NCERT Exemplar Class 12 Chemistry Chapter 3 Electrochemistry

Question1: Which cell will measure standard electrode potential of copper electrode?

$(i) \; Pt (s)| H_{2}(g,0.1 bar)| H^+ (aq.,1 M)|| Cu^{2+}(aq.,1M) |Cu$
$(ii) \; Pt (s) |H_{2}(g,1 bar) |H^+ (aq.,1 M)|| Cu^{+2}(aq.,2M)| Cu$
$(iii) \; Pt (s) | H_{2}(g,1 bar)| H^+ (aq.,1 M) || Cu^{2+}(aq.,1M) |Cu$
$(iv) \; Pt (s) |H_{2}(g,1 bar) |H^+ (aq.,0.1 M) ||Cu^{2+}(aq.,1M)| Cu$

Answer:

The answer is option (iii). On connecting the copper electrode to a standard hydrogen electrode, it acts as the cathode, and its standard electrode potential can be measured.
$E^{0}=E_{R}^{0}-E_L^0=E_R^0-0=E_R^0$
$Pt (s)\left | H_{2}(g,1\; bar) \right |H^{+}(aq., 1 M)\parallel Cu^{2+}(aq., 1\; M)\mid Cu$
will measure the standard electrode potential of the copper electrode.
The standard electrode potential is measured for a given cell by coupling it with the standard hydrogen electrode, where the pressure of hydrogen gas is maintained at one bar. The concentration of the H⁺ ion in the solution is one molar, and so is the concentration of the oxidized and reduced forms of the species.

Question 2: Electrode potential for Mg electrode varies according to the equation
$E_{Mg^{2+}/Mg}=E^{\circleddash }_{Mg^{2+}/Mg}-\frac{0.0591}{2}\; log \frac{1}{[Mg^{2+}]}$ The graph of $E_{Mg^{2+}/Mg}$ Vs log $[Mg^{2+}]$ is

Answer:

The answer is the option (ii) $E_{\frac{Mg2+}{Mg}}=E_{{\frac{Mg^{2+}}{Mg}}}^o-\frac{0.059}{2}log[Mg^{2+}]$
The given equation is that of a straight line with a positive slope and a non-zero intercept

Question 3: Which of the following statements is correct?
(i) $\mathrm{E}_{\text {Cell }}$and $\Delta_{\mathrm{r}} \mathrm{G}$ of the cell reaction are both extensive properties.
(ii) $\mathrm{E}_{\text {Cell }}$ and $\Delta_{\mathrm{r}} \mathrm{G}$ of the cell reaction are both intensive properties.
(iii) $\mathrm{E}_{\text {Cell }}$ is an intensive property, while? $\Delta_{\mathrm{r}} \mathrm{G}$ of the cell reaction is an extensive property.
(iv) $\mathrm{E}_{\text {Cell }}$ is an extensive property, while Δ_{$\Delta_{\mathrm{r}} \mathrm{G}$} of the cell reaction is an intensive property.

Answer:

The answer is option (iii). While $\mathrm{E}_{\text {Cell }}$ is independent of the mass of species or the number of particles, it is an intensive property, whereas $\Delta_{\mathrm{r}} \mathrm{G}$ depends on the number of particles and is an extensive property.

Question 4: The difference between the electrode potentials of two electrodes when no current is drawn through the cell is called ___________.
(i) Cell potential
(ii) Cell emf
(iii) Potential difference
(iv) Cell voltage

Answer:

The answer is the option (ii) When no current is drawn through the cell, the difference between the electrode potentials of two electrodes is EMF.

Question 5: Which of the following statement is not correct about an inert electrode in a cell?
(i) It does not participate in the cell reaction.
(ii) It provides a surface either for oxidation or for the reduction reaction.
(iii) It provides a surface for the conduction of electrons.
(iv) It provides a surface for a redox reaction.

Answer:

The answer is the option (iv) Inert electrodes act only as source or sink for electrons. They do not undergo redox reactions and merely provide the surface for the reaction.

