Problem 7 - Chemistry and Chemical Reactivity

Question

Write balanced chemical equations for the following reactions. (a) The reaction of aluminum and iron(III) oxide to form iron and aluminum oxide (known as the thermite reaction). (b) The reaction of carbon and water at high temperature to form a mixture of gaseous \(\mathrm{CO}\) and \(\mathrm{H}_{2}\) (known as water gas and once used as a fuel). (c) The reaction of liquid silicon tetrachloride and magnesium forming silicon and magnesium chloride. This is one step in the preparation of ultrapure silicon used in the semiconductor industry.

Step-by-Step Solution

Answer

(a) \(2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3\), (b) \(\text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2\), (c) \(\text{SiCl}_4 + 2\text{Mg} \rightarrow \text{Si} + 2\text{MgCl}_2\).

Step 1: Identifying Reactants and Products for Reaction (a)

In the thermite reaction, aluminum (\(\text{Al}\)) reacts with iron(III) oxide (\(\text{Fe}_2\text{O}_3\)) to form iron (\(\text{Fe}\)) and aluminum oxide (\(\text{Al}_2\text{O}_3\)). Write the unbalanced equation: \(\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow \text{Fe} + \text{Al}_2\text{O}_3\).

Step 2: Balancing Chemical Equation for Reaction (a)

To balance the chemical equation, equalize the number of aluminum, iron, and oxygen atoms on both sides. Start by balancing the aluminum atoms. We need 2 \(\text{Al}\) atoms on the left, so place a coefficient of 2: \(2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow \text{Fe} + \text{Al}_2\text{O}_3\). Next, balance the iron atoms by placing a coefficient of 2 in front of \(\text{Fe}\) on the right: \(2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3\). This equation is now balanced.

Step 3: Identifying Reactants and Products for Reaction (b)

For the water-gas reaction, carbon (\(\text{C}\)) reacts with water (\(\text{H}_2\text{O}\)) to form carbon monoxide (\(\text{CO}\)) and hydrogen gas (\(\text{H}_2\)). Write the unbalanced equation: \(\text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2\).

Step 4: Balancing Chemical Equation for Reaction (b)

To balance this reaction, notice that each side of the equation already has 1 carbon atom and 2 hydrogen atoms. Place a coefficient of 1 in front of each reactant and product: \(\text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2\). The equation is already balanced.

Step 5: Identifying Reactants and Products for Reaction (c)

In the final reaction, liquid silicon tetrachloride (\(\text{SiCl}_4\)) reacts with magnesium (\(\text{Mg}\)) to form silicon (\(\text{Si}\)) and magnesium chloride (\(\text{MgCl}_2\)). Write the unbalanced equation: \(\text{SiCl}_4 + \text{Mg} \rightarrow \text{Si} + \text{MgCl}_2\).

Step 6: Balancing Chemical Equation for Reaction (c)

Balance this equation by starting with chlorine atoms. There are 4 chlorine atoms on the left, so place a 2 in front of \(\text{MgCl}_2\) to have 4 chlorines on the right: \(\text{SiCl}_4 + 2\text{Mg} \rightarrow \text{Si} + 2\text{MgCl}_2\). Now, balance magnesium by ensuring there are 2 \(\text{Mg}\) atoms on both sides, finalizing the equation.

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is essential to accurately represent chemical reactions. It ensures that matter is conserved in accordance with the law of conservation of mass. This means the same number of atoms for each element must appear on both sides of the equation.

• Identify each reactant and product involved in the reaction.
• Write down the unbalanced equation.
• Count atoms for each element on both sides of the equation.
• Adjust coefficients to balance the atoms. Remember, coefficients are numbers placed before molecules to indicate how many units of that substance are involved.
• Iteratively check and re-balance as needed until all atoms are balanced.

It's crucial to only change coefficients, never subscripts in the chemical formula, as this would change the substance itself.
Balancing requires practice, but it helps ensure the stoichiometry of reactions is correct, making it a fundamental step in chemistry problem-solving.

Thermite Reaction

The thermite reaction is a highly exothermic reaction involving aluminum and iron(III) oxide. When these substances react, they form molten iron and aluminum oxide.

• Unbalanced reaction: \[ \text{Al} + \text{Fe}_2\text{O}_3 \rightarrow \text{Fe} + \text{Al}_2\text{O}_3 \]
• Balanced reaction: \[ 2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3 \]
• This reaction produces significant heat, enough to melt iron. It is often used for welding and other applications requiring high heat production.
• Its process releases energy due to the strong bond formation in the aluminum oxide product.

Iron produced from the thermite reaction is typically in a molten state, showcasing tremendous temperature increase.

Water-Gas Reaction

The water-gas reaction, involving carbon and water vapor at high temperatures, forms a mixture known as "water gas," which is rich in carbon monoxide and hydrogen.

• Unbalanced equation:\[ \text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2 \]
• Balanced equation:\[ \text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2 \]
• This reaction effectively converts solid carbon fuels into a gaseous form that can be used as a fuel source.
• Water gas was a vital fuel source in the past, particularly before the advent of cleaner energy alternatives.

Given its composition, water gas served multiple purposes, such as in producing heat or as a precursor in chemical manufacturing.

Silicon Tetrachloride and Magnesium Reaction

Involves a reaction between liquid silicon tetrachloride and magnesium to produce silicon and magnesium chloride. This is crucial in producing ultrapure silicon for the semiconductor industry.

• Unbalanced equation: \[ \text{SiCl}_4 + \text{Mg} \rightarrow \text{Si} + \text{MgCl}_2 \]
• Balanced equation: \[ \text{SiCl}_4 + 2\text{Mg} \rightarrow \text{Si} + 2\text{MgCl}_2 \]
• The balanced equation indicates that two magnesium atoms are necessary to fully react with one silicon tetrachloride molecule.
• Magnesium serves as a reducing agent, helping to extract pure silicon from its compound.

This reaction is a step in the purification process of silicon, fundamental for making electronic chips and solar cells.