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.

Verified step by step guidance

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: \[ \text{SiHCl}_3 (g) + \text{H}_2 (g) \rightarrow \text{Si} (s) + 3\text{HCl} (g) \].

Key Concepts

Silica Sand and Silicon Production

Silica sand, primarily composed of silicon dioxide (SiO2), is the starting material for silicon production. The reduction of silica to silicon typically involves a high-temperature reaction with carbon in an electric arc furnace, where SiO2 reacts with carbon to produce silicon and carbon dioxide. This process is essential for obtaining metallurgical-grade silicon, which serves as the foundation for further purification.

Purification of Silicon

To achieve the high purity required for semiconductor applications, metallurgical-grade silicon undergoes further purification processes, such as the Siemens process or the use of chemical vapor deposition (CVD). These methods involve converting silicon into a gaseous form, such as trichlorosilane (SiHCl3), and then redepositing it to yield high-purity silicon. This purification is crucial for enhancing the electrical properties of silicon in electronic devices.

Balanced Chemical Equations

Balanced chemical equations are essential for accurately representing chemical reactions, ensuring that the number of atoms for each element is conserved. In the context of silicon production, the reaction can be represented as: SiO2 + 2C → Si + 2CO. Writing balanced equations helps in understanding the stoichiometry of the reactions involved in both the production and purification of silicon, which is vital for industrial applications.