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ch 9 - Chapter 9 Thermodynamics The First Law Work/Heat...

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Chapter 9 Thermodynamics: The First Law Work/Heat Reaction Enthalpies
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Combustion of Gasoline 2C 8 H 18 (l) + 25O 2 (g) Æ 18H 2 O (l) + 16CO 2 (g) H rxn = -1.09 x 10 4 kJ 5,450 kJ of energy released per mole of octane Combustion of 1 gallon of gas Æ 136 MJ energy
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Energy: capacity to do work or to produce heat Kinetic energy: motion of object, ½ mv 2 Potential energy: due to position or composition (bonds, height of a mass) Ex. Combustion of methane (CH 4 ): potential energy stored in bonds in converted to kinetic energy (thermal energy) via heat
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Kinetic energy versus potential energy
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CH 4 (g) + 2O 2 (g) Æ CO 2 (g) + 2H 2 O(l) H rxn = -890 kJ
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CH 4 (g) + 2O 2 (g) Æ CO 2 (g) + 2H 2 O(l) H rxn = -890 kJ
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N 2 (g) + O 2 (g) + energy Æ 2NO(g) endothermic
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H 2 O(s) + 6.02 kJ Æ H 2 O(l) Melting of ice
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First law of thermodynamics: The total energy of the universe is constant. Also known as “Law of Conservation of Energy”: energy is neither created nor destroyed in ordinary chemical reactions Energy in Joules
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A “system” Ex. System is a reaction mixture Surroundings = atmosphere around the reaction mixture
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Exothermic versus Endothermic
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The internal energy of a system E is the sum of kinetic and potential energies of all particles. The internal energy is the capacity of a system to do work; its total store of energy. E = q + w q is heat w is work Energy: Heat + Work
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Work • Work: change in energy when an object is moved through a distance Example of work: – compression or expansion of a gas W = -P ex V
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Varieties of Work
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A system doing work (i.e. expansion of a gas). Work is proportional to the external pressure and change in volume.
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w = -P V system In (b), the gas has expanded. The system has done work (-w).
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A gas in a cylinder of an engine expands by 500.0 mL against a pressure of 1.20 atm. How much work is done?
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Key Concepts of Thermodynamics E = q + w = heat + work w = motion against an opposing force = -P ex V exothermic: reactions that release energy as heat First Law: energy of universe is constant
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A balloon is inflated by heating the air inside it. The volume changes from 4.00 x 10 6 L to 4.50 x 10 6 L by addition of 1.3 x 10 8 J of energy as heat at 1 atm. Calculate E. A car engine does 520. kJ of work and loses 220 kJ. of heat. What is the change in energy of the engine?
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Enthalpy Heat: q, the energy transferred (in or out) as a result of a temperature difference q = C T w here q is heat, and C is heat capacity. For chemical reactions at constant pressure, H = change in enthalpy = q (heat released or absorbed) H > 0Endothermic H < 0 Exothermic
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