Question 6: An electrochemical cell can behave like an electrolytic cell when ____________.
(i) $\mathrm{E}_{\text {Cell }}$ = 0
(ii) $\mathrm{E}_{\text {Cell }}$ > $\mathrm{E}_{\text {ext }}$
(iii) $\mathrm{E}_{\text {ext }}$ > $\mathrm{E}_{\text {Cell }}$
(iv) $\mathrm{E}_{\text {Cell }}$_l = $\mathrm{E}_{\text {ext }}$

Answer:

The answer is the option (iii) An electrochemical cell can behave like an electrolytic cell when there is an application of an external opposite potential on the galvanic cell and the reaction is not inhibited until the opposing voltage reaches the value 1.1 V. No current flows through the cell when this happens. Reaction will function in the opposite direction on increasing the external potential any further.

Question 7: Which of the statements about solutions of electrolytes is not correct?
(i) The conductivity of the solution depends upon the size of ions.
(ii) Conductivity depends upon the viscosity of the solution.
(iii) Conductivity does not depend upon the solvation of ions present in the solution.
(iv) The conductivity of the solution increases with temperature.

Answer:

The answer is option (iii) Conductivity decreases with an increasing salvation of ions.

Question 8: Use the data given below to find out the strongest reducing agent.
$E^{\circleddash }_{Cr_{2}O{_{7}}^{2-}/Cr^{3+}}=1.33 \; V$
$E^{\circleddash }_{Cl_{2}/Cl^{-}}=1.36 \; V$
$E^{\circleddash }_{MnO{_{4}}^{-}/Mn^{2+}}=1.51 \; V$
$E^{\circleddash }_{Cr^{3+}/Cr}=-0.74\; V$
$(i)\; Cl^{-}$
$(ii)\; Cr$
$(iii)\; Cr^{3+}$
$(iv)\; Mn^{+2}$

Answer:

The answer is that option $(ii)\; Cr$ is the strongest reducing agent as it is the only one with a negative standard reduction potential

Question 9: Use the data given in Q.8 and find out which of the following is the strongest oxidizing agent.
$(i) \; Cl^{-}$
$(ii) \: Mn^{2+}$
$(iii) \; MnO^{-}_{4}$
$(iv) Cr^{3+}$

Answer:

The answer is the option (iii). Oxidizing capacity increases with a positive increase in the standard reduction potential. $MnO^{4-}$ with the highest standard reduction potential is the strongest oxidizing agent.

Question10: Using the data given in Q.8 find out in which option the order of reducing power is correct.
$(i) Cr^{3+} < Cl^{-} < Mn^{2+} < Cr$
$(ii) Mn^{2+} < Cl^{-}< Cr^{+3} < Cr$
$(iii) Cr^{3+} < Cl^{-}< Cr_{2}O_{7}^{2-} < MnO^{-}_{4}$
$(iv) Mn^{2+} < Cr^{3+} < Cl^{-}< Cr$

Answer:

The answer is option (ii). A decrease in reduction potential corresponds with an increase in reduction power. The order of reducing power is $(ii) Mn^{2+} < Cl^{-}< Cr^{+3} < Cr$

Question 11: Use the data given in Q.8 and find out the most stable ion in its reduced form.
$(i) \; Cl^{-}$
$(ii) \; Cr^{3+}$
$(iii) \; Cr$
$(iv)\; Mn^{2+}$

Answer:

The answer is option (iv). Due to a higher $E^{o}$ value, $Mn^{2+}$ is most stable in its reduced form.

Question 12: Use the data of Q.8 and find out the most stable oxidized species.
$(i) \; Cr^{3+}$
$(ii) \; MnO^{-}_{4}$
$(iii)\; Cr_{2}O_{7}^{2-}$
$(iv) \; Mn^{2+}$

Answer:

The answer is option (i). Due to the lowest $E^{o}$value, $Cr^{3+}$ is most stable in its oxidized form.

Question 13: The quantity of charge required to obtain one mole of aluminium from $Al_{2}O_{3}$ is ___________.
(i) 1F
(ii) 6F
(iii) 3F
(iv) 2F

Answer:

The answer is the option (iii) $Al_{2}O_{3}\rightarrow 2Al^{3+}+ 3O^{2-}$
$Al^{3+}+3e^{-} \rightarrow Al$ (for 1 mole)
To obtain 1 mole $Al$ from $Al_{2}O_{3}$, we require 3F charge

Question 14: The cell constant of a conductivity cell _____________.
(i) changes with the change of electrolyte.
(ii) changes with the change of concentration of electrolyte.
(iii) changes with the temperature of the electrolyte.
(iv) remains constant for a cell.

