Textbook Question
The standard enthalpies of formation of gaseous propyne (C3H4), propylene (C3H6), and propane (C3H8) are +185.4, +20.4, and -103.8 kJ/mol, respectively. (c) Which is the most efficient fuel in terms of heat evolved per unit mass?
Ch.5 - Thermochemistry
Chapter 5, Problem 98
The automobile fuel called E85 consists of 85% ethanol and 15% gasoline. E85 can be used in the so-called flex-fuel vehicles (FFVs), which can use gasoline, ethanol, or a mix as fuels. Assume that gasoline consists of a mixture of octanes (different isomers of C8H18), that the average heat of combustion of C8H18 is 5400 kJ/mol, and that gasoline has an average density of 0.70 g/mL. The density of ethanol is 0.79 g/mL. (b) Assume that the density and heat of combustion of E85 can be obtained by using 85% of the values for ethanol and 15% of the values for gasoline. How much energy could be released by the combustion of 1.0 L of E85? (c) How many liters of E85 would be needed to provide the same energy as 40 L of gasoline?
Verified step by step guidance1
Step 1: Calculate the mass of ethanol and gasoline in 1.0 L of E85. Use the densities provided: ethanol has a density of 0.79 g/mL and gasoline has a density of 0.70 g/mL. Since E85 is 85% ethanol and 15% gasoline, calculate the mass of each component in 1.0 L (1000 mL) of E85.
Step 2: Convert the mass of ethanol and gasoline to moles. Use the molar mass of ethanol (C2H5OH, approximately 46.07 g/mol) and the molar mass of octane (C8H18, approximately 114.22 g/mol) to find the number of moles of each component in the calculated masses from Step 1.
Step 3: Calculate the energy released by the combustion of the moles of ethanol and gasoline. Use the heat of combustion values: ethanol has a heat of combustion of approximately 1367 kJ/mol, and gasoline (octane) has a heat of combustion of 5400 kJ/mol. Multiply the moles of each component by their respective heat of combustion to find the energy released.
Step 4: Calculate the total energy released by the combustion of 1.0 L of E85. Since E85 is a mixture, sum the energy contributions from both ethanol and gasoline calculated in Step 3.
Step 5: To find how many liters of E85 are needed to provide the same energy as 40 L of gasoline, first calculate the energy released by 40 L of gasoline using its density and heat of combustion. Then, divide this energy by the total energy released per liter of E85 (calculated in Step 4) to find the required volume of E85.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Heat of Combustion
The heat of combustion is the amount of energy released when a substance undergoes complete combustion with oxygen. It is typically expressed in kJ/mol and varies for different fuels. For example, the average heat of combustion for octane (C8H18) is 5400 kJ/mol, which indicates how much energy can be harnessed from burning a specific amount of this fuel.
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Guided course
02:24
Combustion Apparatus
Density and Volume Relationships
Density is defined as mass per unit volume and is crucial for converting between mass and volume in chemical calculations. In the context of E85, knowing the densities of ethanol (0.79 g/mL) and gasoline (0.70 g/mL) allows us to calculate the mass of each component in a given volume of E85, which is essential for determining the total energy released during combustion.
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01:09Relationship of Volume and Moles Example
Mixture Properties
When dealing with mixtures like E85, the properties of the mixture can be derived from the properties of its components. In this case, the density and heat of combustion of E85 can be calculated by taking a weighted average based on the proportions of ethanol and gasoline (85% and 15%, respectively). This approach is fundamental for analyzing the energy output of mixed fuels.
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02:54Physical Properties
Related Practice
Textbook Question
It is interesting to compare the 'fuel value' of a hydrocarbon in a hypothetical world where oxygen is not the combustion agent. The enthalpy of formation of CF4(g) is -679.9 kJ/mol. Which of the following two reactions is the more exothermic?
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
CH4(g) + 4 F2(g) → CF4(g) + 4 HF(g)
Textbook Question
At the end of 2012, global population was about 7.0 billion people. What mass of glucose in kg would be needed to provide 1500 Cal/person/day of nourishment to the global population for one year? Assume that glucose is metabolized entirely to CO2(𝑔) and H2O(𝑙) according to the following thermochemical equation: C6H12O6(s) + 6 O2(𝑔) → 6 CO2(𝑔) + 6 H2O(𝑙) ΔH° = -2803 kJ
Textbook Question
The automobile fuel called E85 consists of 85% ethanol and 15% gasoline. E85 can be used in the so-called flex-fuel vehicles (FFVs), which can use gasoline, ethanol, or a mix as fuels. Assume that gasoline consists of a mixture of octanes (different isomers of C8H18), that the average heat of combustion of C8H18(l) is 5400 kJ/mol, and that gasoline has an average density of 0.70 g/mL. The density of ethanol is 0.79 g/mL. (a) By using the information given as well as data in Appendix C, compare the energy produced by combustion of 1.0 L of gasoline and of 1.0 L of ethanol.
Textbook Question
The air bags that provide protection in automobiles in the event of an accident expand because of a rapid chemical reaction. From the viewpoint of the chemical reactants as the system, what do you expect for the signs of q and w in this process?
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Textbook Question
Consider a system consisting of the following apparatus, in which gas is confined in one flask and there is a vacuum in the other flask. The flasks are separated by a valve. Assume that the flasks are perfectly insulated and will not allow the flow of heat into or out of the flasks to the surroundings. When the valve is opened, gas flows from the filled flask to the evacuated one. (a) Is work performed during the expansion of the gas? (b) Why or why not?
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