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Lecture 17 - Enthalipies, Fuel and Hess's Law

# Lecture 17 - Enthalipies, Fuel and Hess's Law - Chapter 5...

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1 Chapter 5 Enthalpies, Fuel and Hess’s Law Fuel and food values Until now, we have compared enthalpies on a per-mole basis For consumer products, we usually make decisions based on masses or volumes We can do simple unit conversions in order to calculate energy produced or consumed in terms of mass or volume Use molar mass to get grams, and then use density if you need volume

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2 Fuel value : energy per gram Which produces more heat of combustion per gram , CH 4 or C 3 H 8 ? 1) Calculate heat of combustion per mole 2) Convert to units of grams 3) Compare results CH 4 + 2O 2 Æ CO 2 + 2H 2 O Which produces more heat of combustion per gram , CH 4 or C 3 H 8 ? CH 4(g) + 2O 2 (g) Æ CO 2 (g) + 2H 2 O(g) ' H = -393.5 +2(-241.8) – (-74.8) -2(0) = -802.3kJ/mol -802.3kJ/mol (1mol/16.04g) = 50.08 kJ/g C 3 H 8 (g) + 5O 2 (g) Æ 3CO 2 (g) + 4H 2 O(g) ' H = 3(-393.5)+4(-241.8) –(-103.8) -5(0) = -2043.9kJ/mol -2043.9kJ/mol (1mol/44.1g) = 46.35 kJ/g
3 Fuel Values * Based on the formation of H 2 O (g) 45.8 C 4 H 10 Butane 46.3 C 3 H 8 Propane 47.6 C 2 H 6 Ethane 50.0 CH 4 Methane Fuel Value ( kJ/g )* Molecular Formula Compound Does the fuel value help at the pump?

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4 Fuel Density – energy per liter Which produces more energy per liter , gasoline (C 9 H 20 ) or diesel (C 14 H 30 )? C 9 H 20 ' H combust = 6160 kJ/mol, density=0.718 g/ml C 14 H 30 ' H combust =7940 kJ/mol, density=0.763 g/ml C 9 H 20 : 6160 kJ/mol (1mol/128.25g)(0.718g/ml)(1000ml/L) = 34,500 kJ/L C 14 H 30 : 7940 kJ/mol (1mol/198.38g)(0.763g/ml)(1000ml/L) = 30,500 kJ/L Which has the better value of kJ per dollar , regular gasoline (C 9 H 20 ) or diesel (C 14 H 30 )? (1gal=3.79L) C 9 H 20 ' H combust = 6160 kJ/mol, density=0.718 g/ml
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