Answer:

The answer is option (iv). The cell constant of a conductivity cell remains constant for a cell.

Question 15: While charging the lead storage battery ______________.
(i) $PbSO_{4}$ anode is reduced to $Pb$
(ii) $PbSO_{4}$ cathode is reduced to $Pb$
(iii) $PbSO_{4}$ cathode is oxidised to $Pb$
(iv) $PbSO_{4}$ anode is oxidised to $PbO_{2}$

Answer:

The answer is the option (i)
Reduction at Anode: $PbSO_4 (s)+2e^-\rightarrow Pb(s)+SO_{4}^{2-} (aq) (Reduction)$
Oxidation at Cathode: $PbSO_4 (s)+2H_2 O\rightarrow PbO_2 (s)+SO_{4}^{2-}+4H^++2e^-$
Overall reaction : $2PbSO_4 (s)+2H_2 O\rightarrow Pb(s)+PbO_2 (s)+4H^++2SO_{4}^{2-}$

Question 16: $\wedge ^{0}_{m(NH_{4}OH)}$ is equal to ______________.
$(i) \wedge ^{0}_{m(NH_{4}OH)} + \wedge ^{0}_{m(NH_{4}Cl)}- \wedge ^{0}_{(HCl)}$
$(ii) \wedge ^{0}_{m(NH_{4}Cl)} + \wedge ^{0}_{m(NaOH)}- \wedge ^{0}_{(NaCl)}$
$(iii) \wedge ^{0}_{m(NH_{4}Cl)} + \wedge ^{0}_{m(NaCl)}- \wedge ^{0}_{(NaOH)}$
$(iv) \wedge ^{0}_{m(NaOH)} + \wedge ^{0}_{m(NaCl)}- \wedge ^{0}_{(NH_{4}Cl)}$

Answer:

The answer is the option (ii) $NH_{4}Cl\rightleftharpoons NH_4^++Cl^-$
$NaCl\rightleftharpoons Na^{+}+Cl^{-}$
$NaOH\rightleftharpoons Na^{+}+OH^{-}$
$NH_{4}OH\rightleftharpoons NH_{4}^{+}+OH^{-}$

So
$\wedge ^{0}_{m(NH_{4}Cl)} + \wedge ^{0}_{m(NaOH)}- \wedge ^{0}_{(NaCl)}$

Question 17: In the electrolysis of aqueous sodium chloride solution which of the half-cell reactions will occur at the anode?

$(i) Na^{+}(aq) + e^{-} \rightarrow Na (s); E^{o}_{Cell} = -2.71V$

$(ii) 2H_{2}O (l) \rightarrow O_{2}(g) + 4H^+(aq) + 4e^{-} ; E^{o}_{Cell} = 1.23V$

$(iii) H^{+}(aq) + e^{-}\rightarrow \frac{1}{2}H_{2} (g); E^{o}_{Cell} = 0.00V$

$(iv) Cl^{-}(aq) \rightarrow \frac{1}{2} Cl_{2}(g) + e^{-} ; E^{o}_{Cell} = 1.36V$

Answer:

The answer is option (ii). Upon electrolysis, H₂O gives-

$2H_2 O\rightarrow O_2+4H^++4e^-;E_{cell}^{0}=1.23V$

Since, $E_{cell}^{0}=1.23V$. Hence, this half-cell reaction occurs at the anode.

Question 19: $E^{\Theta }_{cell}$ for some half cell reactions are given below. Based on these mark the correct answer.
$(i) H^{+}(aq) + e^{-} \rightarrow \frac{1}{2}H_{2}(g) ; E^{o}_{cell} = 0.00V$
$(ii) 2H_{2}O (l) \rightarrow O_{2} (g) + 4H^{+}(aq) + 4e^{-}; E^{o}_{cell} = 1.23V$
$(iii) 2SO_{4}^{2-} (aq) \rightarrow S_{2}O_{8}^{2-} (aq) + 2e^{-} ; E^{o}_{cell} = 1.96V$

(i) In dilute sulphuric acid solution, hydrogen will be reduced at the cathode.
(ii) In concentrated sulphuric acid solution, water will be oxidized at the anode.
(iii) In dilute sulphuric acid solution, water will be oxidized at the anode.
(iv) In dilute sulphuric acid solution, $SO_{4}^{2-}$ ion will be oxidized to tetrathionate ion at the anode.

