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1 THERMODYNAMICS I. TERMS AND DEFINITIONS A. Review of Definitions 1. Thermodynamics = Study of the exchange of heat, energy and work between a system and its surroundings . a. System = That part of universe of interest. (reaction vessel, etc.) b. Surroundings = Rest of universe. c. Some types of systems. 1) Open systems = Mass can be exchanged in addition to heat, energy and work. These are important to engineers in flow systems 2) Closed systems = No mass can be exchanged, only heat, energy and work. These are usually encountered in the laboratory 3) Isolated system = No mass, heat or work can be exchanged. The universe is an example of an isolated system. d. Exchange takes place at boundary between system and surroundings during a change in state of system. 1) State = That condition in which all variables are fixed and unvarying. 2) When one or more of these variables are changed, the system changes state. 3) Examples: Temperature Changes Gas at V 1 , P 1 , T 1 -----------> Gas at V 2 ,P 2 , T 2 ( PV = nRT is the equation of state for an ideal gas ) Phase changes ( solid ----> liquid; liquid ---> gas: etc. ) Reactants ----------> Products 2. Can control some variables or conditions during a change. a. Isothermal Change = One at constant temperature (supply or take away just enough heat so that the temperature remains constant). b. Adiabatic Change = No heat is exchanged during change, that is , the system is insulated from its surroundings. c. Isobaric Change = One at constant pressure.
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2 d. Normal laboratory conditions are isothermal and isobaric ones. e. Reversible change = an idealized change carried out very slowly by allowing one restraining variable to be infinitesimally different from it’s equilibrium value; the system is essentially at equilibrium at all times during the change. B. State Functions ( or State Variables ). 1. Variables (properties) whose value depend only on the state of the system. a. Define the state. b. Value does not depend on the past history of the system. c. Examples: State functions -------- T, P, V, Energy. Not State functions --- work(W), heat change(q) 2. Thermodynamics is concerned with how the state variables change during a change of state. It views these changes in the light of three laws. Recall that in science a law is simply a summary of experience. C. Work ( W ) 1. Sense of W. W is positive when work is done on the system by the surroundings. When the system does work, W is negative; the system has to expend energy. 2. Some types of work. a. Mechanical work - exert a force through a distance. W = –( force )x( distance ) or W = – f ( Δ x) If force is not constant, W = – f d x b. Work of expansion of a gas under constant pressure. W = – P V 3. Dimension conversion. a. Because of the different formulas for calculating work, a number of different
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