Q21.121 CP - Chemistry: Molecular Nature Of Matter And Change

Question

Commercial electrolysis is performed on both molten  and aqueous  solutions. Identify the anode product, cathode product, species reduced, and species oxidized for the anode. 

(a) molten electrolysis and 

(b) aqueous electrolysis.

Step-by-Step Solution

Answer

(a) For molten electrolysis the anode product and the cathode product are ${\text{Cl}}_2$and $\text{Na}$respectively.

(b) For aqueous electrolysis the anode product and the cathode product are ${\text{Cl}}_2$ and ${\text{H}}_{\text{2}}{\text{ + OH}}^{\text{ - }}$respectively.

Step 1: Concept Introduction

The process of dissolving ionic compounds into their constituent components by delivering a direct electric current through the complex in a fluid form is known as electrolysis. At the cathode, cations are reduced, whereas anions are oxidised.

Step 2: (a) Molten Electrolysis

In molten electrolysis of $\text{NaCl}$, sodium metal is formed at the cathode, as it is reduced from ${\text{Na}}^{\text{ + }}$to $\text{Na}$ metal, while Chlorine gas is formed at the anode, as the ${\text{Cl}}^{\text{ - }}$ atoms are oxidized and form gas.

Due to overvoltage, not all ions can be converted into their free element form via electrolysis, including these:

• Cations of metals from Group $\text{1A}$ .
• Cations of metals from Group $\text{2A}$ and Aluminium from group $\text{3A}$.
• The fluoride ion cannot be oxidized, while the rest of the halides can be.
• Common oxyanions such as ${\text{SO}}_{\text{4}}^{\text{2 - }}{\text{, CO}}_{\text{3}}^{\text{2 - }}{\text{, NO}}_{\text{3}}^{\text{ - }}{\text{, PO}}_{\text{4}}^{\text{3 - }}$cannot be oxidized because they are at their highest oxidation states as they are. Water is oxidized to ${\text{O}}_{\text{2}}{\text{, H}}^{\text{ + }}$instead.

 

Therefore, at the anode, chlorine is the product, and at the cathode, sodium is the product.

Step 3: (b) Aqueous Electrolysis

In the electrolysis of aqueous $\text{NaCl}$ , metal will be formed at the cathode. This is because it ${\text{Na}}^{\text{ + }}$is a group $\text{1A}$ cation, water is reduced instead. However, at the anode, chlorine gas is formed from chloride ions.

 

Therefore, at the anode, chlorine is the product, and at the cathode, hydrogen and hydroxide ion is the products.