Answer:

The answer is the option (ii, iii) In dilute sulphuric acid solution -
Reduction at cathode: $H^++e^-\rightarrow \frac{1}{2} H_2$
Oxidation at anode : $2H_2 O \rightarrow O_2+4H^++4e^-$
In concentrated sulphuric acid solution, sulphate $(SO{_{4}}^{2-})$ ions oxidize to form tetrathionate $(S_{2}O{_{8}}^{2-})$ ions.

Question 20: $E^{\Theta }_{Cell} = 1.1V$ for Daniel cell. Which of the following expressions are correct description of state of equilibrium in this cell?
$(i) \; 1.1 = K_{c}$
$(ii) \;\frac{2.303RT}{2 F} logK_{c} = 1.1$
$(iii)\; log K_{c} = 2.2/0.059$
$(iv) log K_{c} = 1.1$

Answer:

The answer is option (ii,iii)
$E_{cell}^{0}=\frac{2.303\; RT}{nF} logK_{c}$

$=\frac{0.059}{2}logK_{c}$

$1.1=\frac{0.059}{2}logK_{c}$

$logK_{c}=\frac{2.2}{0.059}$

Question 21: The conductivity of an electrolytic solution depends on ____________.
(i) nature of electrolyte.
(ii) the concentration of electrolyte.
(iii) power of the AC source.
(iv) distance between the electrodes.

Answer:

The answer is the option (i, ii). Mobile ions are responsible for the conductivity of electrolyte solution and this phenomenon is termed ionic conductance. It depends only on the following factors–

1. The nature of the electrolyte added

2. size of the ion produced and its solvation

3. concentration of electrolyte

4. Nature of the solvent and its viscosity

5. temperature

Question 22: $\wedge _{m(H_{2}O)}^{0}$ is equal to ______________.
$(i) \wedge_{m(HCl)}^{0} + \wedge _{m(NaOH)}^{0} - \wedge_{m(NaCl)}^{0}$
$(ii) \wedge_{m(HNO_{3})}^{0} + \wedge _{m(NaNO_{3})}^{0} - \wedge_{m(NaOH)}^{0}$
$(iii) \wedge_{m(HNO_{3})}^{0} + \wedge _{m(NaOH)}^{0} - \wedge_{m(NaNO_{3})}^{0}$
$(iv) \wedge_{m(NH_{4}OH)}^{0} + \wedge _{m(HCl)}^{0} - \wedge_{m(NH_{4}Cl)}^{0}$

Answer:

The answer is option (i,iv)
$\wedge _{m(H_{2}O)}^{0}=\wedge _{m(HCl)}^{0}+\wedge _{m(NaOH)}^{0}-\wedge _{m(NaCl)}^{0}$
$\wedge_{m(NH_{4}OH)}^{0} + \wedge _{m(HCl)}^{0} - \wedge_{m(NH_{4}Cl)}^{0}= \wedge _{m(H_{2}O)}^{0}$

The molar conductivity of water is equal to the sum of the molar conductivities of constituent ions. However, $NH_{4}OH$ doesn’t undergo complete decomposition as it is a weak electrolyte.

Question 23: What will happen during the electrolysis of an aqueous solution of $CuSO_{4}$ by using platinum electrodes?
(i) Copper will deposit at the cathode.
(ii) Copper will deposit at the anode.
(iii) Oxygen will be released at the anode.
(iv) Copper will dissolve at the anode.

Answer:

The answer is option (i,iii)
$CuSO_4\rightleftharpoons Cu^{2+}+SO_{4}^{2-}$
$H_{2}O\rightleftharpoons H^{+}+OH^{-}$
At cathode :

$Cu^{2+}+2e^{-}\rightarrow Cu; E_{cell}^{0}=0.34 \; V$
$H^{+}+e^{-}\rightarrow\; \frac{1}{2} H_{2}; E_{cell}^{0}=0.00 \; V$
At cathode, the reaction with higher $E^{0}$ is preferred
At anode :

$2SO_{4}^{2-}-2e^{-}\rightarrow S_{2}O_{8}^{2-}; E_{cell}^{0}=1.96 V$
$2H_{2}O\rightarrow O_{2}+4H^{+}+4e^{-};E_{cell}^{0}=1.23V$
At the anode, the reaction with lower $E^{o}$ is preferred.

