Ch.5 - Thermochemistry

Problem 116

Chapter 5, Problem 116

Depending on their specific usage, fuels are judged in part on energy released per unit volume and energy released per unit mass. Three prospective fuels are listed in the following table, along with their densities and molar enthalpies of combustion.
a. Rank the three fuels according to their enthalpy produced per gram.
b. Rank them according to their enthalpy produced per cm3:

Verified step by step guidance

Step 1: Identify the given data for each fuel, including its density and molar enthalpy of combustion.

Step 2: To rank the fuels by enthalpy produced per gram, calculate the enthalpy per gram for each fuel. Use the formula: \( \text{Enthalpy per gram} = \frac{\text{Molar enthalpy of combustion}}{\text{Molar mass}} \).

Step 3: Determine the molar mass of each fuel if not provided, using the chemical formula and atomic masses from the periodic table.

Step 4: To rank the fuels by enthalpy produced per cm³, calculate the enthalpy per cm³ for each fuel. Use the formula: \( \text{Enthalpy per cm}^3 = \text{Density} \times \text{Enthalpy per gram} \).

Step 5: Compare the calculated values from Steps 2 and 4 to rank the fuels accordingly for both enthalpy per gram and enthalpy per cm³.

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Key Concepts

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

Enthalpy of Combustion

Enthalpy of combustion is the amount of energy released when a substance undergoes complete combustion with oxygen. It is typically expressed in kilojoules per mole (kJ/mol) and is a crucial measure for evaluating the energy content of fuels. Understanding this concept allows for the comparison of different fuels based on their energy output during combustion.

Density

Density is defined as mass per unit volume, usually expressed in grams per cubic centimeter (g/cm³). In the context of fuels, density affects how much energy can be stored in a given volume. When ranking fuels based on energy produced per cm³, density becomes a critical factor, as it influences the total energy available in a specific volume of fuel.

Energy Density

Energy density refers to the amount of energy stored in a given system or region of space per unit volume or mass. It can be expressed as energy per unit mass (e.g., kJ/g) or energy per unit volume (e.g., kJ/cm³). This concept is essential for comparing fuels, as it helps determine which fuel provides more energy for a given weight or volume, impacting efficiency and practicality in various applications.

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980

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