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Published byGavin Sutton Modified over 6 years ago

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

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

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System & Surroundings l System Part of world we are looking at l Surroundings Rest of the universe

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

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

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Heat Transfer l Surroundings ---->>> System Positive quantity T surr > T sys System --->>> Surroundings Negative quantity T surr < T sys Example: “hot” coffee cup, “cool” surroundings… heat flow: cup-->surroundings

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

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

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PV work w = - P ext dV l at constant P ext w = - P ext (V final - V init ) If compression, V final 0 If expansion, V final > V init & w < 0

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Work l depends upon the path l PV work depends upon value of P ext

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

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Reversible work: minimum amount for compression

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Ideal Gas & PV Work In general w = - P ext dV l for any reversible process P = f(V) in order to integrate l for IG P = RT/V m w = - P ext dV = - (RT/V)dV l If T = const (isothermal)

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Isothermal Reversible PV work for an IG w = -RT dV/V = - RT ln(V fin /V init ) l Value of w depends on the path between V init & V fin

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2-Stage compression at constant-P

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Energy l A property of the system l A state function Path independent

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1st Law l 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

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3 paths to the same end-point

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Adiabatic Process l Adiabatic process: q = 0 No heat transfer l Example: styrofoam cup

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Energy & Ideal Gas l For IG, U only depends on T U = f (T) (prove this later) Specifically: dU = C v dT C = heat capacity l U = C v (T f - T i ) l For isothermal process, U for IG is constant

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3 paths to the same end-point for an IG

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Next Time l Adiabatic Processes & T l Enthalpy l More on Heat Capacity l Heats of Transition

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