Skip to main content
Pearson+ LogoPearson+ Logo
Ch.5 - Thermochemistry
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 98a
Chapter 5, Problem 98a

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.

Verified step by step guidance
1
Calculate the mass of 1.0 L of gasoline using its density. Use the formula: mass = density × volume. Here, the density of gasoline is 0.70 g/mL, and the volume is 1000 mL (since 1.0 L = 1000 mL).
Calculate the number of moles of octane (C8H18) in 1.0 L of gasoline. Use the molar mass of octane, which is approximately 114 g/mol. The number of moles can be calculated using the formula: moles = mass / molar mass.
Calculate the total energy released from the combustion of the octane in 1.0 L of gasoline. Use the heat of combustion of octane, which is 5400 kJ/mol. The total energy can be calculated using the formula: total energy = moles × heat of combustion.
Calculate the mass of 1.0 L of ethanol using its density. Use the formula: mass = density × volume. Here, the density of ethanol is 0.79 g/mL, and the volume is 1000 mL.
Refer to Appendix C for the heat of combustion of ethanol, and calculate the total energy released from the combustion of ethanol in 1.0 L. Use the molar mass of ethanol, which is approximately 46 g/mol, to find the number of moles, and then calculate the total energy using the formula: total energy = moles × heat of combustion.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
6m
Was this helpful?

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 kilojoules per mole (kJ/mol). For gasoline, the average heat of combustion is 5400 kJ/mol, which indicates how much energy can be obtained from burning a specific amount of gasoline. Understanding this concept is crucial for comparing the energy output of different fuels.
Recommended video:
Guided course
02:24
Combustion Apparatus

Density and Volume Relationships

Density is defined as mass per unit volume and is a critical factor in determining how much of a substance can fit into a given volume. In this context, the densities of gasoline (0.70 g/mL) and ethanol (0.79 g/mL) will help calculate the mass of each fuel in 1.0 L. This relationship is essential for converting volume measurements into mass, which is necessary for calculating the total energy produced from combustion.
Recommended video:
Guided course
01:09
Relationship of Volume and Moles Example

Combustion Reaction Stoichiometry

Combustion reaction stoichiometry involves the balanced chemical equations that describe the combustion of fuels, such as ethanol and gasoline. For each fuel, the stoichiometry allows us to determine the moles of reactants and products involved in the reaction. This is important for calculating the total energy produced from a specific volume of fuel, as it links the amount of fuel burned to the energy released during combustion.
Recommended video:
Guided course
02:24
Combustion Apparatus
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 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?

1
views
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?

1
views
Textbook Question

A sample of gas is contained in a cylinder-and-piston arrangement. There is an external pressure of 100 kPa. The gas undergoes the change in state shown in the drawing. (b) Now assume that the cylinder and piston are made up of a thermal conductor such as a metal. During the state change, the cylinder gets colder to the touch. What is the sign of q for the state change in this case? Describe the difference in the state of the system at the end of the process in the two cases. What can you say about the relative values of E?