Question Set B

CBSE 12th Standard Chemistry Electrochemistry HOT Questions Study Material - QB365

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Questions:

Generic 2

4 x 2 = 8

The resistance of a conductivity cell containing 0.001M KCl solution at 298 K is 1500 $\Omega$. What is the cell constant if conductivity of 0.001M KCl solution at 298 K is 0.146 x 10^-3 S cm^-1 ?

Define limiting molar conductivity. Why conductivity of an electrolyte solution decreases with decrease in concentration ?

Why in a concentrated solution, a strong electrolyte shows deviations from Debye-Huckel-Onsager equation ?

The conductivity of 0.20 M KCl at 298 K is 0.025 S cm^-1. Calculate its molar conductivity.

Generic 3

3 x 3 = 9

(i) Write two advantages of H₂$\rightarrow$O₂ fuel cell over ordinary cell.
(ii) Equilibrium constant (K_C) for the given cell reaction is 10. Calculate E⁰_cell.
$A(s)+B^{ 2+ }(aq)\rightleftharpoons A^{ 2+ }(aq)+B(s)$

Predict whether the following reaction (s) is (are) feasible or not
(i) $Fe+{ Zn }^{ 2+ }\longrightarrow { Fe }^{ 2+ }+Zn,{ E }_{ Zn }^{ o }=-0.76V,{ E }^{ o }_{ Fe }=-0.44V$
(ii) $Zn+{ 2Ag }^{ + }\longrightarrow { Zn }^{ 2+ }+2Ag,{ E }_{ Zn }^{ o }=-0.76V,{ E }^{ o }_{ Ag }=-0.80V$

Calculate ${ \Lambda }_{ m }^{ o }$ for CaCl₂ and MgSO₄ from the data given below:
${ \lambda }^{ o }({ Ca }^{ 2+ })=119.0 \ S \ { cm }^{ 2 }{ mol }^{ -1 }$
${ \lambda }^{ o }(Cl^{ - })=76.3 \ S \ { cm }^{ 2 }{ mol }^{ -1 }$
${ \lambda }^{ o }({ Mg }^{ 2+ })=106 \ S \ { cm }^{ 2 }{ mol }^{ -1 }$
${ \lambda }^{ o }({ SO }_{ 4 }^{ 2- })=160.0 \ S \ { cm }^{ 2 }{ mol }^{ -1 }$

Generic 5

3 x 5 = 15

(i) On the basis of E^o values, identify which amongst the following is the strongest oxidising agent
$\mathrm{Cl}_2(\mathrm{~g})+2 e^{-} \longrightarrow 2 \mathrm{Cl}^{-} ; \mathrm{E}^{o}=+1.36 \mathrm{~V} $
$\mathrm{MnO}_4^{-}+8 \mathrm{H}^{+}+5 \mathrm{e}^{-} \longrightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_2 \mathrm{O}_i \mathrm{E}^{\mathrm{o}}=1.5 \mathrm{IV} $
$\mathrm{Cr}_2 \mathrm{O}_7^{2-}+14 \mathrm{H}^{+}+6 \mathrm{c}^{-} \longrightarrow 2 \mathrm{Cr}^{\prime \prime}+7 \mathrm{H}_2 \mathrm{O}_4 \mathrm{E}^{\circ}=+1.33 \mathrm{~V} $
(ii) The following figure, represents variation of (∧m) vs √C for an electrolyte. Here A is the molar conductivity and C is the concentration of the electrolyte.

(a) Define molar conductivity.
(b) Identify the nature of electrolyte on the basis of the above plot. Justify your answer.
(c) Determine the value of Am for the electrolyte.
(d) Show how to calculate the value of A

(a) Define the following terms:
(i) Limiting molar conductivity
(ii) Fuel cell
(b) Resistance of a conductivity cell filled with 0.1 mol L^-1 KCI solution is $\mathbf{1 0 0} \Omega$. If the resistance of the same cell when filled with 0.02 mol L^-1 KCI solution is $\mathbf{520} \Omega$, calculate the conductivity and molar conductivity of 0.02 mol L^-1 KCI solution. The conductivity of 0.1 mol L^-1 KCI solution is 1.29 x $10^{-2} \Omega^{-1}$ cm^-1.

10.

(a)When a bright silver object is placed in the solution of gold chloride, it acquires a golden tinge but nothing happens when it is placed in a solution of copper chloride. Explain this behaviour of silver.
[Given: E_cu²⁺/cu= + 0.34; E^o_Ag⁺ /Ag = + 0.80 V,
E_Au²⁺/Au= + 1.40 V]
(b)
Consider the figure given above and answer the following questions:
(i) What is the direction of flow of electrons?
(ii) Which one is anode and which one is cathode?
(iii) What will happen if the salt bridge is removed?
(iv) How will concentration of Zn²⁺ and Ag ⁺ions be affected when the cell functions?
(v) How will concentration of these ions be affected when the cell becomes dead?

