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Ch.5 - Thermochemistry
Chapter 5, Problem 111

From the following data for three prospective fuels, calculate which could provide the most energy per unit mass and per unit volume:

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insert step 1: Identify the data provided for each fuel, such as the energy content per unit mass (e.g., kJ/g) and the density (e.g., g/mL or kg/L).
insert step 2: Calculate the energy per unit mass for each fuel by using the given energy content per unit mass. This will help determine which fuel provides the most energy per gram.
insert step 3: Calculate the energy per unit volume for each fuel by multiplying the energy per unit mass by the density of the fuel. This will help determine which fuel provides the most energy per liter or milliliter.
insert step 4: Compare the calculated energy per unit mass and energy per unit volume for each fuel to identify which one provides the most energy in each category.
insert step 5: Conclude which fuel is the most efficient based on both energy per unit mass and energy per unit volume, considering the context of the application (e.g., transportation, storage).

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Key Concepts

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

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 is a crucial concept in evaluating fuels, as it determines how much energy can be extracted from a specific quantity of fuel. Higher energy density means that less fuel is needed to produce the same amount of energy, making it more efficient for applications like transportation.
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Calorimetry

Calorimetry is the science of measuring the heat of chemical reactions or physical changes. In the context of fuels, calorimetry helps determine the heat of combustion, which is the energy released when a fuel is burned. Understanding calorimetry allows for the comparison of different fuels based on their energy output, which is essential for identifying the most efficient fuel.
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Specific Energy vs. Volumetric Energy Density

Specific energy is the amount of energy per unit mass of fuel, while volumetric energy density is the amount of energy per unit volume. These two metrics are important for evaluating fuels in different contexts; for instance, specific energy is critical for weight-sensitive applications like aviation, whereas volumetric energy density is more relevant for storage and transportation considerations. Analyzing both helps in making informed decisions about fuel selection.
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Related Practice
Open Question
The corrosion (rusting) of iron in oxygen-free water includes the formation of iron(II) hydroxide from iron by the following reaction: Fe(s) + 2 H2O(l) → Fe(OH)2(s) + H2(g). (b) Calculate the number of grams of Fe needed to release enough energy to increase the temperature of 250 mL of water from 22 to 30 °C.
Open Question
Burning acetylene in oxygen can produce three different carbon-containing products: soot (very fine particles of graphite), CO(g), and CO2(g). (a) Write three balanced equations for the reaction of acetylene gas with oxygen to produce these three products. In each case assume that H2O(l) is the only other product. Determine the standard enthalpies for the reactions in part (a).
Textbook Question

We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethane: 2 CH4(g) → C2H6(g) + H2(g) Calculate the ΔH° for this reaction using the following thermochemical data: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH° = -890.3 kJ 2 H2(g) + O2(g) → 2 H2O(l) H° = -571.6 kJ 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) ΔH° = -3120.8 kJ

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Open Question
The hydrocarbons cyclohexane (C6H12), ΔHf° = -156 kJ/mol, and 1-hexene (C6H12), ΔHf° = -74 kJ/mol, have the same empirical formula. (a) Calculate the standard enthalpy change for the transformation of cyclohexane to 1-hexene. (b) Which has greater enthalpy, cyclohexane or 1-hexene?
Textbook Question
When magnesium metal is burned in air (Figure 3.6), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. (e) The standard enthalpy of formation of solid magnesium nitride is -461.08 kJ>mol. Calculate the standard enthalpy change for the reaction between magnesium metal and ammonia gas.
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Open Question
Three hydrocarbons that contain four carbons are listed here, along with their standard enthalpies of formation: Hydrocarbon Formula ΔHfº (kJ/mol) Butane C4H10(g) -125 1-Butene C4H8(g) -1 1-Butyne C4H6(g) 165. (a) For Butane, calculate the molar enthalpy of combustion to CO2(g) and H2O(l). (kJ/mol) (b) For 1-Butene, calculate the molar enthalpy of combustion to CO2(g) and H2O(l). (kJ/mol) (c) For 1-Butyne, calculate the molar enthalpy of combustion to CO2(g) and H2O(l).