Problem 21 - Chemistry The Central Science In Si Units

Question

The purification process of silicon involves the reaction of silicon tetrachloride vapor \(\left(\mathrm{SiCl}_{4}(g)\right)\) with hydrogen to \(1250^{\circ} \mathrm{C}\) to form solid silicon and hydrogen chloride. (a) Write a balanced equation for this reaction. (b) What is being oxidized, and what is being reduced? (c) Which substance is the reductant, and which is the oxidant?

Step-by-Step Solution

Answer

(a) \( \text{SiCl}_4(g) + 2\text{H}_2 \rightarrow \text{Si} + 4\text{HCl} \). (b) Silicon is reduced, hydrogen is oxidized. (c) Hydrogen is the reductant and \( \text{SiCl}_4 \) is the oxidant.

Step 1: Identify Reactants and Products

The reaction involves silicon tetrachloride vapor \( \text{SiCl}_4(g) \) reacting with hydrogen \( \text{H}_2 \) to form solid silicon \( \text{Si} \) and hydrogen chloride \( \text{HCl} \). These are the reactants and products we need to use in our balanced equation.

Step 2: Balance the Chemical Equation

Based on the reactants \( \text{SiCl}_4(g) \) and \( \text{H}_2 \) and products \( \text{Si} \) and \( \text{HCl} \), write the unbalanced equation: \[ \text{SiCl}_4(g) + \text{H}_2 \rightarrow \text{Si} + \text{HCl}. \]Balance chlorine (Cl) atoms by placing a coefficient of 4 in front of \( \text{HCl} \), resulting in \( \text{SiCl}_4(g) + \text{H}_2 \rightarrow \text{Si} + 4\text{HCl} \).Balance hydrogen (H) atoms by placing a coefficient of 2 in front of \( \text{H}_2 \), resulting in \[ \text{SiCl}_4(g) + 2\text{H}_2 \rightarrow \text{Si} + 4\text{HCl}. \] Now the equation is balanced.

Step 3: Determine Oxidation and Reduction

Identify oxidation states. In \( \text{SiCl}_4 \), silicon is \(+4\) and chlorine is \(-1\). In \( \text{H}_2 \), hydrogen is 0. In \( \text{Si} \), silicon is 0, and in \( \text{HCl} \), hydrogen is \(+1\) and chlorine is \(-1\).Silicon's oxidation state changes from \(+4\) to 0 (reduction) and hydrogen's from 0 to \(+1\) (oxidation). Thus, silicon is reduced, and hydrogen is oxidized.

Step 4: Identify the Oxidant and Reductant

The reductant is the substance that is oxidized and the oxidant is the substance that is reduced. Since hydrogen undergoes oxidation, it acts as the reductant, while silicon tetrachloride is the oxidant as it undergoes reduction.

Key Concepts

Chemical Equation Balancing

Balancing a chemical equation ensures that the number of atoms of each element is the same on both sides of the equation. This is crucial to uphold the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

In the reaction between silicon tetrachloride and hydrogen, the goal is to create a balanced chemical equation that reflects the reaction accurately. The unbalanced equation is:

• \[ \text{SiCl}_4(g) + \text{H}_2 \rightarrow \text{Si} + \text{HCl} \]

To balance the equation, follow these steps:

1. Count the number of each type of atom in the reactants and products.2. Balance the chlorine atoms first by placing a 4 in front of \( \text{HCl} \). This gives:

• \[ \text{SiCl}_4(g) + \text{H}_2 \rightarrow \text{Si} + 4\text{HCl} \]

3. Next, balance the hydrogen atoms by placing a 2 in front of \( \text{H}_2 \). The balanced equation is:

• \[ \text{SiCl}_4(g) + 2\text{H}_2 \rightarrow \text{Si} + 4\text{HCl} \]

Now, all atoms are balanced, with each element having the same number of atoms on both sides of the equation.

Oxidation States

Oxidation states help chemists determine the transfer of electrons in a reaction. They provide insight into which substances are oxidized and which are reduced, a central concept in redox reactions.

In the reaction given, identify the oxidation states of each element to understand the electron transfer:

• In \( \text{SiCl}_4 \), silicon has an oxidation state of \(+4\), and chlorine is \(-1\).
• For \( \text{H}_2 \), hydrogen is at an oxidation state of 0.
• In the products, \( \text{Si} \) is in its elemental form with an oxidation state of 0, while in \( \text{HCl} \), hydrogen becomes \(+1\) and chlorine remains \(-1\).

Notice the changes:

- Silicon goes from \(+4\) in \( \text{SiCl}_4 \) to 0 in \( \text{Si} \), indicating a reduction as it gains electrons. - Hydrogen increases from 0 in \( \text{H}_2 \) to \(+1\) in \( \text{HCl} \), showing it is oxidized as it loses electrons.

This exchange signifies the core of redox reactions: one substance loses electrons (oxidation) and the other gains electrons (reduction). Understanding this helps identify the substances involved as oxidants or reductants.

Silicon Purification

Silicon purification is a vital process in producing high-purity silicon for electronic and solar applications. The method discussed uses a reaction between silicon tetrachloride \( (\text{SiCl}_4) \) and hydrogen \( (\text{H}_2) \) to achieve this goal.

By reacting at high temperatures, around 1250°C, silicon is obtained in its elemental form from \( \text{SiCl}_4 \). This is crucial for producing the ultrapure silicon required in photovoltaic cells and semiconductors.

The process involves these steps:

• Silicon tetrachloride is vaporized and introduced with hydrogen gas.
• The mixture is heated to a very high temperature, causing a reaction that separates pure silicon and hydrogen chloride gas.

The chemical reaction is monitored to ensure complete conversion and extraction of silicon. This method effectively separates silicon from its compound, allowing its use in advanced technological applications, demonstrating the importance of chemical processes in industrial and technological advancements. The result is high-purity silicon critical for ongoing advancements in technology.