Thermodynamics of Reactions

1
Thermodynamics of Reactions

• Spontaneity, Entropy, and Free Energy
• Chapter 16

2
1st Law of Thermodynamics

• The energy of the universe is constant
• (conservation of energy)

3
Spontaneous Processes

• Can be fast OR slow.
• Occurs without outside intervention

4
True, or Not True?
Why??

• Ball rolls downhill spontaneously.
• Ball rolls uphill spontaneously.
• Wood burns spontaneously in O2.
• CO2 and H2O spontaneously form wood.
• Steel rusts spontaneously.
• Rust spontaneously turns into iron and water.

5
Its All About Entropy

• Entropy is
• All about the ability of energy to spread out
• Related to probability.
• Over-simplified to be about disorder.

6
Positional Entropy

• When a substance has more opportunities to exist,
  (ie, particles can have more than one
  arrangement) it is also said to have more
  positional entropy.
• Increasing moles of gas
• Increasing volume
• Decreasing pressure
• Changing state of matter ( s ? l ? g )
• Exothermic rxns

7
Try Me Conceptual

• Determine which of the following pairs has the
  most positional entropy
• 1 mol H2 at STP or 1 Mol H2 at 100oC, 0.5 atm
• 1 mol N2 at STP or 1 mol N2 at 100 K 2atm
• 1 mol H2O(s) at 0oC or 1 mole H2O(l) at 20oC

8
2nd Law of Thermodynamics

• Spontaneous processes increase the entropy of the
  universe.

DSuniverse DSsystem DSsurroundings
9
The Value of S

• Sign is determined by enthalpy
• exothermic
• endothermic
• Magnitude is determined by temperature

10
Try Me Calculation

• The melting point of tungsten (W) is the second
  highest among the elements, at 3680 K. The
  enthalpy of fusion for this metal is 35.2 kJ/mol.
  What is the entropy of fusion?

11
3rd Law of Thermodynamics

• The entropy of a perfect crystal at 0 Kelvin is
  zero.

In Phase Out of Phase Angle Bending Rotation Tr
anslation
vibrations
12
Entropy for a chemical reaction

• DSorxn SnSop - SnSor

13
Try Me

• Calculate the change in entropy at 25oC for the
  reaction
• 2 NiS 3 O2 ? 2 SO2 2 NiO
• Given Entropy Values
• SO2 248 J/Kmol
• NiO 38 J/Kmol
• O2 205 J/Kmol
• NiS 53 J/Kmol

14
Free Energy

• Gibbs Free Energy- another method for determining
  spontaneity.
• Also indicates the amount of available energy
  that is capable of doing work.
• As an energy source is used, the energy is not
  destroyed, only converted to a non-usable form.

15
DG DH - TDS

• Defines Gibbs energy in terms of enthalpy and
  entropy.
• All three factors will contribute to reaction
  spontaneity.
• When DG is negative, the reaction is spontaneous.

16
Rearrange that formula

• Lets start with the following 3 formulae
• DG DH TDS
• DSsurr -DH/T
• DSuniv DSsurr DSsyst

DSuniverse - DG /T
17
Fancy Pants Charts
DO NOT MEMORIZE
DH DS DG Spontaneous?
__ __ Always
__ __ /- When temp is low
/- When temp is high
__ Never
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Free Energy in Chemical Reactions

• Standard Free energy is used so that we can
  compare the relative tendency to occur.
• DGo DHo TDSo

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2SO2(g) O2(g) ? 2SO3(g)

• The above rxn. Occurs at 25oC and 1 atm.
  Calculate DHo, DSo, and DGo using the following
  data
• substance DHof So
• kJ/mol J/Kmol
• SO2(g) -297 248
• SO3(g) -396 257
• O2(g) 0 205

20
It also works like Hesss Law problems

• What is the DGo for the reaction if the mechanism
  is
• Cdi(s) O2(g) ? CO2(g) DGo -397kJ
• Cgr(s) O2(g) ? CO2(g) DGo -394 kJ

21
Dependence on Pressure

• Enthalpy does not depend on pressure.
• Entropy does depend on temperature.
• S low pressure gt S high pressure

22
DG DGo RT ln(P)

• This can be adapted to reflect partial pressures
  for the reaction it describes. When that occurs,
  the formula can be re-written

DGo -RT ln(Q)
23
_at_ Equilibrium

• K Q
• The free energy is the lowest possible it will
  ever be for the system.

24
Try Me Out

• The overall rxn for rusting iron by oxygen is
• 4Fe(s) 3O2(g) ?? 2 Fe2O3(s)
• _at_25oC, find the equilibrium constant given
• Substance DHo So
• Fe2O3(s) -826 90
• Fe(s) 0 27
• O2(g) 0 205

25
Free Energy and Work

• Achieving the maximum amount of work from a
  process is highly unlikely because of transfers
  of energy.
• DG represents the maximum possible quantity of
  work a system is capable of doing.