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Chem 300 - Ch 19/#1 Today’s To Do List

Chem 300 - Ch 19/#1 Today’s To Do List. Start Chapter 19: 1st Law P-V work State Functions 1st Law Adiabatic Processes. Thermodynamics. Based on 3 fundamental laws Natural laws Summaries of experimental facts No known exceptions Macroscopic Concerned with change in a system.

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Chem 300 - Ch 19/#1 Today’s To Do List

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  1. Chem 300 - Ch 19/#1 Today’s To Do List • Start Chapter 19: 1st Law • P-V work • State Functions • 1st Law • Adiabatic Processes

  2. Thermodynamics • Based on 3 fundamental laws • Natural laws • Summaries of experimental facts • No known exceptions • Macroscopic • Concerned with change in a system

  3. System & Surroundings • System • Part of world we are looking at • Surroundings • Rest of the universe

  4. 1st Law of Thermo • Deals with: • Conservation of Energy • Changes in energy of a system • expressible in terms of work and heat

  5. work & heat • Methods of energy transfer between a system and its surroundings: • Heat: • due to a temperature difference • Work: • due to unbalanced forces

  6. Heat Transfer • Surroundings ---->>> System • Positive quantity • Tsurr > Tsys • System --->>> Surroundings • Negative quantity • Tsurr < Tsys • Example: “hot” coffee cup, “cool” surroundings… • heat flow: cup-->surroundings

  7. Work is a Process: (a) by the system, (b) on the system

  8. PV Work • Consider a gas in a container (system) • apply an external force (in surroundings) to compress the gas • work (w) = force x displacement • pressure (P) = force/area

  9. PV work • w = -PextdV • at constant Pext • w = - Pext (Vfinal - Vinit) • If compression, Vfinal < Vinit & w > 0 • If expansion, Vfinal > Vinit & w < 0

  10. Work • depends upon the path • PV work depends upon value of Pext

  11. PV work: isothermal const-P compression at 2 different P’s

  12. Reversible work: minimum amount for compression

  13. Ideal Gas & PV Work • In general w = -PextdV • for any reversible process • P = f(V) in order to integrate • for IG • P = RT/Vm • w = -PextdV = -(RT/V)dV • If T = const (isothermal)

  14. Isothermal Reversible PV work for an IG • w = -RTdV/V = - RT ln(Vfin/Vinit) • Value of w depends on the path between Vinit & Vfin

  15. 2-Stage compression at constant-P

  16. Energy • A property of the system • A state function • Path independent

  17. 1st Law • U = q + w • U is state function (path independent) • q & w not state functions (they are path functions) • A system contains an amount of energy (U) but no work or heat. • For a process where q is transferred & w is done, the energy change for the system is U = q + w

  18. 3 paths to the same end-point

  19. Adiabatic Process • Adiabatic process: q = 0 • No heat transfer • Example: styrofoam cup

  20. Energy & Ideal Gas • For IG, U only depends on T • U = f (T) (prove this later) • Specifically: dU = Cv dT • C = heat capacity • U = Cv (Tf - Ti) • For isothermal process, U for IG is constant

  21. 3 paths to the same end-point for an IG

  22. Next Time • Adiabatic Processes & T • Enthalpy • More on Heat Capacity • Heats of Transition

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