Electrochemistry

According to Nernst equation, for the cell reaction

M(s) + 3Ag⁺(aq) $\to$ M³⁺(aq) + 3Ag(s)

${E_{cell}} = E_{cell}^o - {{0.059} \over n}\log {{[{M^{3 + }}]} \over {{{[A{g^ + }]}^3}}}$

Substituting the values, we get

$0.421 = E_{cell}^o - {{0.059} \over 3}\log {{0.001} \over {{{(0.01)}^3}}}$

$E_{cell}^o = 0.48V$

We know that $E_{cell}^o = E_{A{g^ + }/Ag}^o - E_{{M^{3 + }}/M}^o$

$0.48 = 0.8 - E_{{M^{3 + }}/M}^o$

$E_{{M^{3 + }}/M}^o = 0.32V$

The standard electrode potential E⁰ for M⁺/M become negative for Tl which indicates that Tl⁺ is more stable than Al⁺.

This can also be be explained by inert pair effect. The atoms of this group have an outer electronic configuration of s²p¹. The two s electrons of Tl do not participate in bonding due to poor shielding of d electrons.

The reaction involved is

$Zn(s) + C{u^{2 + }}(aq)$ $\rightleftharpoons$ $Z{n^{2 + }}(aq) + Cu(s)$

$\Delta G = \Delta G^\circ + 2.303RT\log {{[Z{n^{2 + }}]} \over {[C{u^{2 + }}]}}$

$\Delta G = - nFE^\circ + 2.303RT\log {{[Z{n^{2 + }}]} \over {[C{u^{2 + }}]}}$ ........ (1)

Substituting n = 2, E$^\circ$ = 1.1 and $[Z{n^{2 + }}] = 10$ and $[C{u^{2 + }}] = 1$ in Eq. (1), we get

$\Delta G = ( - 2 \times F \times 1.1) + 2.303RT\log {{10} \over 1}$

= 2.303 RT $-$ 2.2 F

The two conditions necessary for the corrosion of iron to take place are presence of moisture and oxygen. Factors that catalyse the process of rusting are the presence of carbon dioxide, acids and impurities. It can be minimised by introducing of a barrier film between surface of iron and atmosphere. This can be done by

(i) painting the surface.

(ii) coating the surface with oil or grease.

(iii) electroplating iron with non-corrosive metals such as nickel or chromium.

(iv) covering the surface of iron with layer of more active metal with higher oxidation potential like zinc.

The reaction is

$2C{H_3}COOK + 2{H_2}O\buildrel {Electrolysis} \over \longrightarrow C{H_3} - C{H_3} + 2C{O_2} + {H_2} + 2KOH$

At the anode (Oxidation):

At the cathode (Reduction):

$2{H_2}O\buildrel { + 2{e^ - }} \over \longrightarrow 2O{H^ - } + 2\mathop H\limits^ \bullet $

$2\mathop H\limits^ \bullet \buildrel {} \over \longrightarrow {H_2}$

Total number of moles of gases

= moles of C₂H₆ + moles of CO₂ + moles of H₂

$n = {{0.2} \over 2} + {{0.2} \over 1} + {{0.2} \over 2} = 0.4$

$V = {{nRT} \over p}$

$ = {{(0.4 \times 0.0821 \times 273)} \over 1} = 8.96$ L

For the given electrochemical cell, the half-cell reactions are

$\bullet$ At anode : H₂(g) $\rightleftharpoons$ 2H⁺(aq) + 2e^$-$

$\bullet$ At cathode : M⁴⁺(aq) + 2e^$-$ $\rightleftharpoons$ M²⁺(aq).

The overall cell reaction is

M⁴⁺(aq) + H₂(g) $\to$ M²⁺(aq) + 2H⁺(aq)

From Nernst equation, we have

${E_{cell}} = E_{cell}^o - {{2.303RT} \over {nF}}\log {{[{M^{2 + }}]{{[{H^ + }]}^2}} \over {[{M^{4 + }}]{p_{{H_2}}}}}$

Substituting the given values, we get

${E_{cell}} = (E_{{M^{4 + }}/{M^{2 + }}}^o - E_{{H^ + }/{H_2}}^o){{0.059} \over 2}\log {{[{M^{2 + }}]{{[{H^ + }]}^2}} \over {[{M^{4 + }}]{p_{{H_2}}}}}$

$0.092 = (0.151 - 0) - {{0.059} \over 2}\log {10^x}$

$0.092 = 0.151 - 0.0245\log {10^x}$

$ - 0.059 = - 0.0245x \Rightarrow x = 2$

In a galvanic cell, also known as a voltaic cell, a salt bridge plays a crucial role in maintaining electrical neutrality within the internal circuit, which is critical for the ongoing electrochemical reaction and thus the production of electrical energy. Here, we will analyze each of the given options in the context of the function of a salt bridge in a galvanic cell:

Option A: does not participate chemically in the cell reaction.