Case Study Questions

2 x 4 = 8

11.

The study of the conductivity of electrolyte solutions is important for the development of electrochemical devices, for the characterization of the dissociation equilibrium of weak electrolytes, and for the fundamental understanding of charge transport by ions. The conductivity of the electrolyte is measured for electrolyte solution with concentrations in the range of 10^–3 to 10^–1 mol L^–1, as a solution in this range of concentrations can be easily prepared. The molar conductivity (Λm)
of strong electrolyte solutions can be nicely fit by the Kohlrausch equation.
Λ_m = Λ°_m – K √C …(i)
Where Λ°m is the molar conductivity at infinite dilution and C is the concentration of the solution. K is an empirical proportionality constant to be obtained from the experiment. The molar conductivity of weak electrolytes, on the other hand, is dependent on the degree of dissociation of the electrolyte. At the limit of a very dilute solution, the Ostwald dilution law is expected to be followed,

where CA is the analytical concentration of the electrolyte and Kd is the dissociation constant. The molar conductivity at infinite dilution can be decomposed into the contributions of each ion.

Where, λ₊ and λ_–are the ionic conductivities of positive and negative ions, respectively and v₊ and v^– are their stoichiometric coefficients in the salt molecular formula.
1.Which statement about the term infinite dilution is correct?
(a) Infinite dilution refers to a hypothetical situation when the ions are infinitely far apart.
(b) The molar conductivity at infinite dilution of NaCl can be measured directly in solution.
(c) Infinite dilution is applicable only to strong electrolytes.
(d) Infinite dilution refers to a real situation when the ions are infinitely far apart.
2.Which of the following is a strong electrolyte in aqueous solution?
(a) HNO₂
(b) HCN
(c) NH₃
(d) HCl
3.Which of the following is a weak electrolyte in aqueous solution?
(a) K₂SO₄
(b) Na₃PO₄
(c) NaOH
(d) H₂SO₃
4.If the molar conductivities at infinite dilution for NaI, CH₃COONa and (CH₃COO)₂Mg are 12.69, 9.10 and 18.78 S cm2 mol^–1 respectively at 25°C, then the molar conductivity of MgI₂ at infinite dilution is
(a) 25.96 S cm² , mol^–1 (b) 390.5 S cm² mol^–1
(c) 189.0 S cm² mol^–1
(d) 3.89 × 10–2 S cm² mol^–1
5.Which of the following is the correct order of molar ionic conductivities of the following ions in aqueous solutions?
(a) Li⁺ < Na⁺ < K⁺ < Rb⁺
(b) Li⁺> Na⁺ > K⁺> Rb⁺
(c) Rb⁺ < Na⁺ < Li⁺ < K⁺
(d) Li⁺ < Rb⁺< Na⁺ < K⁺

12.

Observe the graph shown in figure between Am (molar conductivity) Vs $\sqrt{\mathrm{C}}$ (Molar concentration) and answer the questions based on graph.

(a) The curve 'V' is for KCI or CH₃COOH?
(b) What is intercept on $\Lambda$_m axis for 'X' equal to?
(c) Give mathematical equation representing straight line.
(d) What is slope equal to?
(e) What happens to molar conductivity on dilution in case of weak electrolyte and why?

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Answers Key:

Generic 2

4 x 2 = 8

Cell constant = Conductivity/Conductance = Conductivity x Resistance
= (0.146 x 10^-3) S cm^-1 x 1500 $\Omega$
= 0.219 cm^-1

The molar conductivity of a solution at infinite dilution, i.e., when concentration approaches zero is called limiting molar conductivity. With decrease in concentration, i.e., with dilution, number of ions per cm³ decreases due to large increase in volume of the solution. As conductivity is the conductance per cm³ of the solution, hence it decreases.

In concentrated solution of a strong electrolyte, the interionic forces of attraction are large.

${ \Lambda }_{ m }=\frac { 1000\times k }{ M } =\frac { 1000\times 0.025\times { 10 }^{ -2 } }{ 0.20 } =125 \ S{ cm }^{ 2 }{ mol }^{ -1 }$

Generic 3

3 x 3 = 9

i) Advantages of fuel cells
(a) Because of the continuous supply of fuels, these cells never become dead. These cells are usually operated at a temperature of 70-140°C and gives a potential of about 0.9V.
(b) Fuel cells do not cause any pollution unlike thermal plant which burns fossil fuels like coal gas, oil, etc.
(ii) The reaction
$A(s)+B^{ 2+ }(aq)\rightleftharpoons A^{ 2+ }(aq)+B(s)$
By using Nernst equation
$E_{ cell }=E^{ 0 }_{ cell }-\frac { 0.059 }{ n } log\left[ \frac { A^{ 2+ } }{ B^{ 2+ } } \right] $
At equilibrium E_cell = 0
E_cell= $\frac { 0.059 }{ n } $ log k_c
K_c = 10 and n = 2
$E^{ 0 }_{ cell }-\frac { 0.059 }{ n } $ log10
$E^{ 0 }_{ cell }-\frac { 0.059 }{ n } =0.0295$