Question 24: What will happen during the electrolysis of an aqueous solution of $CuSO_{4}$ in the presence of $Cu$ electrodes?
(i) Copper will deposit at the cathode.
(ii) Copper will dissolve at the anode.
(iii) Oxygen will be released at the anode.
(iv) Copper will deposit at the anode.

Answer:

The answer is option (i,ii)
At Cathode :

$Cu^{2+}+2e^{-}\rightarrow Cu(s)$
At anode :

$Cu(s)\rightarrow Cu^{2+}+2e^{-}$
The cathode will witness deposition of $Cu$ and the Anode will witness its dissolution

Question 25: Conductivity κ , is equal to ____________.
$(i) \frac{1}{R} \frac{l}{A}$

$(ii) \frac{G^{*}}{R}$

$(iii) \wedge_{m}$

$(iv) \frac{l}{A}$

Answer:

The answer is option (i,ii)

$(i) \frac{1}{R} \frac{l}{A}$

$(ii) \frac{G^{*}}{R}$

Question 26: Molar conductivity of ionic solution depends on ___________.
(i) temperature.
(ii) distance between electrodes.
(iii) the concentration of electrolytes in solution.
(iv) the surface area of electrodes.

Answer:

The answer is option (i, iii). Temperature and concentration of electrolytes determine the molar conductivity of an ionic solution

Question 27: For the given cell, $Mg\left | Mg^{2+} \right |\left | Cu^{2+} \right |Cu$
(i) $Mg$ is cathode
(ii) $Cu$ is cathode
(iii) The cell reaction is $Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu$
(iv) Cu is the oxidizing agent

Answer:

The answer is the option (ii, iii). At the cathode, Cu is reduced and at the anode, Mg is oxidized.
(ii) $Cu$ is cathode
(iii) The cell reaction is $Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu$

Question 29: Can $E^{\Theta }_{cell}$ or $\Delta _{r}G^{\Theta }$ for cell reaction ever be equal to zero?

Answer:

$E^{o}$and $\Delta G^{o}$ can never be zero for a cell reaction.

Question 30: Under what condition is $E_{cell} = 0$ or $\Delta _{r}G= 0 \;?$

Answer:

Upon completely discharging or at equilibrium, $E_{cell} = 0$ and $\Delta _{r}G= 0$.

Question 31: What does the negative sign in the expression $E^{\Theta }_{Zn^{2+}/Zn}=-0.76\; V$ mean ?

Answer:

$E^{\Theta }_{Zn^{2+}/Zn}=-0.76\; V$
Zinc has a higher reducing power than Hydrogen it has a negative $E^{0}$ while it is zero for Hydrogen
$Zn+H_{2}SO_{4}\rightarrow ZnSO_{4}+H_{2}$

Question 32: Aqueous copper sulfate solution and aqueous silver nitrate solution are electrolyzed by 1-ampere current for 10 minutes in separate electrolytic cells. Will, the mass of copper and silver, deposited on the cathode be the same or different? Explain your answer.

Answer:

According to Faraday's second law of electrolysis amount of different substances liberated by the same quantity or electricity that passes through an electrolyte solution is directly proportional to their chemical equivalent weight.
W₁/W₂=E₁/E₂
where E₁ and E₂ have different values depending on a number of electrons required to reduce the metal ion.
Hence, the masses of Cu and Ag deposited will be different.

Question 33: Depict the galvanic cell in which the cell reaction is $Cu + 2Ag^+\rightarrow 2Ag + Cu^{2+}$

Answer:

$Cu\left | Cu^{2+} \right |\left | Ag^{+} \right |Ag$

Question 34: Value of standard electrode potential for the oxidation of $Cl^{-}$ ions is more positive than that of water, even then in the electrolysis of aqueous sodium chloride, why is $Cl^{-}$ oxidized at anode instead of water?

Answer:

Under the conditions of electrolysis of aqueous sodium chloride, oxidation of water at the anode requires overpotential hence Cl^- is oxidized instead of water

Question 35: What is electrode potential?

Answer:

The electrical potential difference set up between the metal and its solution is called the electrode potential.