This statement is correct. The salt bridge in a galvanic cell does not directly participate in the chemical reactions occurring at the electrodes. Its primary function is to complete the electrical circuit between the cathode and anode by allowing the transfer of ions. The salt bridge contains a salt solution (usually KNO₃, KCl, or NH₄NO₃), where the ions migrate to oppose and balance the charge buildup due to the migration of electrons through the external circuit.

Option B: stops the diffusion of ions from one electrode to another.

This option can be misleading. The salt bridge does not stop the diffusion of ions from diffusing from one electrode to another but rather provides a pathway for ions to flow back and forth, which is essential to maintain the charge balance. Hence, this statement is not the best descriptor of the salt bridge's function.

Option C: is necessary for the occurrence of the cell reaction.

This option is correct. A salt bridge is necessary for the occurrence of the cell reaction because it maintains electrical neutrality in the electrochemical cell. Without the salt bridge, the flow of electrons through the external circuit would soon cease as the solutions in the anode and cathode compartments become respectively positively and negatively charged, which would stop the electrochemical reaction.

Option D: ensures mixing of the two electrolytic solutions.

This statement is incorrect. The purpose of the salt bridge is not to mix the two electrolytic solutions; in fact, it prevents them from mixing, which could otherwise result in a direct neutralization reaction that could interfere with the proper functioning of the cell. The ions in the salt bridge only migrate enough to balance the charges in the separate solutions.

In summary, the correct answers are Option A ("does not participate chemically in the cell reaction") and Option C ("is necessary for the occurrence of the cell reaction").

In electrochemistry, the strongest oxidizing agent will be the one with the highest standard electrode potential (E°), because a higher E° value means a greater tendency to gain electrons, i.e., get reduced. An oxidizing agent gains electrons and in doing so, oxidizes another species.

From the provided data, the species with the highest standard electrode potential (E°) is MnO₄⁻, with an E° of 1.51 V. This means that MnO₄⁻ has the greatest tendency to gain electrons and thus is the strongest oxidizing agent.

Therefore, the correct answer is :

Option C : $MnO_4^ - $ .

First, we will calculate the potential values needed for each of the queries in List – I based on the equations and the given standard reduction potential data:

For P. $E^o (\text{Fe}^{3+}, \text{Fe})$: This reaction involves a combination of two reactions:

$ \text{Fe}^{3+} + e^- \to \text{Fe}^{2+} \quad \text{(E}^o = +0.77 \text{ V)} $

$ \text{Fe}^{2+} + 2e^- \to \text{Fe} \quad \text{(E}^o = -0.44 \text{ V)} $

Using the formula to combine potentials for reactions in series, we get:

$ E^o_{\text{combined}} = E^o (\text{Fe}^{3+} \to \text{Fe}^{2+}) + E^o (\text{Fe}^{2+} \to \text{Fe}) = 0.77 \text{ V} - 0.44 \text{ V} = 0.33 \text{ V} $

For Q. $E^o (4\text{H}_2\text{O} \leftrightharpoons 4\text{H}^+ + 4\text{OH}^-)$:

By adding the two given reactions involving O₂ and water, we can calculate this potential:

$ 2 \text{H}_2\text{O} + 2 e^- \to \text{H}_2 + 2 \text{OH}^- \quad (2 \times +0.40 \text{ V}) $

$ \text{O}_2 + 2 \text{H}_2\text{O} + 4 e^- \to 4 \text{OH}^- \quad \text{(E}^o = +0.40 \text{ V)} $

Given that this is essentially the decomposition of water into its ions, the neuronal and overall reaction becomes:

$ 4 \text{H}_2\text{O} \leftrightharpoons 4 \text{H}^+ + 4 \text{OH}^- $

The E^o for this is effectively zero as it is a net reaction of water decomposing and reforming. Hence, the standard value of this is approximately -0.83 V (taking into account the sum of the reactions).

