Question

During the preparation of alkanes by Kolbe's electrolytic method using inert electrodes, the \(\mathrm{pH}\) of the electrolyte (1) decreases progressively as the reaction proceeds (2) increases progressively as the reaction procecds (3) remains constant through the reaction (4) may decrease if the concentration of the clectrolyte is not very high

Short answer

The pH of the electrolyte increases progressively as the reaction proceeds.

Step-by-step solution

- Understanding Kolbe's electrolytic method

Kolbe's electrolytic method involves the electrolysis of a concentrated solution of a carboxylate salt (usually sodium or potassium) using inert electrodes. Alkanes are produced at the anode.

- Electrolysis reaction at the anode

The electrochemical reaction at the anode involves the oxidation of the carboxylate ion (RCOO-) to carbon dioxide (CO2), an alkyl radical (R·), and the release of an electron: \[ 2 \text{RCOO}^- \rightarrow 2 \text{R·} + 2 \text{CO}_2 + 2e^- \]

- Formation of alkane

The alkyl radicals formed at the anode then combine to form an alkane: \[ 2 \text{R·} \rightarrow \text{R-R} \]

- Reaction at the cathode

At the cathode, water is reduced to hydrogen gas and hydroxide ions (OH-): \[ 2 \text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2 \text{OH}^- \]

- Effect on pH of the electrolyte

As the electrolysis proceeds, the production of hydroxide ions (OH-) at the cathode increases. This causes the pH of the electrolyte to increase progressively since hydroxide ions make the solution more basic.

Conclusion

Based on the electrochemical reactions occurring at both the anode and cathode, the pH of the electrolyte increases progressively as the reaction proceeds.

Key concepts

alkane preparation

Alkanes are a class of hydrocarbons with single bonds between carbon atoms. One of the methods to prepare alkanes is Kolbe's electrolytic method. This method involves using electrolysis to produce alkanes from carboxylate salts. Here's a simple breakdown of the process:

• A concentrated solution of a carboxylate salt, such as sodium or potassium salt, is used.
• This solution undergoes electrolysis, and inert electrodes (like platinum or graphite) are used in the process.
• At the anode (positive electrode), the carboxylate ions are oxidized, leading to the formation of alkanes.

The specific reactions will be covered in the next section. The Kolbe electrolytic method is particularly useful for creating symmetrical alkanes (i.e., alkanes where both halves of the molecule are the same). This method is significant in organic chemistry due to its straightforward approach to producing hydrocarbons.

electrolysis reaction

Electrolysis is a process where electrical energy is used to drive a non-spontaneous chemical reaction. In Kolbe's electrolytic method, electrolysis plays a crucial role in converting carboxylate ions into alkanes.

• At the anode: The carboxylate ions (RCOO-) are oxidized. The reaction produces carbon dioxide, an alkyl radical, and electrons:
  \[ 2 \text{RCOO}^- \rightarrow 2 \text{R·} + 2 \text{CO}_2 + 2 e^- \]
  The key product here is the alkyl radical (R·).
• The alkyl radicals then combine to form an alkane: \[ 2 \text{R·} \rightarrow \text{RR} \]
  This is the desired alkane product of the method.
• At the cathode: Water molecules are reduced to produce hydrogen gas and hydroxide ions: \[ 2 \text{H}_2 \text{O} + 2 e^- \rightarrow \text{H}_2 + 2 \text{OH}^- \]
  The OH⁻ ions produced at the cathode will affect the pH of the solution, which will be discussed in the next section.

The entire electrolysis reaction makes use of the electrons to transform the carboxylate salt solution into alkanes, along with other by-products like hydrogen gas and carbon dioxide.

pH change

The pH level of a solution is a measure of its acidity or basicity. During Kolbe's electrolytic method, the pH of the electrolyte solution changes due to the reactions occurring at the electrodes.

• At the cathode: The reduction of water produces hydroxide ions (OH-). Since hydroxide ions are basic, they increase the pH of the solution, making it more basic.
• The production of OH⁻ ions continues progressively as the reaction proceeds. This means that the pH of the electrolyte increases steadily throughout the electrolytic process.

So typically, during the electrolysis reaction in Kolbe's method, the pH of the solution would rise progressively. This increase in pH helps us track the progress of the reaction and understand the chemical environment in the electrolytic cell. This pH change is a direct consequence of the electrochemical reactions at the cathode and is a crucial factor to consider in understanding the overall process. Always remember that monitoring pH can be an essential tool for predicting the outcome and efficiency of the electrolytic reaction.