations from Debye-Huckle- Onsagar equation?
Ans:- Because interionic forces of
attractions are large.
2.
What
is the use of Platinum foil in the hydrogen electrode?
A: It is used for inflow and outflow of electrons.
3. Corrosion of motor cars is a
greater problem in winter when salts arespread on roads to melt ice and snow.
Why?
Saline water increases the electrical Conductivity
of electrolytic solution formed on the metal surface. Hence rusting is quicker
in Saline water.
4.
Is
it safe to stir AgNO3 solution with copper spoon? (E0Ag+/
Ag = 0.80 Volt; E0
Cu+/ Cu = 0.34 Volt)
Ans: No it is not safe because Cu reacts with AgNO3
Solution (emf will be positive.)
5. Why is
it necessary to use salt bridge in a galvanic cell?
Ans: To
complete internal circuit and to maintain electrical neutrality of the
solution.
2 mark questions:-
1.
Why
Li is best reducing agent whereas Fluorine is best oxidizing agent ?
2.
Equilibrium
constant is related to Eo cell but not to Ecell. Explain.
3. Why is sodium metal not obtained at
cathode when aqNaCl is electrolyzed with Pt electrodes but obtained when molten
NaCl is electrolyzed?
4. Zn rod weighing 25 g was kept in
100 mL of 1M copper sulphate solution. After certain time interval, the
molarity of Cu2+ was found to be 0.8 M. What is the molarity of SO4-2
in the resulting solution and what should be the mass of Zn rod after cleaning
and drying ?
Sol A. 1mol KCl dissolved in 200 cc of the solution or Sol B. 1 mol KCl
dissolved in 500cc of the solution.
Long
Answer types Questions:-
1. Which cell is generally used in
hearing aids? Name the material of the anode, cathode and the electrolyte.
Write the reactions involved.
2. Iron does not rust even if Zinc
coating is broken in a galvanized iron pipe but rusting occurs much faster if
tin coating over iron is broken .Explain.
3.
Corrosion is an electrochemical phenomenon’,explain.
4. Calculate the pH of following cell:
Pt, H2/ H2SO4, if its electrode potential is
0.03V.
5. Crude
copper containing Fe and Ag as contaminations was subjected to electro refining
by using a current of 175 A for 6.434 min. The mass of anode
was found
to decrease by 22.260 g, while that of cathode was increased by 22.011 g.
Estimate the % of copper, iron and silver in crude copper.
6. Zinc
electrode is constituted at 298 K by placing Zn rod in 0.1 M aq solution of
zinc sulphate which is 95 % dissociated at this concentration. What will be the
electrode potential of the electrode given that EoZn2+/Zn=
- 0.76 V.
7.At what
pH will hydrogen electrode at 298 K show an electrode potential of -0.118 V,
when Hydrogen gas is bubbled at 1 atm. pressure?
8.How many moles of mercury will be
produced by electrolyzing 1.0 M. Hg(NO3)2 solution with a
current of 2.00 A for 3 hours?
9.A voltaic cell is set up at 25° C with the following
half-cells Al3+ (0.001 M) and Ni2+ (0.50 M). Write an
equation for the reaction that occurs when the cell generates an electric
current and determine cell potential.
10. A copper-silver cell is set up. The
copper ion concentration in it is 0.10 M. The concentration of silver ion is
not known. The cell potential measured 0.422 V. Determine the concentration of
silver ion in the cell.
VALUE
BASED QUESTIONS
Q.1 In Apollo Space programs, hydrogen-oxygen fuel cell was
used
(a) Explain why, fuel cell is preferred in space programme.?
(b) Mention the values associated with the decision of
using fuel cell?
(c) Write the values associated with preference of using
fuel cells to fossil fuel.
Q.2 Ira a student of science went with her grandfather to
buy a battery for their inverter and camera.
They found two types of batteries, one a lead storage battery and other
a Nickel-Cadmium storage battery. Later
was more expensive but lighter in weight. Ira insisted to purchase
costlier Nickel-Cadmium battery.
(a) In your opinion, why Ira insisted for Nickel-Cadmium
battery? Give reasons
(b) Write the values associated with above decision?
Q.3 Shyam’s father wants to buy a
new car. In the market various options are available. Shyam persuades his
father to buy a hybrid car which can run both on electricity as well as on
petrel.
(i) Mention the values associated with this decision.
(ii) Name the battery used for running the car.
(iii) Write the reactions taking place at the anode and
cathode of battery.
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