Question 36: Consider the following diagram in which an electrochemical cell is coupled to an electrolytic cell. What will be the polarity of electrodes ‘A’ and ‘B’ in the electrolytic cell?


Answer:

‘A’ has negative and ‘B’ has positive polarity.

Question 37: Why is alternating current used for measuring the resistance of an electrolytic solution?

Answer:

Direct current will lead to electrolysis of the solution, which will change the concentration of ions in the solution. The use of an Alternating current will prevent this from happening.

Question 38: A galvanic cell has an electrical potential of 1.1V. If an opposing potential of 1.1V is applied to this cell, what will happen to the cell reaction and current flowing through the cell?

Answer:

Cell reaction ceases if the opposing potential is equal to the electrical potential.

Question 39: How will the pH of brine (aq. NaCl solution) be affected when it is electrolyzed?

Answer:

Upon electrolysis of Brine solution, the following reactions take place:
Cathode :$2H^{+}+2e^{-}\rightarrow H_{2}$
Anode : $2Cl^{-}\rightarrow Cl_{2}+2e^{-}$
The remaining $Na^{+}$ and $OH^{-}$ ions are responsible for turning the solution basic and thus increasing the pH.

Question 40: Unlike dry cells, the mercury cell has a constant cell potential throughout its useful life. Why?

Answer:

The mercury cell has a constant cell potential throughout its useful life because the Ions are not involved in the overall cell reaction of mercury cells.

Question 41: Solutions of two electrolytes ‘A’ and ‘B are diluted. The $\Lambda _{m}$ of ‘B’ increases 1.5 times while that of A increases 25 times. Which of the two is a strong electrolyte? Justify your answer.

Answer:

The lower increase of $\wedge _{m}$ for Electrolyte B is due to the complete ionization of the same. Hence, It is the stronger electrolyte.

Question 42: When acidulated water (dil.$H_{2}SO_{4}$ solution) is electrolyzed, will the pH of the solution be affected? Justify your answer.

Answer:

The following reaction occurs:
At Anode : $2H_{2}O\rightarrow O_{2}+4H^{+}+4e^{-}$
At cathode : $4H^{+}+4e^{-}\rightarrow 2H_{2}(\uparrow)$
As the concentration of H⁺ ions is maintained, there will be no change in pH.

Question 43: In an aqueous solution, how does the specific conductivity of electrolytes change with the addition of water?

Answer:

The concentration of ions will decrease in the addition of water, which in turn will reduce the electrical conductivity.

Question 44: Which reference electrode is used to measure the electrode potential of other electrodes?

Answer:

The standard hydrogen electrode is used as the reference electrode. For other electrodes, we measure the electrode potential considering the electrode potential for standard hydrogen electrodes to be zero.

Question 45: Consider a cell given below
$Cu\left | Cu^{2+} \right |\left | Cl^{-} \right |Cl_{2},Pt$
Write the reactions that occur at the anode and cathode.

Answer:

The given cell is :
$Cu (s)\left | Cu^{2+} \right |\left | Cl^{-} \right |Cl_{2}(pt)$
Anode : $Cu (s)\rightarrow Cu^{2+}+2e^{-}$
Cathode : $Cl_{2}(g)+2e^{-}\rightarrow 2Cl^{-}$

Question 46: Write the Nernst equation for the cell reaction in the Daniel cell. How will the $E_{\text {cell }}$ be affected when the concentration of $Zn^{2+}$ ions is increased?

Answer:

Daniel Cell :
$Zn\; (s)\left |Zn^{2+} \right |\left | Cu^{2+} \right |Cu\; (s)$
Anode :

$Zn(s)\rightarrow Zn^{2+}+2e^{-}$
Cathode :

$Cu^{2+}+2e^{-}\rightarrow Cu (s)$
Overall cell reaction :

$Zn (s)+Cu^{2+}\rightleftharpoons Zn^{2+}+Cu (s)$
$Q=\frac{[Zn^{2+}]}{[Cu^{2+}]}$
$E_{cell}=E_{cell}^{0}-\frac{0.059}{2}log \; \frac{[Zn^{2+}]}{[Cu^{2+}]}$
Therefore, with increasing concentration of $Zn^{2+}$, the cell potential decreases.

Question 47: What advantage do the fuel cells have over primary and secondary batteries?