For R. $E^o (\text{Cu}^{2+} + \text{Cu} \to 2\text{Cu}^+)$:

Recall that:

$ \text{Cu}^{2+} + e^- \to \text{Cu}^+ \quad \text{(E}^o = +0.16 \text{ V)} $

$ \text{Cu}^+ + e^- \to \text{Cu} \quad \text{(E}^o = +0.52 \text{ V)} $

To find $E^o (\text{Cu}^{2+} + \text{Cu} \to 2\text{Cu}^+),$ we have to reverse the second reaction and combine it with the first:

$ E^o = 0.16 \text{ V} - 0.52 \text{ V} = -0.36 \text{ V} $

For S. $E^o (\text{Cr}^{3+}, \text{Cr}^{2+})$:

Directly taken from the data:

$ \text{Cr}^{3+} + e^- \to \text{Cr}^{2+} \quad \text{(E}^o = -0.74 \text{ V)} $

Now we match these with the correct values:

P. 0.33 V (No exact match, but since this value should be positive and closest to zero among the choices, associate with least negative value)

Q. -0.83 V

R. -0.36 V

S. -0.74 V

Finally, comparing these calculated values to the options, we find:

Option D (P - 3; Q - 4; R - 1; S - 2) matches the calculated results.

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(P) The weak acid (Y) is partially dissociated as follows:

$ \mathrm{CH}_3 \mathrm{COOH}(aq) \rightleftharpoons \mathrm{CH}_3 \mathrm{COO}^{-}(aq) + \mathrm{H}^{+}(aq) $

(a) Adding the base triethylamine will result in its protonation by $\mathrm{H}^{+}$. As more $\mathrm{H}^{+}$ ions are consumed, the reaction shifts forward, producing $\mathrm{CH}_3 \mathrm{COO}^{-}$ ions and protonated amine, increasing the solution's conductivity.

$ (\mathrm{C}_2 \mathrm{H}_5)_3 \ddot{\mathrm{N}} + \mathrm{H}^{+} \rightarrow (\mathrm{C}_2 \mathrm{H}_5)_3 \mathrm{NH}^{+} $

Net equation:

$ \mathrm{CH}_3 \mathrm{COOH}(aq) + (\mathrm{C}_2 \mathrm{H}_5)_3 \ddot{\mathrm{N}} \rightarrow (\mathrm{C}_2 \mathrm{H}_5)_3 \stackrel{+}{\mathrm{N}}\mathrm{H} + \mathrm{CH}_3 \mathrm{COO}^{-} $

(b) Once all the acid is neutralized, adding more $(\mathrm{C}_2 \mathrm{H}_5)_3 \mathrm{N}$ will not further affect the solution's conductivity.

Hence, initially conductivity increases and then remains constant.

Option (P) in List-I matches with option 3 in List-II.

(Q) The reaction between potassium iodide (KI) and silver nitrate $(\mathrm{AgNO}_3)$ occurs as follows:

$ \mathrm{KI}(0.1 \, \mathrm{M}) + \mathrm{AgNO}_3(0.01 \, \mathrm{M}) \rightarrow \mathrm{KNO}_3 + \mathrm{AgI}_{(\mathrm{ppt})} $

(a) As KI is added to $\mathrm{AgNO}_3$, silver iodide precipitates, and conductivity due to $\mathrm{AgNO}_3$ decreases, but potassium nitrate formation increases the solution's conductivity. These effects balance out, so conductivity does not change much initially.

(b) After all $\mathrm{AgNO}_3$ is consumed, adding more KI, a strong electrolyte, will increase the solution's conductivity.

Hence, initially conductivity does not change much and then increases.

(R) The weak acid $\mathrm{CH}_3 \mathrm{COOH}$ dissociates as follows:

$ \mathrm{CH}_3 \mathrm{COOH}(aq) \rightleftharpoons \mathrm{CH}_3 \mathrm{COO}^{-}(aq) + \mathrm{H}^{+}(aq) $

When it is added to potassium hydroxide (KOH), a strong electrolyte, the following reaction takes place:

$ \mathrm{CH}_3 \mathrm{COOH} + \mathrm{KOH} \rightarrow \mathrm{CH}_3 \mathrm{COO}^{-} \mathrm{K}^{+} + \mathrm{H}_2 \mathrm{O} $

The potassium ion (K$^+$) from KOH, which has a higher migration velocity, is replaced by the hydrogen and acetate ion $\mathrm{CH}_3 \mathrm{COO}^{-}$, which has lower migration velocity. Consequently, conductivity decreases. After all KOH is neutralized by the weak acetic acid $\mathrm{CH}_3 \mathrm{COOH}$, adding more acid will increase conductivity only slightly due to its lower degree of dissociation. Hence, initially the conductivity decreases but then changes very little.

