Ch.4 - Chemical Reactions and Chemical Quantities

Tro - Chemistry: A Molecular Approach 5th Edition

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Tro 5th Edition

Ch.4 - Chemical Reactions and Chemical Quantities

Problem 31a

Chapter 4, Problem 31a

Hydrobromic acid dissolves solid iron according to the reaction:
Fe(s) + 2 HBr(aq) → FeBr₂(aq) + H₂(g)
What mass of HBr (in g) do you need to dissolve a 3.2-g pure iron bar on a padlock?

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Identify the balanced chemical equation: \( \text{Fe(s)} + 2 \text{HBr(aq)} \rightarrow \text{FeBr}_2\text{(aq)} + \text{H}_2\text{(g)} \).

Determine the molar mass of iron (Fe) and hydrobromic acid (HBr). The molar mass of Fe is approximately 55.85 g/mol, and the molar mass of HBr is approximately 80.91 g/mol.

Calculate the number of moles of iron in the 3.2 g sample using the formula: \( \text{moles of Fe} = \frac{\text{mass of Fe}}{\text{molar mass of Fe}} \).

Use the stoichiometry of the balanced equation to find the moles of HBr needed. According to the equation, 1 mole of Fe reacts with 2 moles of HBr.

Convert the moles of HBr to grams using the formula: \( \text{mass of HBr} = \text{moles of HBr} \times \text{molar mass of HBr} \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced equation. It allows us to determine the proportions of substances involved in a reaction. In this case, the stoichiometric coefficients from the balanced equation indicate that 2 moles of HBr are required for every mole of Fe, which is essential for calculating the mass of HBr needed.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is crucial for converting between the mass of a substance and the number of moles. For this problem, knowing the molar mass of HBr (approximately 80.91 g/mol) and Fe (approximately 55.85 g/mol) will help in determining how much HBr is needed to react with the given mass of iron.

Chemical Reaction Types

Understanding the type of chemical reaction is important for predicting the products and the behavior of reactants. The reaction between iron and hydrobromic acid is a single displacement reaction, where iron displaces hydrogen from HBr. Recognizing this helps in understanding the overall process and the stoichiometric relationships involved in the reaction.

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Textbook Question

Balance each chemical equation. b. Co(NO₃)₃(aq) + (NH₄)₂S(aq) → Co₂S₃(s) + NH₄NO₃(aq)

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Textbook Question

Consider the balanced equation:

SiO₂(s) + 3 C(s) → SiC(s) + 2 CO(g)

Complete the table showing the appropriate number of moles of reactants and products. If the number of moles of a reactant is provided, fill in the required amount of the other reactant, as well as the moles of each product that forms. If the number of moles of a product is provided, fill in the required amount of each reactant to make that amount of product, as well as the amount of the other product that forms.

Find the limiting reactant for each initial amount of reactants. 4 Al(s) + 3 O₂( g) → 2 Al₂O₃(s)

a. 1 mol Al, 1 mol O₂

b. 4 mol Al, 2.6 mol O₂

c. 16 mol Al, 13 mol O₂

d. 7.4 mol Al, 6.5 mol O₂

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