(i) No (ii) yes

${ { \wedge }^{ ° } }_{ m }\left( { CaCl }_{ 2 } \right) ={ \lambda }^{ ° }\left( { Ca }^{ 2+ } \right) +2{ \lambda }^{ ° }\left( { Cl }^{ - } \right) $
$=119.0S{ cm }^{ 2 }{ mol }^{ -1 }+2\times 76.3S{ cm }^{ 2 }{ mol }^{ -1 }=271.6S{ cm }^{ 2 }{ mol }^{ -1 }$
${ { \wedge }^{ ° } }_{ m }\left( { MgSO }_{ 4 } \right) ={ \lambda }^{ ° }\left( { Mg }^{ 2+ } \right) +2{ \lambda }^{ ° }\left( { { SO }_{ 4 } }^{ 2- } \right) $
$=106 \ S{ \ cm }^{ 2 }{ \ mol }^{ -1 }+160 \ S{ \ cm }^{ 2 }{ \ mol }^{ -1 }=266 \ S{ \ cm }^{ 2 }{ \ mol }^{ -1 }$

Generic 5

3 x 5 = 15

(i) $\mathrm{MnO}_4^{-}$is the strongest oxidising agent. Oxidising power of metals decrease with an increase in their $E^{\prime}$ values.
ii) (a) Molar conducivity of a solution is the conductance of that volume of solution containing one mole of electrolyte, kept between two electrodes having unit length between them and large cross-sectional areas so as to contain the electrolyte.
(b) The nature the electrolyte on the basis of plot is strong electrolyte. For strong electrolyte, $\Lambda_{\text {m }}^{\circ}$ increases slowly with dilution.
(c) Using Debye-Huckel Onsager equation,
$\Lambda_m=A_m^2-A \sqrt{C}$
$\Lambda^{\circ}$ value from graph is $150 \mathrm{~S} \mathrm{~cm}^2 \mathrm{~mol}^{-1}$.

(a) (i) Limiting molar conductivity $\left(\Lambda_{m}^{\circ}\right)$ : It is defined as the maximum molar conductance of an electrolyte when solution is infinitely dilute, i.e. concentration approaches zero when electrolyte solution is kept in cell, unit distance apart having large area of cross-section to hold enough electrolyte.
(ii) Fuel cell is a cell in which chemical energy of a fuel is converted into electrical energy.
(b) R = 100 ohms, M = 0.1 mol L^-1 of KCI
$\kappa=\frac{1}{R} \times \frac{l}{A}$
$\Rightarrow 1.29 \times 10^{-2} \mathrm{ohm}^{-1} \mathrm{~cm}^{-1}=\frac{1}{100 \mathrm{ohm}} \times \frac{l}{A}$
$\Rightarrow \ \frac{l}{A}=1.29 \mathrm{~cm}^{-1}$
Again, $\kappa=\frac{1}{R} \times \frac{l}{A}=\frac{1}{520} \times 1.29$
= $\frac{1.29}{5.20} \times 10^{-2}$
= 2.48 x 10^{- 3} ohm^-1 cm^-1
Now, M = 0.02 mol L^-1
$\Lambda_{m}=\frac{1000 \kappa}{\mathrm{M}}=\frac{1000 \times 2.48 \times 10^{-3}}{0.02}$
= 124 S cm² mol^-1

10.

(a) E^o value of silver (E^o_Ag⁺ /Ag = +0.80V) is lower than Ag lAg that of gold, hence silver displaces gold
( E^o_Au3+_/Au= + 1.40V), which gets deposited on the silver Object
E^ovalue of copper (E°_cu²⁺/cu= +0.34) is lower than that of silver, hence silver cannot displace copper from its solution.
(b) (i) Electrons flow from Zn to Ag plate.
(ii) Zn acts as anode and Ag acts as cathode
(iii) Cell will stop functioning
(iv) Concentration of Zn²⁺ions will increase and that of Ag⁺ ions will decrease.
(v) No change

Case Study Questions

2 x 4 = 8

11.

1. (a) Infinite dilution refers to a hypothetical situation when the ions are infinitely far apart.
2. (d) HCl
3. (d) H2SO3
4. (a) 25.96 S cm2 , mol–1 (b) 390.5 S cm2 mol–1
5. (a) Li+ < Na+ < K+ < Rb+

12.

(a) It is for CH₃COOH.
(b) It is equal $\Lambda$° (limiting molar conductivity).
(c) $\Lambda_{\mathrm{m}}=\Lambda_{\mathrm{m}}^{\circ}-\mathrm{A} \sqrt{\mathrm{C}}$
(d) Slope = -A
(e) $\Lambda$m for weak electrolyte increases sharply on dilution because both number of ions as well as mobility of ions increases.