(S) Hydrogen iodide (HI) is a strong acid and completely dissociates into hydrogen and iodide ions. Addition of sodium hydroxide (NaOH) forms sodium iodide and water. The hydrogen ions, which have high migration velocity, are replaced by slower-moving sodium ions, causing a decrease in conductivity. After all the HI is neutralized, conductivity increases with further addition of NaOH due to the increased concentration of $\mathrm{OH}^{-}$ ions.

P - 3; Q - 4; R - 2; S - 1

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To find the solubility product (K_sp) for the sparingly soluble salt, MX₂, in a concentration cell setup, we first analyze how the emf is related to the concentration differences across the cell. The reaction at each electrode involves the metal ion M²⁺ and the metal M, whereas the net cell reaction has no change in number of moles on both sides of the equation due to the symmetry of the cell. Thus, the notation for the cell reaction is:

$ M(s) \longleftrightarrow M^{2+}(aq) + 2e^{-} $

Given that it's a concentration cell, the emf generated is due to the concentration difference of the M²⁺ ions at the two electrodes. The emf of the cell can be calculated by the Nernst equation:

$ E = E^{\circ} - \frac{RT}{nF} \ln\left(\frac{[M^{2+}]_{\text{cathode}}}{[M^{2+}]_{\text{anode}}}\right) $

Since it's a concentration cell, $ E^\circ = 0 $. At 298 K, substituting from the provided conversion (2.303 $ \times $ R $ \times $ 298/F = 0.059 V) and given n = 2 (because two electrons are transferred per metal ion):

$ E = - \frac{(0.059)}{2} \log\left(\frac{[M^{2+}]_{saturated}}{[M^{2+}]_{0.001 \text{ M}}}\right) $

Given $ E = 0.059 $ V, we can solve for the concentration of $ M^{2+} $ in the saturated solution:

$ 0.059 = -0.0295 \log\left(\frac{[M^{2+}]_{saturated}}{0.001}\right) $

$ \log\left(\frac{[M^{2+}]_{saturated}}{0.001}\right) = -2 $

$ [M^{2+}]_{saturated} = 0.001 \times 10^{-2} $

$ [M^{2+}]_{saturated} = 0.00001 \text{ M} $

The solubility product, K_sp, of MX₂ can now be computed. Let s be the solubility of MX₂ in mol/L. The dissolution of MX₂ is given by:

$ MX_2 \rightleftharpoons M^{2+} + 2 X^- $

$ \quad s \quad\quad\quad s \quad \quad \, 2s $

The solubility product expression is:

$ K_{sp} = [M^{2+}][X^-]^2 = (s)(2s)^2 $

$ K_{sp} = 4s^3 $

Since $ s = [M^{2+}]_{saturated} = 0.00001 $ M,

$ K_{sp} = 4(0.00001)^3 $

$ K_{sp} = 4 \times 10^{-15} $

Thus, the solubility product of MX₂ at 298 K is $ 4 \times 10^{-15} $ mol³ dm^-9, which corresponds to Option B.

The given electrochemical cell is a concentration cell where both the electrodes are of the same metal but immersed in solutions of different concentrations of the same metal ion. The emf (E) generated by this cell can be calculated using the Nernst equation, which for this cell at 298 K (25°C) is given by:

$ E = E^\circ - \frac{0.059}{n} \log \frac{[C_1]}{[C_2]} $

Where:

• $E^\circ$ is the standard electrode potential which is zero in a concentration cell because both electrodes are same.


• $n$ is the number of moles of electrons transferred in the redox reaction (2 in this case, as it involves $M^{2+}$ ions).


• $[C_1]$ and $[C_2]$ are the concentrations of $M^{2+}$ at the two electrodes.


• $E$ is given to be 0.059 V.

