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

Question

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

Step-by-Step Solution

Answer

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

Step 1: Hydrogen Evolution Matching

In the 50% solution of \( \mathrm{H}_{2}\mathrm{SO}_{4} \) using Platinum electrodes, the electrolysis induces hydrogen gas evolution at the cathode. Therefore, (a) matches with (p) \( \mathrm{H}_{2} \) is evolved at cathode.

Step 2: Oxygen Evolution Matching

For a dilute \( \mathrm{NaCl} \) solution with Platinum electrodes, water is reduced and oxidized at the electrodes, leading to oxygen gas evolution at the anode. Thus, (b) matches with (q) \( \mathrm{O}_{2} \) is evolved at anode.

Step 3: Copper Anode Dissolution

In the dilute \( \mathrm{H}_{2}\mathrm{SO}_{4} \) solution using Copper electrodes, the copper anode dissolves, so no gas is primarily evolved. This situation does not match any of the options in column-II, implying a non-spontaneous process. Therefore, (c) matches with (t) non-spontaneous process.

Step 4: Chlorine Evolution Matching

With a concentrated \( \mathrm{LiCl} \) solution using Platinum electrodes, we expect chlorine gas to evolve at the anode due to the high concentration of chloride ions. Therefore, (d) matches with (r) \( \mathrm{Cl}_{2} \) is evolved at anode.

Key Concepts

Electrolysis

Electrolysis is a fascinating process where a chemical compound is split into its constituents by passing an electric current through it. This usually involves an ionic substance, often dissolved in a solution or molten state. By using electrodes, typically made of inert materials like platinum, an electrical current is applied which promotes the movement of ions towards oppositely charged electrodes. This physical movement of ions results in chemical reactions, often yielding elemental gases at the electrodes.

In our original exercise, various solutions undergo electrolysis with platinum electrodes. For instance, the 50% sulfuric acid solution undergoes electrolysis and hydrogen gas evolves as a result. This illustrates the transformative power of electrical energy to induce chemical changes despite the inherent stability of ionic bonds under normal circumstances.

Key points to remember about electrolysis include:

• The process requires a power source to drive the non-spontaneous reaction.
• The type and concentration of solution significantly affect the outcome and products of electrolysis.
• Electrolysis is widely used in industries for electroplating, electrorefining, and in the isolation of pure elements.

Electrode Reactions

Electrode reactions play a crucial role in electrolysis, acting as the sites where reduction and oxidation (redox) reactions occur. Typically, the cathode is where reduction reactions happen, while the anode hosts oxidation reactions. The composition of electrodes can influence the reactions and products of electrolysis.

For example, in the dilute NaCl solution, Platinum electrodes facilitate the splitting of water molecules, leading to oxygen evolution at the anode. In comparison, when using copper electrodes in the dilute sulfuric acid solution, the copper metal itself plays a part by dissolving, showing the diverse outcomes based on electrode choice.

Here are crucial aspects of electrode reactions:

• Reduction at Cathode: Gain of electrons, often resulting in metal or hydrogen gas deposition.
• Oxidation at Anode: Loss of electrons, leading to the release of gases or dissolution of metal.
• Electrode material selection impacts reaction efficiency and type of gas or solid formed.

Gas Evolution

Gas evolution is a quintessential feature of many electrolytic processes. Often visible as bubbles around the electrodes, gas evolution occurs when gaseous products form from the reactions of ions during electrolysis.

In our examples, both hydrogen and oxygen gas were evolved from specific reactions. For instance, hydrogen gas evolved at the cathode when electrolysis was performed on a 50% sulfuric acid solution. Similarly, when a dilute NaCl solution was subjected to electrolysis, oxygen gas evolved at the anode. This phenomenon underlines the electrolytic conversion of ionic to gaseous form.

A few important notes about gas evolution:

• Gas types depend on the electrolyte and specific reactions at electrodes.
• The presence of bubbles can indicate the efficiency and completion of electrolysis.
• Gas evolution is exploited in hydrogen production, oxygen generation, and other industrial applications.

Non-spontaneous Process

A non-spontaneous process in electrochemistry refers to reactions that require external energy input to occur. In electrolysis, the necessity of a power source indicates that these reactions would not happen naturally.

Take, for example, the electrolysis of a dilute solution of sulfuric acid using copper electrodes. Here, the process does not lead to any prominent gas formation but instead requires external electrical energy to dissolve the copper anode, which is otherwise a non-spontaneous reaction.

Key points about non-spontaneous processes include:

• These processes require an energy input, typically from an external electric source, to drive the reaction.
• In electrolysis, non-spontaneous reactions are essential for extracting or modifying elements.
• Understanding these processes is vital in industries such as electroplating, battery manufacturing, and even in some renewable energy technologies.