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Ch.7 Chemical Reactions: Energy, Rate and Equilibrium
McMurry - Fundamentals of GOB 8th Edition
McMurry8th EditionFundamentals of GOBISBN: 9780134015187Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.7 Chemical Reactions: Energy, Rate and EquilibriumProblem 25c
Chapter 7, Problem 25c

The vaporization of Br2 from the liquid to the gas state requires 7.4 kcal/mol (31.0 kJ/mol).
c. How many kilojoules are needed to evaporate 82 g of Br2?

Verified step by step guidance
1
Step 1: Calculate the molar mass of Br2. Bromine (Br) has an atomic mass of approximately 79.9 g/mol. Since Br2 is diatomic, its molar mass is calculated as 2 × 79.9 g/mol = 159.8 g/mol.
Step 2: Convert the given mass of Br2 (82 g) into moles using the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \). Substitute the values: \( \text{moles} = \frac{82}{159.8} \).
Step 3: Use the given heat of vaporization (31.0 kJ/mol) to calculate the total energy required. The formula is: \( \text{Energy} = \text{moles} \times \text{heat of vaporization} \). Substitute the values: \( \text{Energy} = \text{calculated moles} \times 31.0 \).
Step 4: Perform the multiplication to find the total energy in kilojoules. Ensure the units are consistent throughout the calculation.
Step 5: Report the final energy value in kilojoules, which represents the amount of energy needed to evaporate 82 g of Br2.

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Key Concepts

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

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). For bromine (Br2), the molar mass is approximately 159.8 g/mol, which is essential for converting grams of the substance to moles. This conversion is necessary to relate the amount of substance to the energy required for vaporization.
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Energy of Vaporization

The energy of vaporization is the amount of energy required to convert a unit quantity of a substance from liquid to gas at a constant temperature and pressure. For bromine, this value is given as 31.0 kJ/mol. Understanding this concept allows us to calculate the total energy needed for a specific mass of bromine to vaporize.
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Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the conservation of mass. In this context, it involves using the molar mass of Br2 to determine how many moles are present in 82 g, and then applying the energy of vaporization to find the total energy required. This concept is fundamental for solving quantitative problems in chemistry.
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Related Practice
Textbook Question

The following diagram portrays a reaction of the type A(s) → B(g) + C(g), where the different-colored spheres represent different molecular structures. Assume that the reaction has ∆H = +9.1 kcal/mol (+38.1 kJ/mol).

b. Is the reaction likely to be spontaneous at all temperatures, nonspontaneous at all temperatures, or spontaneous at some but nonspontaneous at others?

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Textbook Question

Is the total enthalpy (H) of the reactants for an endothermic reaction greater than or less than the total enthalpy of the products?

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Textbook Question

The vaporization of Br2 from the liquid to the gas state requires 7.4 kcal/mol (31.0 kJ/mol).

a. What is the sign of ∆H for this process? Write a reaction showing heat as a product or reactant.

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Converting liquid water to solid ice releases 1.44 kcal/mol (6.02 kJ/mol).How many kilocalories are released by freezing 32 g of H2O?
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Acetylene (H–C≡C–H) is the fuel used in welding torches.

b. Estimate ∆H for this reaction (in kJ/mol) using the bond energies listed in Table 7.1.

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b. Estimate ∆H for this reaction (in kcal and kJ) using the bond energies from Table 7.1.

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