Problem 79 - Chemistry Matter and Change

Question

Apply During electrolysis, an electrolytic cell releases bromine vapor and hydrogen gas. After electrolysis, the cell is found to contain a concentrated solution of potassium hydroxide. What was the composition of the cell before electrolysis began?

Step-by-Step Solution

Answer

The cell initially contained water and potassium bromide (\(\text{KBr}\)).

Step 1: Identify the reactions

During electrolysis, bromine vapor and hydrogen gas are released from the solution, indicating that component elements of a compound have been separated by the passage of electrical current. Meanwhile, potassium hydroxide solution remains, suggesting the presence of potassium ions and hydroxide ions after electrolysis.

Step 2: Recognize the products

The release of bromine vapor and hydrogen gas indicates that bromide ions and water were decomposed. Bromine would have been released from bromide ions in the anode reaction, while hydrogen gas comes from the reduction of water or hydrogen ions at the cathode, forming the by-products of electrolytic decomposition.

Step 3: Infer the initial composition

Given the production of bromine gas, hydrogen gas, and a solution of potassium hydroxide, initially, the cell contained an aqueous solution of potassium bromide (\(\text{KBr}\)). Potassium ions (\(\text{K}^+\)) combined with the hydroxide ions (\(\text{OH}^-\)) during electrolysis to form the remaining potassium hydroxide solution.

Step 4: Conclusion: Determine initial elements

The initial solution must have contained water and potassium bromide because their component ions and molecules contributed to the products formed via electrolysis. The potassium ions remained in solution while bromide ions were converted into bromine gas, and water contributed to the formation of both hydrogen gas and hydroxide ions.

Key Concepts

Electrolytic Cell

An electrolytic cell is a fascinating setup used to drive a non-spontaneous chemical reaction with the help of electricity. This type of cell may seem complex at first, but it essentially involves a container where an electric current is passed through a solution, leading to chemical changes. Within the cell, electrodes are placed in an electrolyte solution, and these electrodes are crucial as they facilitate the movement of electrons. The key here is that these electrodes have specific roles: one works as the anode and the other as the cathode, each participating in unique reactions during the process of electrolysis.

• **Anode**: This is the positively charged electrode where oxidation occurs. For instance, bromide ions in the solution lose electrons at the anode to form bromine vapor.
• **Cathode**: The negatively charged electrode where reduction takes place. Water molecules gain electrons at the cathode to produce hydrogen gas and hydroxide ions.

Electrolytic cells are pivotal in various industries, aiding in processes such as electroplating, electrolytic refining of metals, and even in water splitting for hydrogen gas production.

Potassium Hydroxide

Potassium hydroxide is a critical component found after the electrolysis process in some cells. Its presence in the remaining solution hints at the reactions that took place during this process. Potassium hydroxide, a strong base with the chemical formula \KOH\, is known for ionizing completely in water, contributing to its strong alkaline nature.
It forms in the electrolytic cell primarily due to the reactions involving potassium ions and hydroxide ions. Initially, the potassium ions are present in the solution as part of the compound potassium bromide (\(\text{KBr}\)). During electrolysis, hydrogen ions are reduced to hydrogen gas, leaving behind hydroxide ions floating free to bond with potassium ions, hence forming potassium hydroxide:
- \text{K}^+ + ext{OH}^- \rightarrow ext{KOH}\
In practical applications, potassium hydroxide is highly valued for its usage in cleaning agents, pH regulation, and even as an electrolyte in alkaline batteries.

Bromine Vapor

Bromine vapor emerges as one of the notable products of the electrolysis in an electrolytic cell containing an aqueous potassium bromide solution. During this process, bromide ions are oxidized at the anode, leading to the formation of bromine gas. This gas then exists in its vapor form due to the conditions within the cell.
The chemical transformation can be represented by the reaction:
- \(2 ext{Br}^- \rightarrow ext{Br}_2 (g) + 2 ext{e}^−\)
This conversion from ions in solution to gaseous state showcases the power of electrolysis to alter states of matter. It's important to handle bromine vapor with caution due to its corrosive nature and potential health risks. In industrial settings, bromine is harnessed for its use in the production of flame retardants, pharmaceuticals, and even in the synthesis of organic bromine compounds.

Hydrogen Gas

Hydrogen gas is one of the key by-products released during the electrolysis of an electrolytic cell. This diatomic molecule is typically produced at the cathode when water molecules accept electrons. In the context of a potassium bromide solution, hydrogen gas formation represents the reduction half-reaction happening in the cell.
The typical reaction at the cathode is:
- \(2 ext{H}_2 ext{O} + 2 ext{e}^- \rightarrow ext{H}_2 (g) + 2 ext{OH}^-\)
Hydrogen gas is known for its tremendous potential as a clean fuel source due to its high energy yield per unit mass and its only combustion by-product being water vapor. This makes it an appealing option in the ongoing quest for sustainable energy solutions. Furthermore, hydrogen plays a vital role in industries such as petroleum refining, ammonia synthesis, and as a fuel for hydrogen fuel cell vehicles. Understanding how hydrogen gas forms at the cathode during electrolysis provides insights into the broader applicability of this versatile element.