CHM 120 CHAPTER 20 Thermodynamics

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CHM 120CHAPTER 20 Thermodynamics

• Dr. Floyd Beckford
• Lyon College

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SPONTANEITY

• Spontaneous process one that proceeds on its
• own without any external influence
• The reverse of any spontaneous process is
• always nonspontaneous
• From a thermodynamic standpoint, spontaneity
• is not related to speed
• - a spontaneous reaction may occur slowly

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• The concept of spontaneity has two parts
• enthalpy and entropy
• Some spontaneous reactions are exothermic and
• others are endothermic
• This implies that entropy plays an important
• part in determining spontaneity
• Molecular disorder or randomness is referred to
• as entropy, S

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• Entropy is a state function. So
• ?S Sfinal Sinitial
• If disorder increases, ?S is positive of
  disorder
• decreases, ?S is negative
• Note molecular systems tend to move
• spontaneously to a state of maximum disorder
• ?Ssys ?Ssurr gt 0
• This is the Second Law of Thermodynamics

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ENTROPY AND TEMPERATURE

• As temperature increases, so does random
• molecular motion
• The Third Law of Thermodynamics establishes
• the zero of the entropy scale
• - the entropy of a perfectly ordered
• crystalline substance is zero at 0K

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STANDARD MOLAR ENTROPIES

• The standard molar entropy of a substance, S,
• is the entropy of 1 mol of the pure substance at
• 1 atm pressure and a specified temperature
• usually 25 C
• Sometimes referred to as absolute entropies
• because they are measured with respect to the
• 3rd law of thermodynamics
• The unit of entropy is J/Kmol

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• We can sometimes predict the sign of ?S
• e.g. for phase changes
• Melting is always accompanied by an entropy
• increase
• Likewise, vaporization takes place with a huge
• increase in entropy
• Any process which leads to an increase in the
• number of gas molecules will have ?S gt 0

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• The standard molar entropy change, ?S, of a
• reaction can be determined from the absolute
• entropies of reactants and products
• ?S Sproducts - Sreactants
• For a reaction
• aA bB ? cC dD
• ?S cSC dSD aSA bSB
• Note that the units of ?S is J/K

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• ?S Sn ?S (products) Sm ?S (reactants)
• When ?S gt 0 reaction is spontaneous
• ?S lt 0 reaction is nonspontaneous
• ?S 0 reaction is at equilibrium
• The entropy of the surrounding can be
• calculated as follows
• ?Ssurr - ?H/T

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FREE ENERGY

• Usually we are interested in the thermodynamics
• of the system
• The purpose of energy changes is to do work
• for this purpose we use free energy
• Free energy G H TS
• Free energy is a state function and at constant
• temperature
• ?G ?H - T ?S

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• It is the sign free energy change, ?G, that
• is the criterion for spontaneity
• ?G ?H - T ?S
• If ?G lt 0 reaction is spontaneous
• ?G gt 0 reaction is nonspontaneous
• ?G 0 reaction is at equilibrium
• Remember that if a reaction is spontaneous in
• one direction it is nonspontaneous in the reverse

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FREE ENERGY CHANGES

• Free energy changes must be compared to
• standard-state conditions
• The standard free-energy, ?G, for a reaction is
• the change in free energy that occurs when
• reactants in their standard states are converted
• to products in their standard states
• ?G is an extensive property it refers to the
• thermochemical equation

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FORMATION REACTIONS

• The standard free energy of formation, ?Gf, of
• a substance is the free-energy for the formation
• of 1 mol of the substance in its standard state
• from the most stable form of the constituent
• elements in their standard states
• N2(g) 3H2(g) ? 2NH3(g) ?G -33.0 kJ
• So ?Gf (NH3) (-33.0 kJ mol-1)/2
• -16.5 kJ/mol

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• The ?Gf measures the thermodynamic stability
• of a substance
• ?Gf lt 0 ? substance is stable and do not
• decompose to its constituents
• ?Gf gt 0 ? substance is unstable and will
• decompose
• - substance will exist for sometime
• if the rate of
  decomposition is
• slow

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• ?Gf for an element in its standard state is
  zero
• ?G Sm ?Gf(products) Sn ?Gf(reactants)
• For our general reaction
• aA bB ? cC dD
• ?G c?Gf(C) d?Gf(D)
• a?Gf(A) - b?Gf(B)

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• The value of ?G will vary with temperature
• ?H -, ?S , reactions are spontaneous at all
• temperatures
• ?H -, ?S -, reactions become spontaneous
• below a definite temperature
• ?H , ?S , reactions become spontaneous
• above a definite temperature
• ?H , ?S -, reactions are nonspontaneous at
• all temperatures

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COMPOSITION OF MIXTRUES

• The free energy change refers to the reaction
• under standard state conditions
• It also assumes the reaction goes to completion
• How does ?G relates to equilibrium mixtures?
• ?G ?G RT lnQ
• At equilibrium, ?G 0 and Q K
• 0 ?G RT lnK
• ?G -RT lnK

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• The above equation is important you can
• measure ?G or K and calculate the other
• ?G lt 0 ln K gt 0 K gt 1 Mainly products
• at equilibrium
• ?G gt 0 ln K lt 0 K lt 1 Mainly reactants
• at equilibrium
• ?G 0 ln K 0 K 1 Mixture contain
• comparable
• amounts of both