Thermodynamics_Notes

Thermodynamics_Notes - Chapter 18 Thermodynamics and...

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Unformatted text preview: Chapter 18 Thermodynamics and Equilibrium 2 Enthalpy, H, introduced in Chapter 6. Change in enthalpy equals the heat of reaction at constant pressure . In this chapter we will define enthalpy more precisely, in terms of the energy of the system. Thermodynamics Thermodynamics: study of relationship between heat and other forms of energy in a chemical or physical process. 3 Internal energy, U : sum of the kinetic and potential energies of the particles making up the system. Internal energy is a state function- depends only on the systems present state. First Law of Thermodynamics Laws of thermodynamics require understanding of the internal energy of a system and how you can change it. 4 Thus, 1 mol of water at 0 o C and 1 atm pressure has a definite quantity of energy. When a system changes from one state to another, its internal energy changes. initial final U U U- = First Law of Thermodynamics Changes in U manifest themselves as exchanges of energy between the system and surroundings. These exchanges of energy are of two kinds; heat and work . 5 Heat is energy that moves into or out of a system because of a temperature difference between system and surroundings. Work , on the other hand, is the energy exchange that results when a force F moves an object through a distance d ; work (w) = F d First Law of Thermodynamics Remembering our sign convention. Work done by the system is negative . Work done on the system is positive . Heat evolved by the system is negative . Heat absorbed by the system is positive . 6 Figure 18.2 Figure 18.2 illustrates the distinction between heat and work. The system in Figure 18.2 gains internal energy from the heat absorbed and loses internal energy via the work done. 7 In general, the first law of thermodynamics states that the change in internal energy, U, equals heat plus work. w q U + = First Law of Thermodynamics 8 When gases are produced, they exert force on the surroundings as pressure. If the reaction is run at constant pressure, then the gases produced represent a change in volume analogous to a distance over which a force is exerted. Heat of Reaction and Internal Energy When a reaction is run in an open vessel (at constant P), any gases produced represent a potential source of expansion work . 9 You can calculate the work done by a chemical reaction simply by multiplying the atmospheric pressure by the change in volume, V. It follows therefore, that V P w - = Heat of Reaction and Internal Energy When a reaction is run in an open vessel (at constant P), any gases produced represent a potential source of expansion work. 10 For example, when 1.00 mol Zn reacts with excess HCl, 1.00 mol H 2 is produced....
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This note was uploaded on 02/26/2012 for the course CHEM 108 taught by Professor Dr.brennan during the Spring '09 term at Binghamton University.

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Thermodynamics_Notes - Chapter 18 Thermodynamics and...

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