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Chemical Thermodynamics: Entropy, Free Energy and Equilibrium

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Chemical Thermodynamics: Entropy, Free Energy and Equilibrium Chapter 18 18.1-18.6 Chemical Thermodynamics Science of interconversion of energy Heat into other forms ... – PowerPoint PPT presentation

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Title: Chemical Thermodynamics: Entropy, Free Energy and Equilibrium


1
Chemical Thermodynamics Entropy, Free Energy and
Equilibrium
  • Chapter 18
  • 18.1-18.6

2
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

3
Spontaneous Processes
  • Main objective
  • Spontaneous Reaction- a reaction does occur under
    specific conditions
  • Non-spontaneous Reaction- a reaction does not
    occur under specific conditions

4
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

5
Spontaneous Processes
6
Spontaneous Processes
  • Does a decrease in enthalpy mean a reaction
    proceeds spontaneously?

Spontaneous reactions
7
Entropy
  • To predict spontaneity we need
  • Change in enthalpy
  • Entropy
  • Entropy- a measure of the randomness or disorder
    of a system.
  • ? Disorder ? Entropy

8
Entropy
  • New Deck Order
  • Shuffled Deck Order
  • Probability
  • Ordered state
  • Disordered State

9
Microstates and Entropy
10
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
11
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
12
Entropy and Disorder
13
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)
14
Entropy and Disorder
15
Standard Entropy
16
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
17
Entropy Changes in the System
  • To calculate ?Suniv, we need both ?Ssys ?Ssurr
  • ?Ssys

18
Entropy Changes in the System
19
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.

20
Entropy Changes in the System
21
Entropy Changes in the Surroundings
22
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

23
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.

24
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.
25
Standard Free-Energy Changes
26
Standard Free-Energy Changes
27
Factors Affecting ?G
28
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
29
Free Energy and Chemical Equilibrium
30
Free Energy and Chemical Equilibrium
31
Free Energy and Chemical Equilibrium
32
Thermodynamics of a Rubber Band
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