Thermodynamics (the other part)

1
Thermodynamics(the other part)

• Topic 19 in Red Book
• Chapter 16 in textbook

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Why does change occur?

• DH change in enthalpy (heat transferred btwn
  system and surroundings)
• DS change in entropy (randomness or disorder of
  the system)
• Systems tend to move towards lower enthalpy
  (releasing heat) and higher entropy (more
  disorder)

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Spontaneous Processes

• reaction that proceeds on its own with no outside
  help
• if a reaction is spontaneous in one direction, it
  is nonspontaneous in the other direction
• does not necessarily mean that the reaction
  happens quickly, just that it happens

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Enthalpy

• DH heat absorbed by the system
• exothermic -DH
• endothermic DH
• 1st law of thermo energy is conserved, heat
  lost by system must equal heat gained by
  environment vice versa
• remember the formula for calculating DH of a rxn
• DHorxn SDHof products SDHof reactants

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Entropy 2nd Law

• measure of randomness (gases are more random than
  liquids, so gases have higher S)
• DS is positive for solid ? liquid ? gas changes
  and negative for the reverse
• DS is higher for more complex molecules (propane
  has higher S than methane)
• 2nd law of thermo states that any spontaneous
  change is always associated with an overall DS

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3rd Law

• states that S of a pure crystalline solid at 0K
  is always zero.
• As the temperature is warmed above 0K, S
  increases gradually
• At phase changes, S increases a lot

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Standard Molar Entropies

• So measures order/disorder of pure substances
  at 1 atm 298 K
• units are J/mol K
• So is largest for gases and heavier/more complex
  substances

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Entropy Changes

• looks just like the equation for DHo
• DSorxn SSo products SSo reactants
• What is the DSorxn for the following rxn?
• 2NO(g) O2(g) ? 2NO2(g)

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Gibbs Free Energy

• DG amt of useful work/energy that can be
  obtained from a process at constant TP
• DG DH TDS
• DGorxn SGof products SGof reactants
• G H are in kJ/mol, S is in J/molK so it must be
  converted to kJ

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What does DG tell you?
If DG is The process is
negative spontaneous
positive nonspontaneous
zero at equilibrium
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What is the standard free energy for the reaction

• 2NO(g) O2(g) ? 2NO2(g)

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Free Energy Temperature
DH DS -TDS DG The rxn is
- - - spontaneous at all T
- nonspontaneous at all T
- - /- Spontaneous only at low T
- /- Spontaneous only at high T
KNOW THIS TABLE!!!
13
Is DG for a burning campfire positive or
negative? Explain citing the positive or
negative signs of DH and DS for the process. Is
there a temperature at which DG for a burning
campfire will change signs? Explain.
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Free Energy Equilibrium

• you can calculate free energy at non-standard
  conditions using the following formula
• DG DGo RT lnQ
• have to use R 8.314 J/molK
• At equilibrium, Q K DG 0, so the eqn
  becomes
• DGo -RT lnK

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• Using the std. free energies of formation for the
  reaction
• 2H2S(g) 3O2(g) ? 2H2O(g) 2SO2(g)
• Calculate DGo and DG for a mixture at 25oC with
  the following composition
• PH2S 1.00 atm, PO2 2.00 atm, PH2O 0.500
  atm, PSO2 0.750 atm

16

• The standard free energy change for the following
  reaction at 25oC is -118.4 kJ/mol
• KClO3(s) ? KCl(s) 3/2 O2(g)
• Calculate Kp for the reaction and the equilibrium
  pressure of O2 gas.