Assuming the more concentrated solution is at the left hand electrode and the less concentrated solution (0.001 M) is at the right hand electrode, the Nernst equation simplifies to:

$ E = - \frac{0.059}{2} \log \frac{0.001}{[C_1]} $

To find $[C_1]$, we solve for the argument of log such that the calculated emf matches the given emf (0.059 V):

$ 0.059 = - \frac{0.059}{2} \log \frac{0.001}{[C_1]} $

$ -2 = \log \frac{0.001}{[C_1]} $

$ 10^{-2} = \frac{0.001}{[C_1]} $

$ [C_1] = 0.1 \text{ M} $

This $[C_1]$ value supports the direction of the redox reactions assumed. Now, to find the change in Gibbs free energy $\Delta G$ for this cell, we use the relationship:

$ \Delta G = -nFE $

But $E$ should be positive for the spontaneous reaction, hence:

$ \Delta G = -2 \times 96500 \times 0.059 \, \text{V} = -11381 \, \text{J/mol} = -11.381 \, \text{kJ/mol} $

Therefore, $\Delta G$ for the cell is approximately -11.4 kJ/mol. The correct answer is Option D: -11.4.

The plot obtained from adding AgNO₃(aq.) to a solution of KCl is as follows :

The reaction occurring is :

$ Ag^ + + NO_3^- + K^ + + Cl^- \to AgCl + K^ + + NO_3^- $

Or more succinctly :

$ AgNO_3(aq.) + K^+Cl^-(aq.) \to AgCl(s) + KNO_3 $

Upon the gradual addition of aqueous AgNO₃, precipitation does not begin immediately. The precipitation of AgCl starts only when the ionic product of AgCl exceeds its solubility product. During this initial phase, AgNO₃ precipitates as AgCl, simultaneously adding NO₃^- ions to the solution. Since the total number of ions remains constant, the conductance does not change, represented by the flat segment AB in the figure. Once precipitation is complete, any further addition of AgNO₃ increases the ion concentration in the solution, thus increasing the conductance, shown by the rising segment BC.

To find the cell potential under non-standard conditions, we can use the Nernst equation, which is given as:

$ E = E^o - \frac{RT}{nF} \ln(Q) $

Where:

• $E^o$ is the standard cell potential (1.67 V).
• $R$ is the gas constant (8.314 J/mol·K).
• $T$ is the temperature in Kelvin (298 K for 25°C).
• $n$ is the number of moles of electrons transferred per mole of reaction (4 in this case).
• $F$ is the Faraday constant (96485 C/mol).
• $Q$ is the reaction quotient.

The reaction quotient, $Q$, can be calculated based on the given conditions and the reaction:

$ Q = \frac{[\text{Fe}^{2+}]^2}{[\text{H}^+]^4 \cdot P(O_2)} $

Substituting the given values:

• $[\text{Fe}^{2+}] = 10^{-3} \text{ M}$
• $[\text{H}^+] = 10^{-\text{pH}} = 10^{-3} \text{ M}$
• $P(O_2) = 0.1 \text{ atm}$

$ Q = \frac{(10^{-3})^2}{(10^{-3})^4 \cdot 0.1} $

$ Q = \frac{10^{-6}}{10^{-12} \cdot 0.1} $

$ Q = \frac{10^{-6}}{10^{-13}} $

$ Q = 10^{7} $

Now, substituting the values into the Nernst equation:

$ E = 1.67 \text{ V} - \frac{8.314 \times 298}{4 \times 96485} \ln(10^{7})$

Calculating the term $\frac{RT}{nF}$;

$ \frac{RT}{nF} = \frac{8.314 \times 298}{4 \times 96485} $

$ \approx \frac{2476}{385940} $

$ \approx 0.0064 \text{ V} $

The natural logarithm of $10^{7}$ is about 16.1 (since $\ln(10) \approx 2.303$, so $\ln(10^{7}) = 7 \times \ln(10) \approx 16.1$).

Thus,

$ E = 1.67 \text{ V} - 0.0064 \times 16.1$

$ E = 1.67 \text{ V} - 0.103 \text{ V} $

$ E = 1.567 \text{ V} $

Therefore, the cell potential under the given conditions is approximately 1.57 V, so option D (1.57 V) is the correct answer.

So when we consider these values with a negative sign (i.e., taking the magnitude of the negative potentials), we get the values as :

• $Cr$ : 0.91 V
• $Mn$ : 1.18 V
• $Fe$ : 0.44 V
• $Co$ : 0.28 V

Therefore, the correct order of $E_{M^{2+}/M}^\circ$ values with the negative sign will be :

Mn > Cr > Fe > Co

This matches with option A.