Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

Which electrolyte is used in the electrolytic method of preparation of fluorine? (a) \(\mathrm{KHF}_{2}+\mathrm{LiF}\) (b) \(\mathrm{CaCl}_{2}+\mathrm{KF}\) (c) \(\mathrm{KF}+\mathrm{LiHF}_{2}\) (d) \(\mathrm{NaF}+\mathrm{CaCl}_{2}\)

Step-by-Step Solution

Answer

The correct electrolyte is option (a) 12KHF_{2}2+12LiF2.

Step 1: Understand the Electrolytic Method for Fluorine Preparation

Fluorine is prepared by the electrolytic oxidation of fluoride ions. The electrolyte should be capable of providing fluoride ions for the process and remain stable under the conditions of electrolysis.

Step 2: Analyze the Electrolyte Properties

Consider the properties of each given electrolyte combination: - (a) d 12KF_{2}+LiF2 - (b) d 12CaCl_{2}+KF2 - (c) d 12KF+LiHF_{2}2 - (d) d 12NaF+CaCl_{2} (c)2 Only those electrolytes providing fluoride ions and being practically used for fluorine production should be considered valid.

Step 3: Confirm the Traditional Electrolyte Used

The commercial preparation of fluorine uses a mixture of potassium fluoride and hydrogen fluoride in the form of potassium hydrogen difluoride (12KF3+12HF12). 12KHF_{2}2 is the traditional electrolyte used in laboratory preparation, with sometimes 12LiF2 added to improve conductance and lower melting temperature.

Step 4: Match the Correct Option

The correct option is the one that most closely matches the traditional electrolyte known for producing fluorine by electrolysis. From step 3, the combination resembling 12KHF_{2}2 with 12LiF2 would work as the ideal electrolyte.

Key Concepts

Electrolyte Properties

Electrolytes play a crucial role in the preparation of fluorine through electrolysis. An electrolyte is a substance that, when dissolved in a solvent like water, dissociates into ions. These ions are responsible for conducting electricity within the solution. For the purpose of preparing fluorine, the chosen electrolyte must efficiently supply fluoride ions (\( \text{F}^- \)) because fluorine is obtained by the oxidation of these ions.
In evaluating the properties of various electrolyte combinations, certain factors are considered important:

• Stability: The electrolyte should remain stable throughout the electrolysis process.
• Dissociation: It must easily dissociate to provide fluoride ions.
• Conductivity: High ionic conductivity is necessary to support the electrochemical reaction.

These properties ensure that the electrolysis process runs smoothly and efficiently, ultimately leading to the successful preparation of fluorine.

Fluoride Ion Oxidation

The core of the electrolytic preparation of fluorine relies on the oxidation of fluoride ions. During electrolysis, electric current causes the fluoride ions present in the electrolyte solution to lose electrons and form fluorine gas (\(\text{F}_2\)). This process can be represented by the half-reaction:\[ 2 \text{F}^- \rightarrow \text{F}_2 + 2e^- \]In this reaction:

• Fluoride ions (\(\text{F}^-\)) are oxidized to produce fluorine gas.
• Electrons are released during the oxidation, completing the circuit of the electrolysis process.

The conditions under which the oxidation occurs are critical. Both the choice of electrolyte and controlling the electrolysis conditions ensure an efficient release of fluorine. Fluorine, being a very reactive element, must be handled with care during preparation.

Potassium Hydrogen Difluoride

Potassium hydrogen difluoride (\(\text{KHF}_2\)) is the traditional electrolyte used in the commercial preparation of fluorine. It is particularly favored because:

• It dissociates to provide the necessary fluoride ions.
• It has a stable structure that supports the conditions of electrolysis.
• It is effective in reducing the melting temperature of the electrolyte mix when combined with lithium fluoride (\(\text{LiF}\)).

The combination of potassium hydrogen difluoride and lithium fluoride creates an efficient medium for conducting electricity and producing fluorine. The ease of handling these chemicals and their conducive properties make them ideal choices for industrial-scale fluorine production.

Electrolysis Process

Electrolysis is the process of using electric current to drive a chemical reaction that would not normally occur on its own. In the preparation of fluorine, electrolysis is used to separate fluoride ions and generate fluorine gas. Here's how it works:

• An electrical current is passed through the electrolyte solution containing \(\text{KHF}_2\) and \(\text{LiF}\).
• The energy from the current causes fluoride ions to migrate to the anode, where they are oxidized to form fluorine gas.
• The simultaneous flow of electrons and ions enables the conversion of ions to neutral molecules, such as \(\text{F}_2\).

This entire procedure is conducted under controlled conditions to ensure safety, given the reactive nature of fluorine. The careful management of the electrolysis process is vital to yield high-purity fluorine gas, suitable for various applications.