Question

The electrolytic decomposition of dilute sulphuric acid with platinum electrode, cathodic reaction is : (a) Reduction of \(\mathrm{H}^{+}\) (b) Oxidation of \(\mathrm{SO}_{4}^{2-}\) (c) Reduction \(\mathrm{SO}_{3}^{2-}\) (d) Oxidation of \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

The cathodic reaction for the electrolytic decomposition of dilute sulfuric acid with platinum electrodes is (a) Reduction of \(\mathrm{H}^{+}\).

Step-by-step solution

Understanding Electrolysis

Electrolysis is the process of causing a chemical reaction through the introduction of an electric current. During electrolysis of dilute sulfuric acid, two reactions - oxidation and reduction - occur at two different electrodes. The cathode is the site of reduction, where positively charged ions gain electrons. The anode is the site of oxidation, where negative ions lose electrons or water is involved.

Identify the Cathodic Reaction

In the case of dilute sulfuric acid, the cations available for the reduction at the cathode are primarily \(\mathrm{H}^{+}\) ions from the acid and \(\mathrm{H}_2\mathrm{O}\) molecules. These species can potentially gain electrons. Since \(\mathrm{SO}_4^{2-}\) and \(\mathrm{SO}_3^{2-}\) are anions, they are not subject to reduction at the cathode.

Determine the Most Likely Reaction

Platinum is considered an inert electrode and does not participate in the reactions. Given the options, \(\mathrm{H}^{+}\) ions are reduced to \(\mathrm{H}_2\) gas at the cathode because the reduction of water is less favorable at lower concentrations of sulfuric acid. Therefore, the reduction of \(\mathrm{H}^{+}\) to hydrogen gas \(\mathrm{H}_2\) is the cathodic reaction.

Key concepts

Electrolytic Decomposition

Electrolytic decomposition, also known as electrolysis, is a fundamental chemical process where electrical energy is used to drive a non-spontaneous chemical reaction. This process is essential in various industrial applications, such as the extraction of metals from their ores, the production of chlorine gas and sodium hydroxide, and the electroplating of metals.

During electrolysis, an electric current is passed through an electrolyte, which is a substance containing free ions, making it electrically conductive. The electrolyte can be either a molten salt or a solution of the substance in water. The electric current enters and leaves the electrolyte through electrodes, causing chemical reactions to occur at the surface of the electrodes which result in the decomposition of the electrolyte. For example, in the electrolysis of dilute sulfuric acid, water is usually decomposed, producing hydrogen gas at the cathode and oxygen gas at the anode.

Cathodic Reaction

The cathodic reaction is a process that occurs at the cathode during electrolysis. It involves the gain of electrons, which is also known as a reduction reaction. In our specific example of dilute sulphuric acid electrolysis, the positively charged hydrogen ions, \(\mathrm{H}^{+}\), are attracted to the negatively charged cathode. Here, they accept electrons from the cathode and get reduced to form hydrogen gas \(\mathrm{H}_2\).

Reduction reactions can involve various species depending on the electrolytic solution and the type of the electrodes. For instance, if other cations were present in higher concentrations, they might be reduced instead. However, in dilute sulphuric acid, the reduction of hydrogen ions to produce hydrogen gas is the most likely reaction due to its lower position in the electrochemical series, which makes it easier to reduce compared to other possible species like water or sulfate ions.

Reduction and Oxidation Reactions

Reduction and oxidation reactions, commonly known as redox reactions, are chemical processes where electrons are transferred between atoms or molecules. Reduction refers to the gain of electrons, whereas oxidation is the loss of electrons. These two processes always occur together; when one substance is oxidized, another is reduced.

During electrolysis, the cathode supports the reduction process as it supplies electrons to the species that is reduced. Conversely, the anode is the site for oxidation, where electrons are removed from the species being oxidized. In our electrolysis example, hydrogen ions \(\mathrm{H}^{+}\) gain electrons (reduction) to form hydrogen gas at the cathode, while at the anode, water molecules lose electrons (oxidation) to form oxygen gas and \(\mathrm{H}^{+}\) ions that maintain the chemical balance in the solution.

Inert Electrodes

Inert electrodes are crucial in certain electrolysis reactions. These electrodes are made from materials that do not participate in the chemical reactions occurring during electrolysis; instead, they offer a conductive path for electrons to enter or leave the electrolyte. Common inert electrode materials include platinum, gold, and graphite. These materials are chosen for their ability to resist corrosion and remain chemically stable under the conditions of the electrolytic process.

In the case of dilute sulfuric acid electrolysis, platinum electrodes are used. The platinum electrode at the cathode does not react with the \(\mathrm{H}^{+}\) ions or the water; rather, it provides a surface for \(\mathrm{H}^{+}\) ions to gain electrons and form \(\mathrm{H}_2\) gas. The inert nature of the platinum electrode is significant because it ensures that the reaction at the cathode is the reduction of \(\mathrm{H}^{+}\) ions rather than any side reactions involving the electrode material itself.