Systems and surroundings, state functions, work and heat, internal energy and the first law of thermodynamics, with work done by an expanding gas.
3 lessons, about 1 h 15 min3 simulations10 problems3 practice questionsPython in every lesson
Created by StemPy LabThermodynamics studies energy changes in bulk matter. We pick a system and call the rest the surroundings. A system is open, closed or isolated, by what can cross its boundary. Its state is fixed by a few state variables. A state function depends only on the state, not on how it was reached; heat and work are not state functions.
Internal energy U is the total energy stored in a system. It is a state function. It changes when work is done on or by the system, or when heat flows in or out. Chemistry counts energy that goes into the system as positive: q > 0 for heat in and w > 0 for work done on the system.
The first law says that energy is conserved: for a closed system, delta U = q + w. A gas expanding against a constant outside pressure does work w = -p_ex delta V, and 1 L bar = 100 J. Into a vacuum it does no work. A reversible isothermal expansion of an ideal gas does the most work, w = -2.303 nRT log(Vf/Vi), and for it delta U = 0.
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