Unformatted text preview: Th=398K. -6 points if you tried to use free energy/enthalpy to calculate
this. 7. (65 points) In the atmosphere, NO 2 can dimerize to create N2O4, according to the reaction: 2 NO2
In areas with a high concentration of NO2/N2O4, on cold days the air is crisp and clear but on hot days it becomes tinged brown. Which molecule, NO2
or N2O4, is responsible for this brownish tinge? To help you answer this question, work through the following:
a) The heat of formation of NO2 is approximately 8 kcal/mol and the heat of formation of N2O4 is approximately 2 kcal/mol. Calculate ∆H
for this reaction. Is the dimerization as written exothermic or endothermic? (circle one) (7.5 points)
ΔH = (2 kcal/mol – 2*8 kcal/mol) = -‐14 kcal/mol (10 points) b) What is the sign of ΔS for the reaction as written? Circle one: + or – or 0 (7.5 points) (Since ΔH for this reaction is negative, this would suggest that the change in entropy is also negative; if it was positive then the equilibrium would always favor N2O4 and we would not expect to see a change in the air quality.) c) What do your answers to (a) and (b)...
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