GenChem1Chap6 - 6 Thermochemistry First Law The energy of...

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6. Thermochemistry First Law : The energy of the universe is constant. three types of sys tems isolated : no interaction at all with surroundings example: Dewar flask with absolutely rigid walls in a Faraday cage on a space station idealization, but useful closed : exchange of energy, no exchange of matter example: photochemical batch reactor open : exchange of matter and energy example: living systems GChem I 6.1
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state of the system: state variables : P , V , T , . . . state function : a quantity that depends on ly on the current state of the system path function : a quantity that depends on the history of the system, i.e., on the process or path taken to achieve the current state P T state function = function( ) path function = function( ) when the state of a system changes, the magnitude of change in any state function depends on ly on the initial and final states and not on how the change is accomplished GChem I 6.2
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example: Δ V = V f - V i state function: internal energy E energy = capacity to do work mechanics: work = force × distance (parallel) unit of energy = unit of work = N m = J Joule, ( N = kg m s - 2 ) energy exchange between system and surroundings: two types heat q and work w First Law of Thermodynamics Δ E = E f - E i = q + w A system contains only internal energy E . We can distinguish between heat and work on ly when the energy is exchanged be- tween the system and the surroundings. sign con ven tion: viewpoint of the system w positive: work done on the system w negative: work done by the system q positive: heat gained by the system q negative: heat evolved (lost) by the system GChem I 6.3
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work w : expansion/compression of a system P P P compression initial expansion pressure-volume work (for P = const) w = - P Δ V = - P ( V f - V i ) use SI units!!
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