Explain the role of and write the equations involved :CO in purification of nickelIodine in refining of titaniumIodine in refining of zirconiumZinc in extraction of silver

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I).

In the Mond Process (also known as the Carbonyl Process), carbon monoxide combines with Nickel at 50-600C to form nickel tetracarbonyl gas. This removes nickel from impurities, none of which form a carbonyl at such a low temperature.

This process has three steps:

1. Nickel oxide is reacted with Syngas at 200 °C to remove oxygen, leaving impure nickel. Impurities include iron and cobalt. NiO (s) + H2 (g) → Ni (s) + H2O (g)

2. The impure nickel is reacted with excess carbon monoxide at 50–60 °C to form nickel carbonyl.

Ni (s) + 4 CO (g) → Ni(CO)4 (g)

3. The mixture of excess carbon monoxide and nickel carbonyl is heated to 220–250 °C. On heating, nickel tetracarbonyl decomposes to give nickel:

Ni(CO)4 (g) → Ni (s) + 4 CO (g)

II).

TiI4 is a rare molecular binary metal iodide, consisting of isolated molecules of tetrahedral Ti(IV) centers. The Ti-I distances are 261 pm. Reflecting its molecular character, TiI4 can be distilled without decomposition at one atmosphere; this property is the basis of its use in the Van Arkel process. The difference in melting point between TiCl4 (m.p. -24 °C) and TiI4 (m.p. 150 °C) is comparable to the difference between the melting points of CCl4 (m.p. -23 °C) and CI4 (m.p. 168 °C), reflecting the stronger intermolecular van der Waals bonding in the iodides.

From the elements, typically using a tube furnace at 425 °C:

Ti + 2 I2 ⇔ TiI4

This reaction can be reversed to produce highly pure films of Ti metal.

III).

Zirconium is produced by adding gaseous zirconium tetrachloride to molten magnesium metal. (In the absence of air). The end product is always contaminated with magnesium and it doesn't take much magnesium to make it brittle, etc.

at one temperature iodine would react with zirconium metal to form zirconium tetraiodide and that at a higher temperature the tetraiodide would decompose to zirconium metal and free iodine.

First step, zirconium in the cladding is reacted with elemental iodine at 300–500oC to form volatile ZrI4, which, in the second step, is then decomposed at >1100oC.
Formation  Zr + 4I2  → ZrI4  300-500oC
Decomposition  ZrI4  → Zr + 4I2  >1100oC

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