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Chemistry · 2024 · 4 marks
CBSE 2024 · Region 4 · Set 1 · Q30
In a galvanic cell, chemical energy of a redox reaction is converted into electrical energy, whereas in an electrolytic cell the redox reaction…
In a galvanic cell, chemical energy of a redox reaction is converted into electrical energy, whereas in an electrolytic cell the redox reaction occurs on passing electricity. The simplest galvanic cell is in which Zn rod is placed in a solution of $\displaystyle \mathrm{ZnSO}_{4}$ and Cu rod is placed in a solution of $\displaystyle \mathrm{CuSO}_{4}$. The two rods are connected by a metallic wire through a voltmeter. The two solutions are joined by a salt bridge. The difference between the two electrode potentials of the two electrodes is known as electromotive force. In the process of electrolysis, the decomposition of a substance takes place by passing an electric current. One mole of electric charge when passed through a cell will discharge half a mole of a divalent metal ion such as $\displaystyle \mathrm{Cu}^{2+}$. This was first formulated by Faraday in the form of laws of electrolysis. Answer the following questions :
(a)
What is the function of a salt bridge in a galvanic cell ?
(b)
When does galvanic cell behave like an electrolytic cell ?
Can copper sulphate solution be stored in a pot made of zinc ? Explain with the help of the value of $\displaystyle \mathrm{E}^{\circ}$ cell. \[\begin{aligned} & \left(\mathrm{E}^{\circ} \mathrm{Cu}^{2+} / \mathrm{Cu}=0.34 \mathrm{~V}\right) \\ & \left(\mathrm{E}^{\circ} \mathrm{Zn}^{2+} / \mathrm{Zn}=-0.76 \mathrm{~V}\right) \end{aligned} \]
How much charge in terms of Faraday is required for the following :
(i)
$\displaystyle 1$ mol of $\displaystyle \mathrm{MnO}_{4}^{-}$to $\displaystyle \mathrm{Mn}^{2+}$
(ii)
$\displaystyle 1$ mol of $\displaystyle \mathrm{H}_{2} \mathrm{O}$ to $\displaystyle \mathrm{O}_{2}$
Marking-scheme solution
(a) It allows flow of ions and the circuit is completed / itmaintains the electrical neutrality. .
(b) When \(\displaystyle \mathrm{E}_{\text {ext }}>\mathrm{E}_{\text {cell }}\)
(c) \[\begin{aligned}
& \mathrm{E}^{\mathrm{o}}{ }_{\mathrm{cell}}=\mathrm{E}^{\mathrm{o}}{ }_{\mathrm{Cu}}{ }^{2+} / \mathrm{Cu}-\mathrm{E}^{\mathrm{o}}{ }_{\mathrm{Zn}}{ }^{2+} / \mathrm{Zn} \\
& =0 \cdot 34-(-0 \cdot 76)=1 \cdot 10 \mathrm{~V}
\end{aligned}
\]
As \(\displaystyle \mathrm{E}^{\mathrm{O}}{ }_{\text {cell }}=+\mathrm{ve}\), the reaction takes place, so copper sulphate cannot be stored ina zinc pot. OR
(c) (i) 5F
(ii) $\displaystyle 2$ F
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CBSE Class 12 Chemistry past-paper question from the 2024 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.