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Chap9_Zumdahl-3 - THERMODYNAMICS Thermodynamics addresses...

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THERMODYNAMICS Thermodynamics addresses three questions: Will a reaction occur? If a reaction occurs, what energy changes are involved? If a reaction occurs, to what extent does it occur (will it go to completion)? Note that thermodynamics is not concerned with the rate of a reaction (how fast it goes)! THE NATURE OF ENERGY Energy is the capacity to do work or to produce heat. The law of conservation of energy states: “ Energy can be converted from one form to another but can neither be destroyed nor created”. Energy can be classifies as either kinetic or potential. Potential energy is energy due to position or composition. Kinetic energy is energy due to motion. Energy can be easily converted from one form to the other. There are two ways to transfer energy work and heat. State Function It refers to a property of the system whose value depends only on the state of the system and not on how it got to this state. A change in a state function describes a difference between the two states and is independent of the process or pathway that leads to this change. Energy is a state function but work and heat are not. The amount of work depends on the process and so does the heat. State functions are represented in capital letters. Properties of a system such as P, V, and T are also state functions. Chemical Energy 1
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System Substance(s) involved in the chemical or physical changes. A system can be open, closed, or isolated. An open system can exchange both matter and energy with its surroundings. Example: the water in an open container. A closed system can exchange only energy with its surroundings. Example: the ice in a closed pack of ice. An isolated system can exchange neither matter nor energy. The ice in a closed pack of ice that is inside a sealed vacuum flask is a good approximation of an isolated system. Surroundings Everything else in the environment, the flask, the air surrounding it etc …. Universe The system and its surroundings…. Internal Energy (E or U) In thermodynamics, the internal energy of the system U or E can be defined as the sum of the kinetic and potential energy of all particles in the system. The absolute internal energy of a system can not be measured because it includes the energy of all the atoms, the electrons, the nuclei. Therefore, we can only talk about change in internal energy. The internal energy of a system changes by doing work and by heat transfer: this U or E = q + w. The First Law of Thermodynamics states that “ the energy of the universe is constant”. Heat The internal energy of a system can be changed by heat transfer. The SI unit for measuring energy (heat is a form of energy) is Joules. However, in 2
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biochemistry related fields, we still see the use of calories (1 cal = 4.184 J).
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