chapter 10.ppt

Chapter 10Spontaneity, Entropy, and Free Energy

Concept for second law of thermodynamic

Isothermal expansion device

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

One-Step ExpansionM1 is replaced by M1/4.

Two-Step ExpansionP11/2P1, V12V1

1/2P11/4P1, 2V14V1

PV diagram two-step expansion

The PV diagram six-step expansion

Infinite-Step Expansion(dV: V0 )

Reversible expansion

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.

A heat engine operating between two temperatures

Hot Reservoir at H

qH

Heat Engine -W

-qC

Cold Reservoir at C

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

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.

A system that violates the second law

Heat Reservoir

Heat q

Cyclic Machine

Work Output=q

Heat Efficiency

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.

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)

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.

Carnot Efficiency

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Carnot Efficiency

Adiabatic Process

U=0 for an isothermal process, q=-w

No heat transfer (q=0) for adiabatic process, U=w

Adiabatic ProcessProcess in which no heat transfer takes place

Application of Carnot CycleCalculate Q, U, W First law: U = QH QL + W W = QL - QH

P (atm)V (L)3101.520125.5212.75

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.

Definition of EntropyS=kBlnkB: Boltzmanns constant : the number of microstatescorresponding to a given state

For one particleS1=kBln1S2=kBln2S=S2-S1= kBln2-kBln1=kBln(2/1)S= kBln(21/1)=kBln2

M104

4224=16M104

M104

Definition of entropy in term of probability

Entropy for Isothermal Process

Quantity of Entropy for Reversible Process

Entropy and Physical ChangesTemperature Dependence

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

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

Entropy and Second Law of ThermodynamicsSuniv= Ssys+Ssurr

Free Energy and Chemical Reactions

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).

(a) T=0 K, S=0(b) T>0 K, S>0

The Dependence of Free Energy on Pressure

Free Energy and Equilibrium

The Temperature Dependence of K

Free Energy and Work

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