Problem 73 - Chemistry The Molecular Science

Question

Three reactions very important to the semiconductor industry are (a) The reduction of silicon dioxide to crude silicon, \(\mathrm{SiO}_{2}(\mathrm{~s})+2 \mathrm{C}(\mathrm{s}) \longrightarrow \mathrm{Si}(\mathrm{s})+2 \mathrm{CO}(\mathrm{g})\) $$ \Delta_{\mathrm{r}} H^{\circ}=689.9 \mathrm{~kJ} / \mathrm{mol} $$ (b) The formation of silicon tetrachloride from crude silicon, $$ \mathrm{Si}(\mathrm{s})+2 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow \mathrm{SiCl}_{4}(\mathrm{~g}) \quad \Delta_{\mathrm{t}} H^{\circ}=-657.01 \mathrm{~kJ} / \mathrm{mol} $$ (c) The reduction of silicon tetrachloride to pure silicon with magnesium, $$ \begin{array}{r} \mathrm{SiCl}_{4}(\mathrm{~g})+2 \mathrm{Mg}(\mathrm{s}) \longrightarrow 2 \mathrm{MgCl}_{2}(\mathrm{~s})+\mathrm{Si}(\mathrm{s}) \\ \Delta_{\mathrm{r}} H^{\circ}=-625.6 \mathrm{~kJ} / \mathrm{mol} \end{array} $$ Calculate the overall enthalpy change when \(1.00 \mathrm{~mol}\) sand, \(\mathrm{SiO}_{2}\), changes into very pure silicon by this series of reactions.

Step-by-Step Solution

Answer

The overall enthalpy change is \(-592.71 \, \mathrm{kJ/mol}\).

Step 1: Understand the Reactions

The problem involves three reactions which transform silicon dioxide (\(\mathrm{SiO}_2\)) into pure silicon. Each reaction has an associated enthalpy change (\(\Delta_r H^\circ\)). We need to combine these reactions to calculate the overall enthalpy change.

Step 2: Write the Reactions and Their Enthalpies

1. \(\mathrm{SiO}_{2} + 2 \mathrm{C} \rightarrow \mathrm{Si} + 2 \mathrm{CO}\), \(\Delta_r H^\circ = 689.9\, \mathrm{kJ/mol}\).2. \(\mathrm{Si} + 2 \mathrm{Cl}_{2} \rightarrow \mathrm{SiCl}_{4}\), \(\Delta_t H^\circ = -657.01\, \mathrm{kJ/mol}\).3. \(\mathrm{SiCl}_{4} + 2 \mathrm{Mg} \rightarrow 2 \mathrm{MgCl}_{2} + \mathrm{Si}\), \(\Delta_r H^\circ = -625.6\, \mathrm{kJ/mol}\).

Step 3: Add the Enthalpy Changes

Combine the enthalpy changes of the three reactions to find the total enthalpy change:\[\Delta H_{\text{total}} = 689.9 + (-657.01) + (-625.6)\, \mathrm{kJ/mol}.\]

Step 4: Calculate the Total Enthalpy Change

Add the enthalpy values:\[\Delta H_{\text{total}} = 689.9 - 657.01 - 625.6 = -592.71\, \mathrm{kJ/mol}.\]

Step 5: Conclude the Overall Enthalpy Change

The overall enthalpy change for converting \(1.00\, \mathrm{mol}\) of silicon dioxide to pure silicon through these reactions is \(-592.71\, \mathrm{kJ/mol}\).

Key Concepts

Enthalpy Change: The Heat of Reaction

Enthalpy change refers to the amount of heat absorbed or released during a chemical reaction under constant pressure. It's a crucial concept in thermochemistry that helps in understanding how energy is transferred in reactions.
For any given reaction, the change in enthalpy, often represented as \( \Delta H \), indicates the heat content change of the system.

In the context of silicon purification, we observe three reactions with specific enthalpy changes:

• The reduction of silicon dioxide to crude silicon has a high enthalpy change \( (\Delta H = 689.9 \text{ kJ/mol}) \), meaning it requires energy input, making it an endothermic reaction.
• The formation of silicon tetrachloride \( (\Delta H = -657.01 \text{ kJ/mol}) \) is exothermic, as it releases energy to the surroundings.
• The reduction of silicon tetrachloride to pure silicon \( (\Delta H = -625.6 \text{ kJ/mol}) \) is also exothermic, further lowering the system's energy content.

By summing up these enthalpy changes, we can determine that the overall process from silicon dioxide to pure silicon is exothermic, with a total change of \(-592.71 \text{ kJ/mol}\). This indicates that the entire sequence releases energy, signifying a net reduction in enthalpy and heat release to the surroundings.

Silicon Purification: From Sand to Silicon

Silicon purification involves transforming raw silicon, often from sand \(\mathrm{SiO}_2\), into highly pure silicon.
This process is vital for industries like semiconductors, where high-purity silicon is essential.

The purification of silicon from its oxide form typically follows these steps:

• Reduction of Silicon Dioxide: Initially, silicon dioxide is reduced using carbon in a high-temperature reaction to produce crude silicon and carbon monoxide.
• Formation of Silicon Tetrachloride: This crude silicon then reacts with chlorine to form silicon tetrachloride, a volatile compound that can be easily purified by distillation.
• Reduction to Pure Silicon: Finally, silicon tetrachloride is reduced using magnesium to yield pure silicon and magnesium chloride as a byproduct.

Each of these stages plays a unique role in increasing the purity level of silicon, highlighting the intricacies and energy demands of silicon purification processes.

Chemical Reactions: Transformation of Matter

Chemical reactions are transformations where substances change into entirely new compounds, often with different properties and energy states.
The understanding of reactions is pivotal in thermochemistry, particularly in how they alter the energy within a system.

In silicon purification:

• Reduction Reaction: Silicon dioxide's reduction with carbon is a classic example, where the reactants (\(\mathrm{SiO}_2\) and carbon) form different products (crude silicon and carbon monoxide). This reaction consumes energy, illustrating the importance of energetic considerations in industrial processes.
• Synthesis Reaction: The formation of silicon tetrachloride involves the direct combination of silicon and chlorine, showcasing synthesis, where simpler substances construct more complex ones while releasing energy.
• Redox Reaction: The transformation from silicon tetrachloride to pure silicon is a reduction-oxidation process, where the silicon is reduced and the magnesium oxidized, embodying electron transfer and further releasing energy.

These reactions are foundational to achieving the transformation from natural to high-purity materials, underscoring the integral role of controlled chemical changes in material science and engineering.