Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \begin{array}{l} \text { (a) } 50 \% \text { solution of } \mathrm{H}_{2} \mathrm{SO}_{4} \\ \text { using Pt electrodes } \end{array} & \text { (p) } \mathrm{H}_{2} \text { is evolved at } \\ \text { cathode } \\ \begin{array}{ll} \text { (b) } \text { Dilute solution } \mathrm{NaCl} & \text { (q) } \mathrm{O}_{2} \text { is evolved at } \\ \text { using Pt electrodes } & \text { anode } \end{array} \\ \begin{array}{ll} \text { (c) Dilute solution of } \mathrm{H}_{2} \mathrm{SO}_{4} \\ \text { using Cu electrodes } \end{array} & \text { (r) } \mathrm{Cl}_{2} \text { is evolved at } \\ \text { (d) Concentrated solution of } & \text { anode } \\ \mathrm{LiCl} \text { using Pt electrodes. } & \text { (s) } \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{8} \text { is } \\ \text { formed at anode } \\ & \text { (t) non-spontaneous } \\ \text { process } \end{array} $$

Step-by-Step Solution

Answer

(a) matches with (s), (b) with (q), (c) with (t), and (d) with (r).

Step 1: Match (a)

For the 50% solution of \( \text{H}_2\text{SO}_4 \), using Pt electrodes, during electrolysis, \( \text{H}_2 \) gas is evolved at the cathode. The high concentration does not produce \( \text{O}_2 \) at the anode but produces \( \text{S}_2\text{O}_8^{2-} \) ions leading to the formation of \( \text{H}_2\text{S}_2\text{O}_8 \). Hence, (a) matches with (s).

Step 2: Match (b)

For a dilute solution of \( \text{NaCl} \) with Pt electrodes, typically, water is reduced at the cathode, producing \( \text{H}_2 \), and water is oxidized at the anode, producing \( \text{O}_2 \). Thus, (b) matches with (q).

Step 3: Match (c)

In a dilute solution of \( \text{H}_2\text{SO}_4 \) using Cu electrodes, copper undergoes oxidation at the anode and dissolves, typically leading to a non-standard reaction condition. This setup is intermediary or an ambiguous case but usually corresponds to a non-spontaneous process when considered with standard electrode dynamics. Hence, (c) matches with (t).

Step 4: Match (d)

For a concentrated solution of \( \text{LiCl} \) with Pt electrodes, \( \text{Cl}_2 \) gas is evolved at the anode because of the high concentration of chloride ions. Therefore, (d) matches with (r).

Key Concepts

Electrode Materials

The choice of electrode materials in an electrolytic setup plays a crucial role in the type of reactions that occur at the electrodes. Electrodes can be made from various materials like platinum (Pt), copper (Cu), and graphite, each exhibiting distinctive properties that influence their performance in electrochemical processes.

- **Platinum (Pt) Electrodes:** Pt is a widely used inert electrode material due to its excellent conductivity and corrosion resistance. It doesn’t participate in the reactions, making it suitable for experiments where the electrode material should remain unchanged.- **Copper (Cu) Electrodes:** Unlike Pt, Cu electrodes can participate in the electrochemical reactions. For instance, in a solution of \( \text{H}_2\text{SO}_4 \), Cu can dissolve by oxidizing at the anode.
The electrode material not only determines the reaction pathway but also the potential complications in the reaction. Choosing the right materials aids in optimizing the efficiency of the electrochemical cell and the desired electrolysis process.

Electrochemical Reactions

Electrochemical reactions are chemical reactions that involve the transfer of electrons between species at the electrodes. These reactions are central to the understanding of electrolysis.

In an electrolytic cell, two half-reactions occur:- **Reduction at the Cathode:** A reduction reaction involves the gain of electrons. For example, in a dilute \( \text{NaCl} \) solution with Pt electrodes, water reduces at the cathode to produce \( \text{H}_2 \) gas.- **Oxidation at the Anode:** Oxidation involves the loss of electrons. In the electrolysis of a concentrated \( \text{LiCl} \) solution, chloride ions oxidize at the anode, releasing \( \text{Cl}_2 \) gas.
The type of electrochemical reaction that occurs is influenced by parameters like ion concentration, the nature of the electrode material, and the voltage applied. These reactions are harnessed in various applications, from industrial electrolysis to energy storage technologies like batteries.

Electrochemical Cells

An electrochemical cell is a device capable of generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. There are primarily two types of electrochemical cells, galvanic (or voltaic) and electrolytic cells.

- **Galvanic Cells:** These cells convert chemical energy into electrical energy through spontaneous redox reactions. They are the basis for batteries in consumer electronics.- **Electrolytic Cells:** Unlike galvanic cells, electrolytic cells require an external voltage source to drive non-spontaneous reactions, as seen in electrolysis.
In electrolytic cells, the setup includes a power supply connected to the electrodes submerged in an electrolyte solution. For example, using Pt electrodes in \( 50\% \) \( \text{H}_2\text{SO}_4 \) leads to \( \text{H}_2 \) gas evolution at the cathode and the formation of \( \text{H}_2\text{S}_2\text{O}_8 \) at the anode.

Understanding the function and design of electrochemical cells is vital in tailoring them for different applications, from large-scale industrial processes to small gadgets.