POGIL.CH6C.Tro

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CHEM 161
Hess’ Law and
Enthalpy Worksheet
Name ________________
Partners ________________
INSTRUCTIONS: Complete the worksheet with your group using a black or blue pen or pencil. Be prepared to
share your answers. When answered are shared with the rest of the class, use only a red pen.
1. One of the principle components of acid rain is sulfuric acid, which is formed from the combustion of
sulfur, a contaminant in coal, in a coal-fired power plant. The overall reaction is:
S(s) + 3/2 O2 (g) + H2O (l) → H2SO4 (aq)
(eqn 1)
The reaction above does not represent what really happens since the acid rain forms in multiple steps as
it is released from the smoke stack and then interacts with water droplets in clouds. The following
reactions represent a more realistic reaction pathway:
S(s) + O2 (g) → SO2 (g)
ΔH2 = -296.8 kJ
(eqn 2)
2 SO2 (g) + O2 (g) → 2 SO3 (g)
ΔH3 = -198.4 kJ
(eqn 3)
SO3 (g) + H2O (l) → H2SO4 (aq)
ΔH4 = -225.7 kJ
(eqn 4)
a. Calculate the ΔHrxn for production of sulfuric acid using the overall reaction (eqn 1) and standard
heats of formation (back of your textbook).
b. Using Hess’s Law, calculate the ΔHrxn for production of sulfuric acid using only the eqn’s 2-4.
c. Do the two calculations produce the same result?
CHEM 161 - F2016
Dr. Overway
Page 1 of 2
106749943
General Chemistry I
2. Ethanol (C2H5OH) production (via fermentation of corn) is currently subsidized with tax dollars in order
to decrease our nation’s dependence on foreign petroleum (at least that is one argument for the subsidy).
Gasoline, which is principally octane (C8H18), is distilled from petroleum. The following questions will
compare the two as a fuel source for your car.
a. Write a balanced combustion reaction for liquid ethanol. Assume water is gaseous in the reaction.
b. Determine the Hrxn for the combustion reaction by using the standard heats of formation.
c. If it takes 118 MJ to move an average car 32 miles down the road, calculate how many gallons of
ethanol are required in the gas tank if the density of ethanol is 0.7892 g/mL and 3.7854 L = 1 gal?
d. Write a balanced combustion reaction for liquid octane. Assume water is gaseous in the reaction.
e. Determine the Hrxn for the combustion reaction by using the standard heats of formation. The H°f
of octane is -249.95 kJ/mol.
f. If it takes 118 MJ to move an average car 32 miles down the road, calculate how many gallons of
octane are required in the gas tank if the density of octane is 0.703 g/mL and 3.7854 L = 1 gal?
g. Which fuel has a higher energy density (more energy per gallon)?
CHEM 161 - F2016
Dr. Overway
Page 2 of 2
106749943
General Chemistry I
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