Ch5 Sample Problem #43

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PROBLEM: The enthalpy changes for the following reactions can be measured:
CH4(g) + 2 O2(g)  CO2(g)+ 2 H2O(g)
DH0r = –802.4 kJ / mol-rxn
CH3OH (g) + 3/2 O2(g)  CO2(g)+ 2 H2O(g)
DH0r = –676 kJ / mol-rxn
(a) Use these values and Hess’s law to determine the enthalpy change for the reaction
CH4 (g) + 1/2 O2(g)  CH3OH
(b) Draw an energy level diagram that shows the relationship between the energy
quantities involved in this problem.
Step 1: Hess’s Law problem – rearrange and add equations together
Take first equations as is; reverse second equation and reverse sign
CH4(g) + 2 O2(g)  CO2(g)+ 2 H2O(g)
DH0r = –802.4 kJ / mol-rxn
CO2(g)+ 2 H2O(g)  CH3OH (g) + 3/2 O2(g)
DH0r = –(–676 kJ / mol-rxn)
CH4 (g) + 1/2 O2(g)  CH3OH
Step 2: Draw the energy
level diagram
DH0r = – 126 kJ / mol-rxn)
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