Chemical Thermodynamics: Entropy, Free Energy and Equilibrium

1
Chemical Thermodynamics Entropy, Free Energy and
Equilibrium

• Chapter 18
• 18.1-18.6

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Chemical Thermodynamics

• Science of interconversion of energy
• Heat into other forms of energy
• Amount of heat gained/released from a system
• Spontaneity of a reaction
• Gibbs free energy function
• Relationship between Gibbs Free Energy and
  chemical equilibrium

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Spontaneous Processes

• Main objective
• Spontaneous Reaction- a reaction does occur under
  specific conditions
• Non-spontaneous Reaction- a reaction does not
  occur under specific conditions

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Spontaneous Processes

• A waterfall runs downhill
• A lump of sugar dissolves in a cup of coffee
• At 1 atm, water freezes below 0ºC and ice melts
  above 0ºC
• Heat flows from a hotter object to a colder
  object
• Iron exposed to oxygen and water forms rust

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Spontaneous Processes
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Spontaneous Processes

• Does a decrease in enthalpy mean a reaction
  proceeds spontaneously?

Spontaneous reactions
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Entropy

• To predict spontaneity we need
• Change in enthalpy
• Entropy
• Entropy- a measure of the randomness or disorder
  of a system.
• ? Disorder ? Entropy

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Entropy

• New Deck Order
• Shuffled Deck Order
• Probability
• Ordered state
• Disordered State

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Microstates and Entropy
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Microstates and Entropy

• Boltzmann, 1868
• S k ln W
• k 1.38 x 10-23 J/K
• ? W ? Entropy
• ? S Sf Si
• ? S k ln Wf
• Wi

Wf gt Wi then DS gt 0
Wf lt Wi then DS lt 0
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Entropy and Disorder
If the change from initial to final results in an
increase in randomness
Sf gt Si
DS gt 0
For any substance, the solid state is more
ordered than the liquid state and the liquid
state is more ordered than gas state
Ssolid lt Sliquid ltlt Sgas
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Entropy and Disorder
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Entropy and Disorder

• How does the entropy of a system change for each
  of the following processes?

(a) Forming sucrose crystals from a
supersaturated solution
Randomness decreases
Entropy decreases (DS lt 0)
(b) Heating hydrogen gas from 600C to 800C
Randomness increases
Entropy increases (DS gt 0)
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Entropy and Disorder
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Standard Entropy
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The Second Law of Thermodynamics

• The entropy of the universe increases in a
  spontaneous process and remains unchanged in an
  equilibrium process.
• Importance?

DSuniv DSsys DSsurr gt 0
Spontaneous process
Equilibrium process
DSuniv DSsys DSsurr 0
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Entropy Changes in the System

• To calculate ?Suniv, we need both ?Ssys ?Ssurr
• ?Ssys

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Entropy Changes in the System
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Entropy Changes in the System

• When gases are produced (or consumed)

• If a reaction produces more gas molecules than
  it consumes, DS0 gt 0.
• If the total number of gas molecules diminishes,
  DS0 lt 0.
• If there is no net change in the total number of
  gas molecules, then DS0 may be positive or
  negative BUT DS0 will be a small number.

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Entropy Changes in the System
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Entropy Changes in the Surroundings
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Entropy Changes in the Surroundings

• ?Ssurr -?Hsys
• T
• Using the information from Example 18.2,
  determine whether or not the reaction is
  spontaneous.
• N2(g) 3H2(g) ? 2 NH3(g) ?Hºrxn -92.6
  kJ/mol
• ?Ssys -199 J/K mol
• ?Ssurr -(-92.6 x 1000) J/mol
• 298 K
• ?Ssurr 311 J/mol
• ?Suniv ?Ssys ?Ssurr
• ?Suniv -199 J/K mol 311 J/mol
• ?SUNIV 112 J/K mol

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The Third Law of Thermodynamics and Absolute
Entropy

• Third Law of Thermodynamics- the entropy of a
  perfect crystalline substance is zero at the
  absolute zero of temperature.

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Gibbs Free Energy

• Predicts the direction of a spontaneous reaction.
• Uses properties of the system to calculate.
• For a constant pressure-temperature process

DG DHsys -TDSsys
DG lt 0 The reaction is spontaneous in the
forward direction.
DG gt 0 The reaction is nonspontaneous as
written. The reaction is
spontaneous in the reverse direction.
DG 0 The reaction is at equilibrium.
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Standard Free-Energy Changes
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Standard Free-Energy Changes
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Factors Affecting ?G
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Free Energy and Chemical Equilibrium
DG DG0 RT lnQ
R is the gas constant (8.314 J/Kmol)
T is the absolute temperature (K)
Q is the reaction quotient
At Equilibrium
DG 0
Q K
0 DG0 RT lnK
DG0 - RT lnK
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Free Energy and Chemical Equilibrium
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Free Energy and Chemical Equilibrium
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Free Energy and Chemical Equilibrium
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Thermodynamics of a Rubber Band