Question

What products are formed during the electolysis of concentrated aqueous solution of sodium chloride? (I) \(\mathrm{Cl}_{2}(g)\) at anode (II) \(\mathrm{NaOH}\) as electrolyte (III) \(\mathrm{H}_{2}(g)\) at cathode (a) I only (b) I and II only (c) I and III only (d) I, II and III

Short answer

The products formed during the electrolysis of concentrated aqueous sodium chloride are chlorine gas ( ) at the anode, hydrogen gas ( ) at the cathode, and sodium hydroxide ( ) in solution. Therefore, the correct answer is (d) I, II and III.

Step-by-step solution

Understand Electrolysis

Electrolysis is a chemical process that uses electricity to drive a non-spontaneous chemical reaction. When an aqueous solution of sodium chloride ( }) is electrolyzed, water ( ) is actually electrolyzed into its component elements, hydrogen and oxygen, while sodium chloride is dissociated into sodium and chloride ions.

Determine the Products at the Anode

During electrolysis, oxidation occurs at the anode. Chloride ions ( ) are oxidized to chlorine gas ( ), which is released at the anode. This corresponds to product (I), .

Determine the Products at the Cathode

At the cathode, reduction occurs. In the presence of a high concentration of ions, water is more likely to be reduced than sodium ions because of its lower reduction potential. Therefore, hydrogen gas ( ) is formed at the cathode, which is product (III).

Determine the Formation of Sodium Hydroxide

As the hydroxide ions ( ) are produced at the cathode and sodium ions ( ) are left in solution, they combine to form sodium hydroxide ( ). Thus, product (II) is also formed as a result of electrolysis.

Key concepts

Electrolysis Process

Electrolysis is an essential technique used in chemistry to bring about a chemical change through the application of electrical energy. At its core, the process involves passing an electric current through a substance to cause a non-spontaneous chemical reaction. It's a method by which ionic substances are decomposed into simpler substances when an electric current is passed through them.

Understanding the Setup

During electrolysis, two electrodes (an anode and a cathode) are submerged in an electrolyte and connected to a power source. The electric current flows through the electrolyte, which is often an aqueous solution, causing chemical reactions at the electrodes and the separation of materials.

Role of Ionic Movement

Key to the electrolysis process is the movement of ions within the solution. Cations, which are positively charged ions, move towards the cathode where they may gain electrons in a reduction reaction. Anions, negatively charged ions, move towards the anode where they lose electrons in an oxidation reaction. This movement allows for the transfer of charges and the formation of new substances.

Chemical Reaction

Chemical reactions are processes where reactant substances transform into products. Electrolysis involves a specific type of chemical reaction known as redox (reduction-oxidation) reactions. Here, the transformation is facilitated using electrical energy.

Redox Reactions During Electrolysis

In the context of electrolysis, a redox reaction occurs in a distinctly compartmentalized manner: oxidation occurs at the anode, and reduction happens at the cathode. For aqueous sodium chloride, or brine, the dissociation of sodium chloride into sodium and chloride ions allows these reactions to proceed when an electric current is applied. Oxidation involves the loss of electrons, and during the electrolysis of brine, chloride ions lose electrons to form chlorine gas. Reduction at the cathode involves gaining electrons, and water molecules gain electrons to form hydrogen gas and hydroxide ions. The net effect of electrolysis is the conversion of electrical energy into chemical energy resulting in the formation of new substances.

Products of Electrolysis

The products of electrolysis depend significantly on the substances being electrolyzed as well as the conditions under which electrolysis is conducted. In the case of concentrated aqueous sodium chloride, otherwise called brine, three main products are formed:

Chlorine Gas

At the anode, chloride ions are oxidized, and chlorine gas (\r\(\mathrm{Cl}_{2}(g)\)) is released. This is a valuable product used in various industries, notably in the production of disinfectants and plastics.

Hydrogen Gas

Hydrogen gas (\r\(\mathrm{H}_{2}(g)\)) is formed at the cathode as water molecules receive electrons through the reduction process. The hydrogen produced can be utilized as a clean fuel or in the manufacturing of hydrochloric acid.

Sodium Hydroxide

The formation of sodium hydroxide (\r\(\mathrm{NaOH}\)) is another important result of the electrolysis of brine. As hydroxide ions (\r\(OH^-\)) are produced at the cathode, they remain in the solution with the unreacted sodium ions (\r\(Na^+\)), resulting in the creation of sodium hydroxide. This compound has extensive use in the chemical industry, particularly in soap making and as a cleaning agent. The identification and understanding of these products are crucial for various applications across different sectors, highlighting the versatility and significance of the electrolysis process.