Chapter 20.3, Problem 29SSC

Interpretation Introduction

Interpretation:

Short summary to explain each of the three objectives of section 20.3 is to be written.

Concept introduction:

Electrochemistry is study of relation between electrical energy and chemical changes that takes place in redox reaction. There are three main aspects of electrochemistry branch, those are:

1. Electrolysis
2. Electrochemical cell
3. Electrolytic conduction.

Answer to Problem 29SSC

• Electrolysis is a process in which decomposition of electrolyte occurs when flow of electricity from its aqueous solution or molten state occurs. In electrolysis external voltage is applied which reverses the movement of electron in cell, so it drives non-spontaneous reaction.
• When molten $\text{NaCl}$ undergo electrolysis, only ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are generated whereas when brine is electrolyzed, apart from ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions, there is an additional possibility of generation of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions. Hence as per the ease of redox reaction or lower discharge potential of ions, product is formed. Therefore, different product formation is observed on electrolysis of brine solution and molten sodium chloride.
• In smelting process, molten $\text{Cu}$ is casted into thick plates which act as anode in electrolytic cell. The electrolyte used is ${\text{CuSO}}_4$ with a thin $\text{Cu}$ sheet as cathode. On flow of current, $\text{Cu}$ from impure anode oxidizes to ${\text{Cu}}^{2+}$ ions. These ions migrate to cathode for reduction into $\text{Cu}$ atoms and impurities leave behind at the bottom of solution or electrolyte.

Explanation of Solution

Objective 1: Mechanism for reversal of spontaneous reaction in an electrochemical cell.

Electrolysis is a process in which decomposition of electrolyte occurs when flow of electricity from its aqueous solution or molten state takes place. In electrolysis external voltage is applied which reverses the movement of electron in cell, so it drives non-spontaneous reaction. When battery generates current in an electrochemical cell, electrons flow readily from anode to cathode through an external circuit. It a spontaneous process and goes until whole of anode gets used up and battery becomes dead. But when battery is recharged, cell operates like electrolytic cell that is electrical energy is supplied by external source and reactions at electrode reverse. External voltage source is used to drive a non spontaneous reaction.

Objective 2: Comparison of electrolysis of molten $\text{NaCl}$ with that of brine.

When molten $\text{NaCl}$ undergo electrolysis, ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are generated. ${\text{Na}}^{+}$ moves to cathode and undergo reduction to $\text{Na}$ and ${\text{Cl}}^{-}$ move to anode and oxidizes to ${\text{Cl}}_2$ gas.

The half cell reactions at electrodes are as follows:

At anode: ${\text{Na}}^{+}\left(\text{l}\right)+{\text{e}}^{-}\to \text{Na}\left(\text{l}\right)$

At cathode: ${\text{2Cl}}^{-}\left(\text{l}\right)\to {\text{Cl}}_2\left(\text{g}\right)+2{\text{e}}^{-}$

And the net cell reaction is as follows:

${\text{2Cl}}^{-}\left(\text{l}\right)+2{\text{Na}}^{+}\left(\text{l}\right)\to {\text{Cl}}_2\left(\text{g}\right)+2\text{Na}\left(\text{l}\right)$

When brine or aqueous $\text{NaCl}$ undergo electrolysis, apart from ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions there is possibility of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions to be generated due to presence of large amount of water. Therefore, at cathode 2 reactions are possible which are as follows:

$\begin{array}{l}{\text{Na}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\to \text{Na}\left(\text{s}\right)\\ {\text{2H}}_{\text{2}}\text{O}\left(\text{l}\right)+{\text{2e}}^{-}\to {\text{H}}_{\text{2}}\left(\text{g}\right){\text{+2OH}}^{-}\left(\text{aq}\right)\end{array}$

Since water is easy to reduce than ${\text{Na}}^{+}$, so ${\text{H}}_{\text{2}}\left(\text{g}\right)$ is evolved at cathode.

And at anode, similarly, 2 reactions are possible which are as follows:

$\begin{array}{l}{\text{2Cl}}^{-}\left(\text{aq}\right)\to {\text{Cl}}_2\left(\text{g}\right)+2{\text{e}}^{-}\\ {\text{2H}}_{\text{2}}\text{O}\left(\text{l}\right)\to {\text{O}}_{\text{2}}\left(\text{g}\right){\text{+4H}}^{+}\left(\text{aq}\right)+4{\text{e}}^{-}\end{array}$

Since ${\text{Cl}}_2$ is desired product so its quantity is kept large at anode hence, oxidation of ${\text{Cl}}^{-}$ takes place preferentially over water.

Objective 3: Significance of electrolysis to smelting process and purification of metals.

One of the applications of electrolysis is also found in purification of metals like $\text{Cu}$. $\text{Cu}$ is mainly mined in form of its sulfide ores as they are most abundant. Examples of copper ores are chalcopyrite $\left({\text{CuFeS}}_{\text{2}}\right)$ and malachite $\left({\text{Cu}}_{\text{2}}{\text{CO}}_{\text{3}}{\left(\text{OH}\right)}_{\text{2}}\right)$.

On heating sulfide ore in presence of oxygen, copper is obtained as follows:

${\text{Cu}}_{\text{2}}\text{S}\left(\text{s}\right)+{\text{O}}_{\text{2}}\left(\text{g}\right)\to \text{2Cu}\left(\text{l}\right)+{\text{SO}}_{\text{2}}\left(\text{g}\right)$

$\text{Cu}$ obtained by this smelting process is refined for purification from impurities. For purification, molten $\text{Cu}$ is casted into thick plates which act as anode in electrolytic cell. The electrolyte used is ${\text{CuSO}}_4$ with a thin $\text{Cu}$ sheet as cathode. On flow of current, $\text{Cu}$ from impure anode oxidizes to ${\text{Cu}}^{2+}$ ions. These ions migrate to cathode for reduction into $\text{Cu}$ atoms and impurities leave behind at the bottom of solution or electrolyte.

Conclusion

• Electrolysis is a process in which decomposition of electrolyte occurs when flow of electricity from its aqueous solution or molten state occurs. In electrolysis external voltage is applied which reverses the movement of electron in cell, so it drives non-spontaneous reaction.
• When molten $\text{NaCl}$ undergo electrolysis, only ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are generated whereas when brine is electrolyzed, apart from ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions, there is an additional possibility of generation of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions. Hence as per the ease of redox reaction or lower discharge potential of ions, product is formed. Therefore, different product formation is observed on electrolysis of brine solution and molten sodium chloride.
• In smelting process, molten $\text{Cu}$ is casted into thick plates which act as anode in electrolytic cell. The electrolyte used is ${\text{CuSO}}_4$ with a thin $\text{Cu}$ sheet as cathode. On flow of current, $\text{Cu}$ from impure anode oxidizes to ${\text{Cu}}^{2+}$ ions. These ions migrate to cathode for reduction into $\text{Cu}$ atoms and impurities leave behind at the bottom of solution or electrolyte.