Question

Over the years, the thermite reaction has been used for welding railroad rails, in incendiary bombs, and to ignite solid-fuel rocket motors. The reaction is $$ \mathrm{Fe}_{2} \mathrm{O}_{3}(s)+2 \mathrm{Al}(s) \longrightarrow 2 \mathrm{Fe}(l)+\mathrm{Al}_{2} \mathrm{O}_{3}(s) $$ What masses of iron(III) oxide and aluminum must be used to produce \(15.0 \mathrm{~g}\) iron? What is the maximum mass of aluminum oxide that could be produced?

Short answer

To produce 15.0 g of iron, 21.45 g of iron(III) oxide and 14.49 g of aluminum are required. The maximum mass of aluminum oxide that could be produced is 13.70 g.

Step-by-step solution

Find moles of Iron

Calculate the moles of iron (Fe) needed by dividing the mass of iron (15.0 g) by its molar mass (55.85 g/mol): moles of Fe = (15.0 g) / (55.85 g/mol) = 0.2686 mol

Find moles of iron(III) oxide and aluminum

Using the stoichiometry of the balanced chemical equation, determine the moles of iron(III) oxide (Fe2O3) and aluminum (Al) required to produce 0.2686 mol of iron (Fe): moles of Fe2O3 = moles of Fe / 2 moles of Fe2O3 = 0.2686 mol / 2 = 0.1343 mol moles of Al = 2 × moles of Fe moles of Al = 2 × 0.2686 mol = 0.5372 mol

Convert moles to mass

Find the masses of iron(III) oxide (Fe2O3) and aluminum (Al) needed by multiplying the moles found in the previous step by their respective molar masses: mass of Fe2O3 = (0.1343 mol) × (159.69 g/mol) = 21.45 g mass of Al = (0.5372 mol) × (26.98 g/mol) = 14.49 g

Find maximum mass of aluminum oxide

The balanced chemical equation states that 1 mole of Fe2O3 reacts with 2 moles of Al to produce 1 mole of Al2O3. Since we already know the moles of iron (Fe) formed, we can find the moles of aluminum oxide (Al2O3) that could be produced. moles of Al2O3 = moles of Fe2O3 = 0.1343 mol mass of Al2O3 = (0.1343 mol) × (101.96 g/mol) = 13.70 g Thus, to produce 15.0 g of iron, 21.45 g of iron(III) oxide and 14.49 g of aluminum are required. The maximum mass of aluminum oxide that could be produced is 13.70 g.

Key concepts

Chemical Reaction Balancing

When engaging with the chemistry of reactions, balancing the equation is a crucial first step. It ensures that the law of conservation of mass is obeyed, stating that mass is neither created nor destroyed in a chemical reaction. For the thermite reaction, the balanced equation is:
\[\begin{equation}\mathrm{Fe}_2\mathrm{O}_3(s) + 2\mathrm{Al}(s) \rightarrow 2\mathrm{Fe}(l) + \mathrm{Al}_2\mathrm{O}_3(s)\end{equation}\]
In this balanced equation, each element has the same number of atoms on both sides of the arrow. For instance, there are two atoms of iron and three of oxygen on the left side, matched with two atoms of iron in the product side and three oxygen atoms within the aluminum oxide molecule. Similar balance is noted with aluminum atoms. This equilibrium of atoms is fundamental for accurate stoichiometric calculations.

Stoichiometric Calculations

After the equation is balanced, stoichiometric calculations can predict the quantities of reactants and products involved in the reaction. Stoichiometry is rooted in the balanced chemical equation and involves the use of conversion factors, which relate the amounts of different substances in a reaction. As shown in the solution, finding the desired quantity of iron, Fe, first required conversion from mass to moles, a stoichiometric step that employs molar mass as a conversion factor. Then, by interpreting the stoichiometric coefficients of the equation, we can determine the ratios in which the reactants combine and the products form. In our case, the stoichiometric ratio between aluminum and iron is 2:2, and between iron(III) oxide and iron it is 1:2. These ratios help in calculating exact amounts of reactants needed for a certain amount of product.

Molar Mass

Molar mass, the mass of one mole of a substance expressed in grams per mole (g/mol), is fundamental to stoichiometry. It acts as a pivotal conversion factor between mass and moles. In the provided solution, the molar mass of iron (55.85 g/mol) was leveraged to convert 15.0 grams of iron to moles. Molar mass values for aluminum (26.98 g/mol), iron(III) oxide (159.69 g/mol), and aluminum oxide (101.96 g/mol) were similarly crucial for translating moles back into masses. Therefore, it's essential to use the correct molar mass for each compound in order to ensure the accuracy of mass-mole conversions which lay the groundwork for the subsequent stoichiometric steps.

Stoichiometric Coefficients

Stoichiometric coefficients are the numbers written in front of reactants and products in a balanced chemical equation. These numbers indicate the proportions in which substances react and form products. In our exercise, the stoichiometric coefficients are 1 for iron(III) oxide, and 2 for aluminum, telling us that one mole of iron(III) oxide reacts with two moles of aluminum. These coefficients are integral in stoichiometric calculations as they guide us in converting moles of one substance to moles of another. They play a critical role in all stoichiometric calculations ranging from predicting how much product will form from given quantities of reactants to calculating the amounts of reactants necessary for a desired quantity of product.