Answer:

Primary batteries come with a limited number of reactants and can’t be reused once they are discharged. Secondary batteries take a considerable amount of time to recharge. Fuel cells is superior to both primary and secondary batteries as they operate without any breaks as long as you keep supplying the reactants

Question 48: Write the cell reaction of a lead storage battery when it is discharged. How does the density of the electrolyte change when the battery is discharged?

Answer:

During Discharge: $Pb+PbO_2+2H_2 SO_4\rightarrow 2PbSO_4+2H_2 O$
As water is formed during the discharge process, the concentration of electrolyte reduces.

Question 49: Why on dilution the $\wedge _{m}$ of $CH_{3}COOH$ increases drastically, while that of $CH_{3}COONa$ increases gradually?

Answer:

For weak electrolytes like $CH_{3}COOH,$ on dilution, the concentration of ions increases due to the increase in the degree of dissociation, but for strong electrolytes like $CH_{3}COONa$, the number of ions remains constant upon dilution

Question 51: Match the terms given in Column I with the items given in Column II.

Answer:

$(\mathrm{i} \rightarrow \mathrm{d}),(\mathrm{ii} \rightarrow \mathrm{a}),(\mathrm{iii} \rightarrow \mathrm{b}),(\mathrm{iv} \rightarrow \mathrm{c})$

Question 52: Match the items of Column I and Column II.

Answer:

$(\mathrm{i} \rightarrow \mathrm{d}),(\mathrm{ii} \rightarrow \mathrm{c}),(\mathrm{iii} \rightarrow \mathrm{a}),(\mathrm{iv} \rightarrow \mathrm{b})$

Question 53: Match the items of Column I and Column II.

Answer:

$(\mathrm{i} \rightarrow \mathrm{d}),(\mathrm{ii} \rightarrow \mathrm{c}),(\mathrm{iii} \rightarrow \mathrm{b}),(\mathrm{iv} \rightarrow \mathrm{a})$

Question 54: Match the items of Column I and Column II.

Answer:

$(\mathrm{i} \rightarrow \mathrm{d}),(\mathrm{ii} \rightarrow \mathrm{c}),(\mathrm{iii} \rightarrow \mathrm{a,e}),(\mathrm{iv} \rightarrow \mathrm{b})$

Question 55: Match the items of Column I and Column II on the basis of data given below:
$E_{F_{2}/F^{-}}^{\circleddash}=2.87\; V,$ $E_{Li^{+}/Li}^{\circleddash}=-3.5\; V,$ $E_{Au^{3+}/Au}^{\circleddash}=1.4\; V,$ $E_{Br_{2}/Br^{-}}^{\circleddash}=1.09\; V,$

Answer:

$(\mathrm{i} \rightarrow \mathrm{c}),(\mathrm{ii} \rightarrow \mathrm{a}),(\mathrm{iii} \rightarrow \mathrm{g}),(\mathrm{iv} \rightarrow \mathrm{e}),(\mathrm{v} \rightarrow \mathrm{d}),(\mathrm{vi} \rightarrow \mathrm{b}), (\mathrm{vii} \rightarrow \mathrm{g,f})$

Question 57: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion:$E_{Cell}$ should have a positive value for the cell to function.

Reason:$E_{cathode}<E_{anode}$

(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false, but the Reason is true.

Answer:

The answer is the option (iii) $E_{cell}=E_{cathode}-E_{anode}$. To have a positive value of $E_{cell}, E_{cathode}>E_{anode}$

Question 58: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: Conductivity of all electrolytes decreases on dilution.
Reason: On dilution number of ions per unit volume decreases.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true, but the Reason is false.
(iv) Both Assertion and Reason are false.
(v)The Assertion is false, but the Reason is true.

Answer:

The answer is option (I). Upon dilution, the concentration of ions decreases, and hence, conductivity also decreases.

Question 59: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: $\wedge _{m}$ for weak electrolytes shows a sharp increase when the electrolytic solution is diluted.
Reason: For weak electrolytes degree of dissociation increases with a dilution of the solution.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not correct explanation for the Assertion
(iii) The assertion is true, but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false, but the Reason is true.

Answer:

The answer is option (I). Degree of dissociation of weak electrolytes increases on dilution, which results in a sharp increase in $\wedge _{m}$ values.

