Question

(a) Write equations for the half-reactions that occur at the cathode and the anode when an aqueous solution of KCl is electrolyzed. Which chemical species is oxidized, and which chemical species is reduced in this reaction? (b) Predict the products formed when an aqueous solution of Csl is electrolyzed.

Short answer

(a) Cathode: H_2O is reduced; Anode: Cl^- is oxidized. (b) Products: H_2 and I_2 are formed.

Step-by-step solution

Understand the Electrolysis Process

In electrolysis, electricity is passed through an aqueous solution to break down the compound into its elements or ions. The cathode attracts cations where reduction occurs, and the anode attracts anions where oxidation occurs.

Write the Half-Reactions for KCl Electrolysis

During the electrolysis of aqueous KCl, water can also be involved in the reaction. At the cathode, potassium ions (K^+) could theoretically reduce, but due to the presence of water, hydrogen ions from water reduce preferentially. Therefore, the half-reaction at the cathode is: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \]. At the anode, chloride ions (Cl^-) oxidize via the reaction: \[ 2Cl^- \rightarrow Cl_2 + 2e^- \].

Determine Oxidation and Reduction Species for KCl

The species oxidized is Cl^-, since it loses electrons to form Cl_2 gas. The species reduced is H_2O, as it gains electrons and forms hydrogen gas (H_2).

Write the Half-Reactions for CsI Electrolysis

In the electrolysis of aqueous CsI, similarly, water is more favorable for reduction at the cathode than Cs^+. Thus, at the cathode, the reaction is: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \]. At the anode, iodide ions (I^-) oxidize as they have a higher oxidation potential than water: \[ 2I^- \rightarrow I_2 + 2e^- \].

Predict Products for CsI Electrolysis

The products formed during the electrolysis of CsI are hydrogen gas (H_2) at the cathode and iodine (I_2) at the anode. Cs^+ and OH^- remain in solution.

Key concepts

Half-Reactions

In electrolysis, half-reactions play a crucial role in understanding how substances are transformed at the electrodes. A half-reaction is a part of the overall electrochemical reaction that happens at an electrode. It either involves oxidation, the loss of electrons, or reduction, the gain of electrons. These separate reactions occur simultaneously and are essential for the reaction's completion.

Consider an aqueous solution of potassium chloride (KCl). At the cathode, where reduction occurs, water is reduced instead of potassium ions. This preference arises due to the easier reduction of water in comparison to potassium. The half-reaction at the cathode can be described as:

• At the cathode: Water is reduced: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \]

Meanwhile, at the anode end, chloride ions undergo oxidation. This results in the release of chlorine gas, shown in the reaction:

• At the anode: Chloride ions oxidize: \[ 2Cl^- \rightarrow Cl_2 + 2e^- \]

By understanding these half-reactions, we can predict the products formed in the electrolysis process.

Cathode and Anode

The cathode and anode are terminals through which a direct current enters or exits a system. They are vital in electrolysis as they determine which half-reactions occur. Each electrode plays a specific role that complements the other.

• The Cathode: This is the negative electrode during electrolysis. It attracts positively charged ions, known as cations. Reduction, where electrons are gained, occurs here. For example, in the electrolysis of aqueous potassium chloride, hydrogen ions from water are reduced to form hydrogen gas.
• The Anode: This is the positive electrode. It attracts negatively charged ions, called anions. At this terminal, oxidation occurs, where electrons are lost. In the KCl electrolysis, chloride ions lose electrons to form chlorine gas.

Understanding the functions of the cathode and anode helps predict the reactions taking place during electrolysis.

Oxidation and Reduction

Oxidation and reduction are core concepts in electrochemical reactions, vital for interpreting electrolysis processes. Often remembered by the acronym "OIL RIG" (Oxidation Is Loss, Reduction Is Gain), these reactions focus on electron exchange.

• Oxidation: This process involves the loss of electrons. During electrolysis, substances at the anode often undergo oxidation. For example, in the electrolysis of KCl, chloride ions (Cl^-) oxidize to produce chlorine gas (\[ Cl_2 \] ), releasing electrons in the process.
• Reduction: Conversely, this process involves the gain of electrons. At the cathode, substances are typically reduced. In aqueous KCl, water molecules gain electrons, resulting in the formation of hydrogen gas (\[ H_2 \] ).

Understanding these processes provides insights into how and why specific reactions occur during electrolysis.

Electrochemical Reactions

Electrochemical reactions form the foundation of electrolysis, driving chemical transformations through electron transfer. These reactions involve the interplay between electrical energy and chemical change, involving two primary parts: the oxidation and reduction reactions at the anode and cathode, respectively.

In the case of electrolyzing aqueous solutions such as CsI or KCl:

• At the cathode, the chemical reaction takes place where reduction occurs, typically involving water molecules. It results in the formation of hydrogen gas.
• At the anode, oxidation takes place, with anions like chloride or iodide losing electrons to form elemental gases like chlorine (Cl_2) or iodine (I_2).

These reactions transform the compounds into different substances, highlighting the conversion of electrical energy into chemical energy and vice versa. Understanding these reactions allows for better manipulation and prediction of products in electrochemical processes.