Question

A solution of sodium sulphate in water is electrolyzed using inert electrodes. The products at the cathode and anode are respectively (a) \(\mathrm{O}_{2}, \mathrm{H}_{2}\) (b) \(\mathrm{O}_{2}, \mathrm{Na}\) (c) \(\mathrm{O}_{2}, \mathrm{SO}_{2}\) (d) \(\mathrm{H}_{2}, \mathrm{O}_{2}\)

Short answer

The products are (d) \( H_2\) at the cathode and \( O_2\) at the anode.

Step-by-step solution

Identifying the Electrolysis Process

In the electrolysis of an aqueous solution of sodium sulphate ( Na_2SO_4 eds{} ), water is electrolyzed at the electrodes. This process involves breaking down water into its constituent elements because sodium and sulphate ions in solution do not discharge at the inert electrodes under normal conditions.

Determining the Reaction at the Cathode

At the cathode, reduction occurs. In the solution, water is reduced instead of sodium ions because water requires less energy to reduce. The half-equation for the reduction of water is:\[2\, \mathrm{H_2O} + 2\, \mathrm{e^-} \rightarrow \mathrm{H_2} + 2\, \mathrm{OH^-}\]This equation shows that the product at the cathode is hydrogen gas (\(H_2\)).

Determining the Reaction at the Anode

At the anode, oxidation takes place. In this case, water is oxidized to oxygen because oxidation of water requires less energy than the oxidation of sulphate ions. The half-equation for this process is:\[2\, \mathrm{H_2O} \rightarrow \mathrm{O_2} + 4\, \mathrm{H^+} + 4\, \mathrm{e^-}\]This indicates that oxygen gas (\(O_2\)) is produced at the anode.

Identifying the Products

Based on the reactions at the cathode and anode, we conclude that the products of the electrolysis are \(H_2\) at the cathode and \(O_2\) at the anode. This corresponds to option (d) in the answer choices.

Key concepts

Aqueous Solution

An aqueous solution is a crucial aspect of electrolysis, particularly when dealing with compounds like sodium sulphate. An aqueous solution essentially means that the compound is dissolved in water. This solvent, water, allows the ions within a compound to dissociate and move freely. Sodium sulphate dissociates into sodium ions (Na⁺) and sulphate ions (SO₄²⁻) when in solution. However, during electrolysis, these ions do not react at the electrodes. Instead, water plays a primary role. The presence of water molecules provides an alternative medium for the reaction because it can also split into hydrogen (H⁺) and hydroxide ions (OH⁻). These aspects of an aqueous solution are vital in understanding why water, and not sodium or sulphate ions, reacts at the electrodes during the process.

Inert Electrodes

Inert electrodes are substances used in electrolysis that do not participate in the chemical reaction. Common materials include platinum and graphite. Their primary role is to conduct electricity efficiently without altering the nature of the electrolyzed solution. In the case of our sodium sulphate solution, the inert electrodes serve as a platform for the exchange of electrons. These electrodes do not react with sodium sulphate or its ions, instead, they facilitate the reduction and oxidation of water. This quality is pivotal, as it ensures that they do not interfere with the desired chemical reactions. These self-contained and stable electrodes assist in safely managing reactions even at high temperatures or in chemically reactive environments.

Cathode Reaction

The reaction that occurs at the cathode during the electrolysis of sodium sulphate in an aqueous solution is a reduction process. In this specific situation, water molecules are reduced rather than the sodium ions. This decision is based on the fact that reduction of water requires less energy. The reduction half-reaction is: \[2\, \mathrm{H_2O} + 2\, \mathrm{e^-} \rightarrow \mathrm{H_2} + 2\, \mathrm{OH^-}\] This reaction clearly indicates that hydrogen gas (H₂) is formed as a product at the cathode. This highlights the importance of energy considerations in determining which species in the solution undergoes the reaction. The electrons required for this reduction are supplied by the battery or power source driving the electrolysis, ensuring continuous production of hydrogen gas.

Anode Reaction

The anode serves as the site where oxidation occurs during electrolysis of an aqueous sodium sulphate solution. At the anode, water molecules are oxidized to form oxygen gas (O₂), rather than the sulphate ions. Oxidation of water is energetically more favorable and requires less energy compared to other possible reactions.The oxidation half-reaction is:\[2\, \mathrm{H_2O} \rightarrow \mathrm{O_2} + 4\, \mathrm{H^+} + 4\, \mathrm{e^-}\]This process produces oxygen gas as the main byproduct, alongside protons (H⁺), which remain in solution. This oxygen gas is collected at the anode and serves as an indication of the successful oxidation process. The liberation of oxygen not only finalizes the chemical conversion occurring at the anode but also compliments the stoichiometry required for the continuous flow of electricity through the solution, balancing the electrons released back to the power source.