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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 97c
Chapter 5, Problem 97c

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?

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1
Identify the molecular weights of propyne (C3H4), propylene (C3H6), and propane (C3H8). Calculate these by summing the atomic masses of carbon (12.01 g/mol) and hydrogen (1.008 g/mol) for each molecule.
Calculate the heat evolved per mole for each compound using their respective standard enthalpies of formation. Remember that a negative value indicates heat is released, thus a more negative value means more heat is evolved.
Convert the heat evolved per mole to heat evolved per gram for each compound. This is done by dividing the heat evolved per mole (from step 2) by the molecular weight (from step 1).
Compare the values obtained in step 3 for each compound. The compound with the highest negative value (or least positive value) per gram will be the most efficient fuel in terms of heat evolved per unit mass.
Conclude which compound is the most efficient fuel based on the calculations and comparisons made in the previous steps.

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Key Concepts

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

Enthalpy of Formation

The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It provides insight into the stability of a compound; a lower (more negative) enthalpy indicates a more stable compound. In this context, it helps compare the energy content of different fuels.

Heat of Combustion

The heat of combustion is the amount of energy released when a substance is burned in oxygen. It is typically expressed in kJ/mol and is crucial for evaluating the efficiency of fuels. The higher the heat of combustion per unit mass, the more energy is available for work, making it a key factor in determining fuel efficiency.
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Energy Density

Energy density refers to the amount of energy stored in a given system or region of space per unit volume or mass. For fuels, it is often expressed in kJ/kg. Understanding energy density is essential for comparing fuels, as it indicates how much energy can be obtained from a specific mass, which is critical for applications in energy production and transportation.
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Related Practice
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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Textbook Question

The heat of combustion of ethanol, C2H5OH(l), is -1367 kJ/mol. A batch of Sauvignon Blanc wine contains 10.6% ethanol by mass. Assuming the density of the wine to be 1.0 g/mL, what is the caloric content due to the alcohol (ethanol) in a 6-oz glass of wine (177 mL)?

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Textbook Question

The heat of combustion of fructose, C6H12O6, is -2812 kJ/mol. If a fresh golden delicious apple weighing 4.23 oz (120 g) contains 16.0 g of fructose, what caloric content does the fructose contribute to the apple?

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Textbook Question

Two positively charged spheres, each with a charge of 2.0⨉10-5 C, a mass of 1.0 kg, and separated by a distance of 1.0 cm, are held in place on a frictionless track. (a) What is the electrostatic potential energy of this system?

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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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