4/3/2015 1 CHAPTER 12: Thermodynamics Why Chemical Reactions Happen Useful energy is being "degraded" in the form of unusable heat, light, etc. A tiny fraction of the sun's energy is used to produce complicated, ordered, high- energy systems such as life • Our observation is that natural processes proceed from ordered, high-energy systems to disordered, lower energy states. • In addition, once the energy has been "degraded", it is no longer available to perform useful work. • It may not appear to be so locally (earth), but globally it is true (sun, universe as a whole).
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CHAPTER 12: Thermodynamics Why Chemical Reactions …postonp/ch223/pdf/Ch12-s15.pdf4/3/2015 6 Thermodynamics: Entropy Second Law of Thermodynamics: • The entropy of the universe
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CHAPTER 12: Thermodynamics
Why Chemical Reactions Happen
Useful energy is being "degraded" in
the form of unusable heat, light, etc.
A tiny fraction of the sun's
energy is used to produce
complicated, ordered, high-
energy systems such as life
• Our observation is that natural processes proceed from ordered,
high-energy systems to disordered, lower energy states.
• In addition, once the energy has been "degraded", it is no longer
available to perform useful work.
• It may not appear to be so locally (earth), but globally it is true (sun,
universe as a whole).
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Thermodynamics - quantitative description
of the factors that drive chemical reactions, i.e.
temperature, enthalpy, entropy, free energy.
Answers questions such as-
will two or more substances react when they are mixed
under specified conditions?
if a reaction occurs, what energy changes are associated
with it?
to what extent does a reaction occur to?
Thermodynamics does NOT tell us the RATE of a reaction
12.1 Spontaneous Processes
12.2 Entropy
12.3 Absolute Entropy and Molecular Structure
12.4 Applications of the Second Law
12.5 Calculating Entropy Changes
12.6 Free Energy
12.7 Temperature and Spontaneity
12.8 Coupled Reactions
Chapter Outline
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Spontaneous Processes A spontaneous process is one that is capable of proceeding in
a given direction without an external driving force
• A waterfall runs downhill
• A lump of sugar dissolves in a cup of coffee
• At 1 atm, water freezes below 0 0C and ice melts above 0 0C
• Heat flows from a hotter object to a colder object
• A gas expands in an evacuated bulb
• Iron exposed to oxygen and water forms rust
Spontaneous chemical and physical changes are frequently
accompanied by a release of heat (exothermic H < 0) -
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l)
Ho = -2200 kJ
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But sometimes a spontaneous process
can be endothermic H > 0 -
Some processes are accompanied by no change in enthalpy
at all (Ho = 0), as is the case for an ideal gas spontaneously
expanding:
spontaneous
nonspontaneous
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There's another factor promoting spontaneity in these processes,
and that's the increasing randomness or disorder of the system
(this is a qualitative description only – quantitative coming up):