Describe the preparation of silicon from silica sand, and explain how silicon is purified for use in semiconductor devices. Write balanced equations for all reactions.

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1. Step 1: Begin with the extraction of silicon from silica sand (SiO₂). Silica sand is primarily composed of silicon dioxide. The first step in the preparation of silicon is to reduce silicon dioxide to silicon. This is typically done using a high-temperature reduction process with carbon in an electric arc furnace.
2. Step 2: Write the balanced chemical equation for the reduction of silicon dioxide with carbon. The reaction is: \[ \text{SiO}_2 (s) + 2\text{C} (s) \rightarrow \text{Si} (s) + 2\text{CO} (g) \]. This reaction involves heating silica sand with carbon to produce silicon and carbon monoxide gas.
3. Step 3: Once silicon is obtained, it needs to be purified for use in semiconductor devices. The most common method for purification is the Siemens process, which involves converting silicon into a volatile compound and then decomposing it to obtain pure silicon.
4. Step 4: In the Siemens process, silicon is reacted with hydrogen chloride gas to form trichlorosilane (SiHCl₃). The balanced equation for this reaction is: \[ \text{Si} (s) + 3\text{HCl} (g) \rightarrow \text{SiHCl}_3 (g) + \text{H}_2 (g) \].
5. Step 5: Finally, trichlorosilane is decomposed to produce high-purity silicon. This is done by heating trichlorosilane in the presence of hydrogen gas, which results in the deposition of pure silicon. The balanced equation for this reaction is: \[ \text{SiHCl}_3 (g) + \text{H}_2 (g) \rightarrow \text{Si} (s) + 3\text{HCl} (g) \].