Question

Metallic potassium was first prepared by Humphrey Davy in 1807 by electrolysis of molten potassium hydroxide: (a) Label the anode and cathode, and show the direction of ion flow. (b) Write balanced equations for the anode, cathode, and overall cell reactions.

Short answer

Cathode: \( K^+ + e^- \rightarrow K \); Anode: \( 4OH^- \rightarrow 2H_2O + O_2 + 4e^- \).

Step-by-step solution

Understanding the Electrolysis Process

In the electrolysis of molten potassium hydroxide (KOH), it is important to understand that this process involves the decomposition of the compound into its constituent elements by passing an electric current through the molten substance.

Identifying Components

In the electrolysis cell, the anode is the positive electrode, and the cathode is the negative electrode. During electrolysis, positive ions move towards the cathode to gain electrons, while negative ions move towards the anode to lose electrons.

Label the Anode and Cathode

In the electrolysis of molten KOH, the potassium ions (\( K^+ \) ions) move towards the cathode, and the hydroxide ions (\( OH^- \) ions) move towards the anode. Thus, the cathode is where K+ ions gain electrons, and the anode is where OH- ions lose electrons.

Write the Cathode Reaction

At the cathode, potassium ions reduce by gaining electrons to form potassium metal. The balanced cathode reaction is:\[ K^+ + e^- \rightarrow K \]

Write the Anode Reaction

At the anode, hydroxide ions oxidize to form oxygen gas, and electrons are released. The balanced anode reaction is:\[ 4OH^- \rightarrow 2H_2O + O_2 + 4e^- \]

Write Overall Cell Reaction

Combine the half-reactions from the cathode and anode to write the overall balanced equation for the electrolysis of molten potassium hydroxide:\[ 4KOH \rightarrow 4K + 2H_2O + O_2 \]

Key concepts

Metallic Potassium Preparation

The preparation of metallic potassium through electrolysis was a groundbreaking achievement by Humphrey Davy in 1807. This process involves the decomposition of potassium hydroxide ( KOH ) into its elements, potassium and oxygen, through the application of an electric current.
By applying this current to molten KOH , Potassium ions ( K^+ ) are reduced at the cathode, forming the metallic potassium.
- This method enables chemists to isolate pure potassium, a highly reactive metal. - The process is significant because it provides a clean and effective way of extracting metallic potassium, which can be challenging due to its high reactivity and the stability of its compounds in nature. Electrolysis, therefore, offers a controlled environment for the extraction of potassium by carefully manipulating electric currents within an electrolytic cell.

Anode and Cathode Labeling

In an electrolytic cell setup, it is vital to correctly identify the anode and cathode, as these labels determine the direction of ion movement and the reactions taking place. The anode is the positive electrode, while the cathode is the negative electrode.
- At the cathode, reduction reactions occur, which involve gaining electrons. - Conversely, at the anode, oxidation reactions occur, involving the loss of electrons. In the context of potassium hydroxide electrolysis: - Potassium ions ( K^+ ) are attracted to the cathode and gain electrons to become potassium metal. - Hydroxide ions ( OH^- ) move to the anode and lose electrons, resulting in the formation of oxygen gas.
This understanding of the movement and transformation of ions is crucial for effectively performing the electrolysis process.

Electrolysis Reactions

Electrolysis reactions involve the conversion of a substance into its constituent components through the application of electricity. In the electrolysis of molten potassium hydroxide, these reactions are divided between the electrodes:- At the cathode, the reduction reaction occurs where potassium ions (K^+) gain electrons to become metallic potassium. The balanced reaction is:\[ K^+ + e^- \rightarrow K \]- At the anode, hydroxide ions (OH^-) lose electrons, leading to their oxidation and the generation of water and oxygen gas. The balanced reaction is:\[ 4OH^- \rightarrow 2H_2O + O_2 + 4e^- \]
This separation of reactions into cathodic and anodic ones enables the operation of an electrolysis cell, effectively breaking down the initial compound (potassium hydroxide) into useful products.

Potassium Hydroxide

The compound potassium hydroxide ( KOH ) plays a key role in the electrolysis process for preparing metallic potassium. It is a solid, white compound that becomes molten at high temperatures. KOH is the starting material that supplies potassium ions ( K^+ ) and hydroxide ions ( OH^- ) when it dissolves or liquefies: - Potassium ions are crucial for forming the metallic potassium at the cathode. - Hydroxide ions are involved in forming oxygen at the anode. When subjected to electrolysis, molten KOH decomposes effectively, offering a direct route to extract valuable elements like potassium. The high melting point and inherent reactivity of KOH make it an ideal candidate for electrolysis. Understanding the role of potassium hydroxide in this context underscores its importance in the broader field of inorganic chemistry.

Chemical Equations in Electrolysis

Chemical equations form the backbone of understanding electrolysis reactions, serving as a concise representation of the chemical processes taking place. In the case of potassium hydroxide electrolysis, two half-reactions occur at the electrodes, which combine to present a complete picture:- The cathodic reaction is represented by:\[ K^+ + e^- \rightarrow K \]- The anodic reaction is represented by:\[ 4OH^- \rightarrow 2H_2O + O_2 + 4e^- \]To find the overall reaction, these half-reactions are combined, ensuring that the electrons gained and lost balance each other out:\[ 4KOH \rightarrow 4K + 2H_2O + O_2 \]
This balanced equation not only illustrates the transformation of reactants to products but also highlights the conservation of mass and charge, which is fundamental in chemical processes. Understanding and writing accurate chemical equations is essential for exploring and optimizing various electrolysis systems.