   Chapter 20, Problem 47GQ

Chapter
Section
Textbook Problem

Here we want to compare ethanol (C2H5OH) and gasoline as fuels. We will use isooctane (C8H18) as a substitute for gasoline. (Isooctane is one of the many hydrocarbons in gasoline, and its enthalpy of combustion will approximate the energy obtained when gasoline bums. See Study Question 22.) (a) Calculate ΔrH° for the combustion of 1.00 kg each of liquid ethanol and liquid isooctane. Which fuel releases more energy per kilogram? [ΔfH° = −259.3 kJ/mol for liquid isooctane at 298 K.] (Assume H2O(ℓ) is one product of combustion.) (b) Compare the two fuels on the basis of the release of CO2, a common greenhouse gas. Which fuel produces more CO2 per kilogram? (c) On the basis of this simple comparison and neglecting the energy costs involved in producing 1.00 kg each of ethanol and isooctane, which is the better fuel in terms of energy production and greenhouse gases?

(a)

Interpretation Introduction

Interpretation: The enthalpy of combustion of ethanol and isooctane, the fuel which release more CO2 and better fuel in terms of energy production should be given.

Concept introduction:

Enthalpy of combustion: At standard conditions when one mole of substance is burnt completely in presence of oxygen, the enthalpy change involved is called enthalpy of combustion.

ΔH0comb=nΔH0fproducts-nΔH0freactants

Explanation

Given the combustion of ethanol and isooctane:

EthanolC2H5OH(l)+3O2(g)2CO2(g)+3H2O(l)

ΔH0fof CO2=-393.509kJ/molΔH0fof H2O=-285.83kJ/molΔH0fofC2H5OH=-277kJ/molΔH0fofO2=0kJ/mol

Enthalpy of combustion can be calculated by the equation:

ΔH0comb=nΔH0fproducts-nΔH0freactants

ΔH0comb=[2×-393.509+3×-285.83]-[-277]=-1367

(b)

Interpretation Introduction

Interpretation: compare the enthalpy of combustion of ethanol and isooctane and the fuel which release more CO2 per kilogram should be determined.

Concept introduction:

Enthalpy of combustion: At standard conditions when one mole of substance is burnt completely in presence of oxygen, the enthalpy change involved is called enthalpy of combustion.

ΔH0comb=nΔH0fproducts-nΔH0freactants

(c)

Interpretation Introduction

Interpretation: The better fuel in terms of energy production and greenhouse gases should be determined.

Concept introduction:

Enthalpy of combustion: At standard conditions when one mole of substance is burnt completely in presence of oxygen, the enthalpy change involved is called enthalpy of combustion.

ΔH0comb=nΔH0fproducts-nΔH0freactants

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