Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound used for the preparation of \(\mathrm{UF}_{6}\) in the enrichment of \({ }_{92} \mathrm{U}^{235}\) is (a) \(\mathrm{HF}\) (b) \(\mathrm{CaF}_{2}\) (c) \(\mathrm{ClF}_{3}\) (d) \(\mathrm{AlF}_{3}\)

Step-by-Step Solution

Answer

The correct compound is (a) \( \mathrm{HF} \).

Step 1: Analyze the Target Compound

The question asks for the compound used to prepare \( \mathrm{UF}_{6} \), which is uranium hexafluoride. It is important to identify that the compound needed must provide fluoride ions to bond with uranium.

Step 2: Consider the Choices

The options provided are: (a) \( \mathrm{HF} \), (b) \( \mathrm{CaF}_{2} \), (c) \( \mathrm{ClF}_{3} \), and (d) \( \mathrm{AlF}_{3} \). We need to determine which of these can produce \( \mathrm{UF}_{6} \) effectively.

Step 3: Eliminate Possible Incorrect Options

Options (b) \( \mathrm{CaF}_{2} \) and (d) \( \mathrm{AlF}_{3} \) involve solid fluorides that do not directly produce \( \mathrm{UF}_{6} \). Option (c) \( \mathrm{ClF}_{3} \) is a chlorofluoride, which is not generally used to directly prepare \( \mathrm{UF}_{6} \).

Step 4: Identify the Correct Compound

Option (a) \( \mathrm{HF} \) is a source of fluoride ions and, when fed to uranium compounds, can help in the production of \( \mathrm{UF}_{6} \). It is commonly used in the "fluorination" step to make \( \mathrm{UF}_{6} \).

Step 5: Confirm Understanding

\( \mathrm{UF}_{6} \) is formed through a reaction where \( \mathrm{UF}_{4} \) reacts with \( \mathrm{F}_{2} \), often facilitated by the presence of \( \mathrm{HF} \), confirming \( \mathrm{HF} \) is a key component for \( \mathrm{UF}_{6} \) production.

Key Concepts

Fluoride Compounds

Fluoride compounds are chemical compounds that contain the element fluorine combined with another element. Fluorine is highly reactive and forms compounds with nearly all elements. One of the primary uses of fluoride compounds is in the preparation of other materials, such as Uranium Hexafluoride (UF6).
UF6 is a compound used in the uranium enrichment process, and it requires fluoride ions for its preparation. In the context of UF6 preparation, hydrogen fluoride (HF) is a significant fluoride compound. It acts as a source of fluoride ions necessary to react with uranium compounds such as uranium tetrafluoride (UF4).
Some common fluoride compounds include:

• Hydrogen Fluoride (HF): A gas at room temperature, essential for producing UF6.
• Calcium Fluoride (CaF2): A solid that does not easily release fluoride ions.
• Aluminum Fluoride (AlF3): Another solid not typically utilized for UF6 preparation.

Among these, HF is key because of its gaseous state and ability to release fluoride ions needed in chemical reactions to form UF6.

Enrichment of Uranium

Enrichment of uranium is a critical step in preparing nuclear fuel. Natural uranium consists primarily of two isotopes: Uranium-238 (U-238) and Uranium-235 (U-235). For most nuclear reactors, a higher concentration of U-235 is required than what is found naturally. This is where enrichment comes into play.
The enrichment process increases the proportion of U-235 in uranium. Uranium Hexafluoride (UF6) is pivotal for this procedure because it can be easily turned into a gas. The gaseous form of UF6 allows for isotopic separation using methods like gas diffusion or gas centrifugation.
Key factors in uranium enrichment include:

• Volatility: UF6 must be volatile to transform from a solid to a gas for separation.
• Density: Differences in mass between U-235 and U-238 enable separation.

Thus, the preparation and use of UF6 are crucial steps in the uranium enrichment process, facilitating the necessary conditions for nuclear fuel production.

Chemical Reactions in Nuclear Chemistry

Chemical reactions in nuclear chemistry involve transformations that produce significant energy changes, often involving radioactive substances. These reactions differ from typical chemical reactions in their mechanisms and energy scales.
A key reaction in nuclear chemistry is the conversion of uranium tetrafluoride (UF4) to uranium hexafluoride (UF6). This reaction crucially involves the introduction of fluoride ions, usually from hydrogen fluoride (HF) and fluorine gas (F2):
\[ \text{UF}_4 + \text{F}_2 \rightarrow \text{UF}_6 \]
Such reactions are unique because they not only involve changes in chemical states but also have implications for atomic nuclei, especially when dealing with isotopes.
Nuclear reactions release energy through two main processes:

• Fission: Splitting heavy nuclei into lighter ones, releasing energy.
• Fusion: Combining light nuclei to form heavier nuclei, also releasing energy.

In nuclear chemistry, reactions that involve compounds like UF6 are central to processes like uranium enrichment and subsequently, nuclear fuel production. These reactions underline the importance of understanding both chemical and nuclear change dynamics.