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Ch. 20: Entropy and Free Energy. Thermodynamics: the science of energy transfer Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored . Section 20.1. Ø Thermodynamics tells us NOTHING about the rate of reaction.

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Ch. 20: Entropy and Free Energy

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Ch 20 entropy and free energy l.jpg

Ch. 20: Entropy and Free Energy

  • Thermodynamics: the science of energy transfer

    • Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored


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Section 20.1

  • ØThermodynamics tells us NOTHING about the rate of reaction.

    • The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.)


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Section 20.2 Entropy

  • Entropy, S:Measure of dispersal or disorder.

  • ØCan be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no disorder and S = 0.

  • ØIf temperature change is very small, can calculate entropy change, DS = q/T (heat absorbed / T at which change occurs)

  • ØSum of DS can give total entropy at any desired temperature. See Table 20.1


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Section 20.2 Entropy

  • In general, the final state is more probable than the initial one if:

  • (1)energy can be dispersed over a greater number of atoms and molecules (hot  cold)

  • (2)the atoms and molecules can be more disordered (dissolving, diffusion of gas)


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Section 20.2 Entropy

  • More specifically,

  • (1)if energy and matter are both more dispersed, it is definitely product-favored

  • (2)if only energy or matter is dispersed, then quantitative information is necessary to decide which effects are greater

  • (3)if neither matter nor energy is more dispersed, then the process will be reactant-favored


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Entropy Examples (positive DS)

  • Boiling water

  • Melting ice

  • Preparing solutions

  • CaCO3 (s)  CaO (s) + CO2 (g)


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Entropy Examples (negative DS)

  • Molecules of gas collecting

  • Liquid converting to solid at room temp

  • 2 CO (g) + O2 (g)  2 CO2 (g)

  • Ag+ (aq) + Cl-(aq)  AgCl (s)


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Entropy Generalizations

  • Sgas > S liquid > Ssolid

  • Entropies of more complex molecules are larger than those of simpler molecules (Spropane > Sethane>Smethane)

  • Entropies of ionic solids are higher when attraction between ions are weaker.

    ØEntropy usually increases when a pure liquid or solid dissolves in a solvent.

    Entropy increases when a dissolved gas escapes from a solution


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Laws of Thermodynamics

  • First law: Total energy of the universe is a constant.

  • Second law: Total entropy of the universe is always increasing.

  • Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.


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Calculating DSosystem

  • DSosystem =  So (products) -  So (reactants)

    Can also relate surroundings to the system!

  • DSosurroundings = q surroundings / T

    = - DHsystem / T


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Calculating DSouniverse

  • DSouniverse = DSosurroundings +DSosystem

  • DSouniverse = - DHsystem / T +DSosystem

  • Can use 2nd law to predict whether a reaction is product-favored or reactant-favored!

    • The higher the temperature, the less important the enthalpy term is!


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  • Roald Hoffmann (1981 Nobel prize): “One amusing way to describe synthetic chemistry, the making of molecules that is at the intellectual and economic center of chemistry, is that it is the local defeat of entropy.”


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20.3 Gibbs Free Energy

  • DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!


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20.3 Gibbs Free Energy

  • DGsystem = - T DSuniverse

    = DHsystem - TDSsystem

  • DGosystem = DHosystem - T DSosystem

  • DGorxn = DHorxn - T DSorxn


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20.3 Gibbs Free Energy

  • DGosystem or DGorxn If negative, then product-favored. If positive, then reactant-favored.

  • DGoreaction =  Gfo (products) -  Gfo (reactants)


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20.3 Gibbs Free Energy

  • DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!

  • Know the meaning of enthalpy-driven vs. entropy-driven reactions.

    DGs are additive!


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20.4 Thermodynamics and K

If not at standard conditions,

DG = DGo + RT ln Q

 (Equilibrium is characterized by the inability to do work.)

At equilibrium, Q = K and DG = O

Therefore, substituting into previous equation gives 0 = DGo + RT ln K and

DGo = - RT ln K(can use Kp or Kc)


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2020.5      Thermodynamics and Time

  • First law: Total energy of the universe is a constant.

  • Second law: Total entropy of the universe is always increasing.

  • Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.

  • Entropy : time’s arrow

  • Absolutely MUST learn table in Chapter highlights!


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20.4 Thermodynamics and K

  • ØUnderstand relationship between DGo, K, and product-favored reactions!

  • DGo<0 K>1product-favored

  • DGo=0 K=1

  • DGo>0 K<1reactant-favored


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