Question 60: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: Mercury cell does not give steady potential.
Reason: In the cell reaction, ions are not involved in the solution.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true, but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false butthe Reason is true.

Answer:

The answer is option (v). Mercury cell maintains a constant cell potential because the electrolyte isn’t consumed in the cell process

Question 61: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: Electrolysis of $NaCl$ solution gives chlorine at anode instead of $O_{2}$.
Reason: Formation of oxygen at the anode requires overvoltage.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true, and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false but Reason is true.

Answer:

The answer is the option (i). Though the $E^{o}$ value for the formation of oxygen is lower than that for the formation of chlorine, it is not formed because it requires overvoltage.

Question 62: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: For measuring the resistance of an ionic solution an AC source is used.
Reason: The concentration of ionic solution will change if a DC source is used.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false but the Reason is true.

Answer:

The answer is option (i). Direct current will lead to electrolysis of the solution, which will change the concentration of ions in the solution. Use of an Alternating current will prevent this from happening

Question 63: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: Current stops flowing when $E_{cell}=0.$
Reason: Equilibrium of the cell reaction is attained.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for the Assertion
(iii) The assertion is true, but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false, but Reason is true.

Answer:

The answer is the option (i) At equilibrium, $E_{cell}=0$ and no current flows.

Question 64: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion:$E_{Ag^{+}/Ag}$ increases with increase in concentration of $Ag^{+}$ ions.
Reason:$E_{Ag^{+}/Ag}$ has a positive value.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
(ii) Both Assertion and Reason are true and the Reason is not correct explanation for the Assertion
(iii) The assertion is true but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false but Reason is true.

Answer:

The answer is option (ii) Both Assertion and Reason are true and the Reason is not the correct explanation for Assertion

Question 65: In the following question, a statement of Assertion (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion: Copper sulfate can be stored in a zinc vessel.
Reason: Zinc is less reactive than copper.
(i) Both Assertion and Reason are true and the Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are true and the Reason is not the correct explanation for Assertion
(iii) Assertion is true but the Reason is false.
(iv) Both Assertion and Reason are false.
(v) Assertion is false but Reason is true.

Answer:

The answer is option (iv) As Zinc is more reactive than Copper, Zinc dissolves in $CuSO_{4}$ solution.

Question 67: Consider the figure given below and answer the questions (i) to (vi) that follow.

(i) Redraw the diagram to show the direction of electron flow.
(ii) Is silver plate anode or cathode?
(iii) What will happen if salt bridge is removed?
(iv) When will the cell stop functioning?
(v) How will concentration of $Zn^{+2}$ ions and $Ag^{+}$ ions be affected when the cell functions ?
(vi) How will the concentration of $Zn^{+2}$ ions and $Ag^{+}$ ions be affected after the cell becomes ‘dead’?

Answer:

(i) The cell reaction can be summarised as:
$Zn(s)\left | Zn^{+2} \right |\left | Ag^{+} \right |Ag$
Electrons move from Zn to Ag.
(ii) Due to a higher standard reduction potential, Silver will act as a Cathode and in the external circuit, electrons will flow from the zinc anode to the silver cathode.
(iii) Removal of the salt bridge will lead to a sudden drop in the potential to zero.
(iv) If the potential reaches zero (or the cell is discharged), all reactions will cease and the cell will stop functioning.
(v) Nernst equation for the cell is: 0.059,
$E=E^{0}-\frac{0.059}{2}log\frac{[Zn^{2+}]}{[Ag^{+}]^{2}}$
With the increase in the concentration of $[Zn^{+2}]$, cell potential will decrease, and with an increase in the concentration of $[Ag^{+}]$, cell potential will increase.

(vi) At equilibrium (discharged state, potential drop to zero), the concentration of $[Zn^{+2}]$ and $[Ag^{+}]$ will not change.

Question 68: What is the relationship between Gibbs free energy of the cell reaction in a galvanic cell and the emf of the cell? When will the maximum work be obtained from a galvanic cell?

Answer:

The required relationship between Gibbs free energy and the emf in a galvanic cell is $\Delta G=-nFE$
The maximum work will be obtained from a galvanic cell $W_{max}=nFE^{0}$ where,
E=cell potential
$E^{0}$ = Standard emf of the cell