chapter 10.ppt
TRANSCRIPT
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Chapter 10Spontaneity, Entropy, and Free Energy
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Concept for second law of thermodynamic
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Isothermal expansion device
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One-Step Expansion (No Work)Mass M1 is removed from the pan, the gas will expand, moving the piston to the right end of the cylinder.P11/4P1, V14V1, No work is done. W0=0Free expansion
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One-Step ExpansionM1 is replaced by M1/4.
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Two-Step ExpansionP11/2P1, V12V1
1/2P11/4P1, 2V14V1
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PV diagram two-step expansion
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The PV diagram six-step expansion
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Infinite-Step Expansion(dV: V0 )
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Reversible expansion
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Reversible ProcessReversible process: the system is always infinitesimally close to equilibrium, and an infinitesimal change in conditions can reverse the process to restore both system and surroundings to their initial states.
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A heat engine operating between two temperatures
Hot Reservoir at H
qH
Heat Engine -W
-qC
Cold Reservoir at C
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Heat EnginesA heat engine converts some of the random molecular energy of heat flow into macroscopic mechanical energy.qH: the working substance from a hot body-w: the performance of work by the working substance on the surroundings-qC: the emission of heat by the working substance to a cold body
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The Second Law of Thermodynamics Kelvin-Planck statement for heat engine It is impossible to extract an amount of heat qH from a hot reservoir and use it all to do work W. Some amount of heat qC must be exhausted to a cold reservoir. This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law.
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A system that violates the second law
Heat Reservoir
Heat q
Cyclic Machine
Work Output=q
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Heat Efficiency
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The Second Law of ThermodynamicsClausius statement for refrigerator It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. The statements about refrigerators apply to air conditioners and heat pumps which embody the same principles.
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Carnots PrincipleNo heat engine can be more efficient than a reversible heat engine when both engines work between the same pair of temperature tH and tC.Isothermal Process: the temperature of the system and the surroundings remain constant at all times. (q=-w)Adiabatic: a process in which no energy as heat flows into or out of the system. (U=w)
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Carnot cycleCarnot cycle is a four stage reversible sequence consisting of 1. isothermal expansion at high temperature T2 2. adiabatic expansion 3. isothermal compression at low temperature T1 4. adiabatic compression5. back to stage 1 and continue.
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Carnot Efficiency
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Carnot Efficiency
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Adiabatic Process
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U=0 for an isothermal process, q=-w
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No heat transfer (q=0) for adiabatic process, U=w
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Adiabatic ProcessProcess in which no heat transfer takes place
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Application of Carnot CycleCalculate Q, U, W First law: U = QH QL + W W = QL - QH
P (atm)V (L)3101.520125.5212.75
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Spontaneous Process and EntropySpontaneous Process: A process occurs without outside intervention.Entropy: In qualitative terms, entropy can be viewed as a measure of randomness or disorder of the atoms or molecules in a substance.
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Definition of EntropyS=kBlnkB: Boltzmanns constant : the number of microstatescorresponding to a given state
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For one particleS1=kBln1S2=kBln2S=S2-S1= kBln2-kBln1=kBln(2/1)S= kBln(21/1)=kBln2
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M104
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4224=16M104
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M104
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Definition of entropy in term of probability
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Entropy for Isothermal Process
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Quantity of Entropy for Reversible Process
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Entropy and Physical ChangesTemperature Dependence
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Entropy and Physical ChangesChange of StateChange of state from solid to liquidqrev=HfusionT=melting point in K Change of state from liquid to gasqrev=HvaporizationT=boiling point in K
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The Second Law of ThermodynamicsThe Third StatementIn any spontaneous process, there is always an increase in the entropy of the universe. dq/T is the differential of a state function S that has the property Suniv 0 for any process
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Entropy and Second Law of ThermodynamicsSuniv= Ssys+Ssurr
- Gibbs Free EnergySuniv>0, so G
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Free Energy and Chemical Reactions
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Third Law of ThermodynamicsThe entropy of a perfect crystal at 0 K is zero.
It is impossible to reach a temperature of absolute zeroIt is impossible to have a (Carnot) efficiency equal to 100% (this would imply Tc = 0).
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(a) T=0 K, S=0(b) T>0 K, S>0
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The Dependence of Free Energy on Pressure
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Free Energy and Equilibrium
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The Temperature Dependence of K
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Free Energy and Work
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