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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 system’s 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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Thermodynamics_Notes - Chapter 18 Thermodynamics and...

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