Which refining method is represented by the following equations?

\(Zr + 2I_2 \longrightarrow ZrI_4\)

\(ZrI_4 \longrightarrow Zr + 2I_2\)

van Arkel method

The correct answer is option 3. van Arkel method.

Let us go through the van Arkel method in detail, including the chemical principles behind it and the specific reactions involved.

The van Arkel method, also known as the iodide process, is a method used for the purification of metals, particularly those that are difficult to purify by other means. This method is especially useful for metals like zirconium (Zr) and titanium (Ti), which are prone to impurities that can affect their properties.

The van Arkel method exploits the ability of certain metals to form volatile compounds with halogens (like iodine) at relatively low temperatures. The key idea is to first convert the impure metal into a volatile compound and then decompose this compound to obtain the pure metal.

Step-by-Step Process

Step 1: Formation of the Volatile Metal Halide

The impure metal is reacted with iodine (\(I_2\)) at a moderate temperature to form a volatile metal iodide. For zirconium (Zr), the reaction is:

\(\text{Zr (solid)} + 2\text{I}_2 \text{(gas)} \longrightarrow \text{ZrI}_4 \text{(gas)}\)

In this reaction: Zirconium (\(Zr\)) reacts with iodine (\(I_2\)) to form zirconium tetraiodide (\(ZrI_4\)). \(ZrI_4\) is volatile, meaning it can easily vaporize at the temperatures used in this process.

Step 2: Decomposition of the Volatile Compound

The volatile metal iodide (\(ZrI_4\)) is then transported to a high-temperature region where it is decomposed back into pure metal and iodine. This decomposition occurs on a heated tungsten filament or similar surface at high temperatures (around 1800 K for zirconium):

\(\text{ZrI}_4 \text{(gas)} \longrightarrow \text{Zr (solid)} + 2\text{I}_2 \text{(gas)}\)

In this step: The zirconium tetraiodide (\(ZrI_4\)) decomposes, depositing pure zirconium (\(Zr\)) on the filament. Iodine (\(I_2\)) is released as a gas and can be reused in the process.

Significance of the Van Arkel Method

The van Arkel method is particularly significant for purifying metals like zirconium and titanium because these metals often contain impurities that are difficult to remove using conventional methods. This process produces metals of very high purity, which are essential in critical applications like nuclear reactors, aerospace engineering, and medical devices. The method is also useful because it operates under relatively mild conditions compared to other refining processes.

Example of the Van Arkel Method in Action

Zirconium Refinement: In industries where ultra-pure zirconium is needed (e.g., for nuclear reactor cladding), zirconium is first converted to zirconium tetraiodide via the reaction with iodine. This \(ZrI_4\) is then decomposed on a hot tungsten filament to yield pure zirconium and iodine gas.

Why Other Methods Are Different

To clarify why the van Arkel method is distinct:

Zone Refining: This method involves melting a small region of a solid metal and moving this molten zone along the length of the solid, carrying impurities with it. It is used for semiconductors and metals like silicon and germanium, not volatile halides.

Mond Process: Used for refining nickel by converting it to volatile nickel carbonyl \(Ni(CO)_4\), which decomposes to yield pure nickel.

Electrolytic Method: This involves using an electric current to reduce metal ions to pure metal at the cathode. It's commonly used for metals like copper and aluminum, not for volatile compounds.

Conclusion

The given equations represent the van Arkel method, which is used to purify metals by converting them to volatile halides (like \(ZrI_4\)) and then decomposing these halides to obtain the pure metal.

Thus, the correct answer is option 3: van Arkel method.