I included a picture of equation 7.15 (Tro pg. 292) which is metioned below
a). What are the three major problems associated with the emissions of fossil fuel combustion?
- One renewable energy fuel source is hydrogen. Hydrogen fuel cells use the chemical energy of hydrogen to cleanly and efficiently produce electricity. However, hydrogen is usually “tied-up” in other molecules such as hydrocarbons and must be harvested. The two main ways this is done is by (1) “splitting” water to make hydrogen gas or (2) reforming natural gas (methane). However, process 2 means we are using fossil fuels (non-renewable energy sources) to create renewable energy.
Using the standard enthalpies of formation in the table below and equation 7.15 (Tro pg. 292), calculate ΔHrxn for reaction 2.
Substance
ΔHf°(kJ/mol)
CH4(g)
-74.6
H2O(g)
-241.8
H2O(l)
-285.8
CO2(g)
-393.5
CO2(aq)
-413.8
b. compare the reaction enthalpies for splitting water and for reforming natural gas.
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As we know from Hess's law, the enthalpy of reaction for the overall reaction is the
sum of the enthalpies of reaction of the individual steps:
(1) CH,(g) Cisgraphite) + 211
-AH= +74.6 kJ/mol
(3)0 (8FO + 5)) (
AH = -393.5 kJ/mol
2 x AH= -483.6 kJ/mol
(2b) 2g) + Og) 2 H,0(g)
2 HO(g)
CH.() + 20g) CO.(g) + 2 H,0(g)
AHA=-802.5 kJ/mol
We can streamline and generalize this process as follows:
To calculate AHan, subtract the enthalpies of formation of the
reactants multiplied by their stoichiometric coefficients from
the enthalpies of formation of the products multiplied by their
stoichiometric coefficients.
In the form of an equation:
AH = E,AH (products) - En AH| (reactants)
In this equation, , represents the stoichiometric coefficients of the products, n,
represents the stoichiometric coefficients of the reactants, and AH; represents the standard
enthalpies of formation. Keep in mind when using this equation that elements in their
standard states have AH¡ = 0. Examples 7.11 and 7.12 demonstrate this process.
IMON HHM
INTERACTIVE WORKED EXAMPLE 7.11
EXAMPLE 7.11
AH and Standard Enthalpies of Formation
Lise the standard enthalpies of formation to determine A/H, for the reaction:
(8)FOS + (8)*HN
()0'H9 + ()ON •
SORT You are given the balanced equation
and asked to find the enthalpy of reaction.
(3)0H9 + ()ON Fos + HN+ N
STRATEGIZE To calculate AH from
standard enthaipies of formation, subtract the
heats of formation of the reactants multiplied
by their stoichiometric coefficients from the
heats of formation of the products multiplied
CONCEPTUAL PLAN
AH = En, AH (products) - ENAH (reactants)
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