CBSE Class 12 Chemistry Electrochemistry Board Exam Notes

 

Frequently asked question
1 Mark
1).What is meant by limiting molar conductivity?
Ans Molar conductivity of an electrolyte at infinite dilution.

2.) Give the relationship between molar conductivity and specific conductivity ?
Ans. ᴧ_m = k (1000\M)

3.) What is meant by limiting molar conductivity ?
Ans. The molar conductivity of a solution at infinite dilution is called limiting molar conductivity

4.) What is the unit of molar conductivity?
Ans . ohm–1cm²mol^–1

5.) What is the effect of dilution of concentration on specific conductance ?
Ans. Specific conductance decrease with dilution because it is the conduction power of ion present in unit volume of solution and number of ion in unit volume decreases on dilution. Specific conductance increases with concentration.

6.) What type of metals can be used in cathodic protection of iron against rusting?
Ans. A metal which is more electropositive than iron i.e. having lower reduction potential. Example- Zn, Mg etc.

7. Why mercury cell gives constant voltage throughout its life?
Ans. As the overall reaction does not involve any ion in solution whose conc. can change during its life.
A statement of assertion is followed by a statement of reason. Mark the correct choice from the options given below:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.

1. Assertion : Current stops flowing when Ecell = 0.
Reason : Equilibrium of the cell reaction is attained. ( Ans - a)

2. Assertion : A standard hydrogen electrode is also called reversible electrode.
Reason : It can act on both as anode as well as cathode in an electrochemical cell.
( Ans - a)

3. Assertion : Kohlrausch law helps to find the molar conductivity of weak electrolyte at infinite dilution.
Reason : Molar conductivity of a weak electrolyte at infinite dilution cannot be determined experimentally. ( Ans - a)

4. Assertion : When a copper wire is dipped in silver nitrate solution, there is no change in the colour of the solution.
Reason : Copper cannot displace silver from its salt solution. ( Ans - d)

5. Assertion : Fluorine is the best oxidising agent.
Reason : Fluorine has highest reduction potential. ( Ans - a)

One - word answer
1. How much electricity in terms of Faraday is required to produce 100g of Ca from molten CaCl₂ ? ( 5F)
2. Name the solid substance produced, during the discharge of lead- storage battery.

( Ans - Lead sulphate)

2 marks
1. What is fuel cell ? Write two chemicals which can be used as fuel.
Ans . The cell which converts combustion energy of fuel into electricity .
Methane and hydrogen can be used as fuel 2H₂(g) + 4OH^–(aq) → 4H₂O(l) + 4e^– O₂ + 2H₂O (l) + 4e^– →4OH^–(aq)

2.) What type of battery is the lead storage battery? Write the anode and the cathode reaction and the overall reaction occurring when current is drawn from it ( reaction during discharge of it )
Ans. Lead storage battery is the secondary cell. So it can be recharged by passing direct current through it .
Discharge reaction –
At anode : Pb(s) + SO₄
2^–(aq) → PbSO₄ (s) +2e^–
At cathode: PbO₂(s) + SO₄
2^–(aq) + 4H⁺(aq) + 2e^– →2PbSO⁴ (aq) +2H₂O(l) Overall solution :
Pb + PbO₂+ 4H⁺ + SO₄
2^–(aq) → 2PbSO₄ + 2H₂O

3.) How many moles of mercury will be produced by electrolysing 1.0 M Hg (NO3)2 solution with a current of 2.00 A for three hours ?
Molar mass of Hg(NO₃)₂= 200.6 g/mol
Ans . Current = 2A
Time = 3h = 3(60)(60)s
w = ZIt
Z for Hg in compound =200.6/2F
w= 200.6x2x (3)(60)(60)/2(96500)
w= 22.45g
number of moles = 22.45\200.6 = 0.112 mol

4.) Represent the galvanic cell in which the following reactions take place
Zn(s) + 2Ag+(aq) → Zn²⁺(aq) + 2Ag(s)
i.) Which one of the electrodes is negatively charged ?
ii.) Write the reaction taking place at each of t he electrodes.
iii.) Name the carrier of current within the cell.
Ans .The cell is represented as
Zn(s) │Zn2+(aq) ││Ag^+(aq) │Ag(s) │
i) Zn electrode is negatively charged
ii) At anode
Zn(s) → Zn2⁺ + 2e^–
At cathode
Ag^+(aq) + e^– → Ag(s)