Download text page 117 2.4 Entropy Change versus

2.4
Entropy Change versus
Enthalpy Change
•
What is Entropy?
•
Chemical Systems and Entropy
•
Chemical Reactions and Entropy Change
•
Natural Thermodynamic Drives
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Enthalpy, Entropy, and Equilibrium
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Enthalpy, Entropy, and Spontaneity
2.4
Entropy Change versus
Enthalpy Change
What is Entropy?
Chemists working definition of entropy (S) is a substance’s
or system’s state of disorganization or randomness.
Low Entropy
Organized
Simple
High Entropy
Disorganized
Complex
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2.4
Entropy Change versus
Enthalpy Change
Chemical Systems and Entropy
Low Entropy
High Entropy
Atom with few electrons
Molecule with few atoms
Atom with many electrons
Molecule with many atoms
Pure Substance
Substance at low temp.
Solid
Mixture
Substance at high temp.
Gas
text pages 117-118
2.4
Entropy Change versus
Enthalpy Change
Chemical Reactions and Entropy Change
A chemical equation alone does not contain enough
information for you to reliably determine whether entropy
increases or decreases during the reaction but:
Entropy usually decreases when gas particles combine into
fewer particles.
text pages 119-120
2.4
Entropy Change versus
Enthalpy Change
Chemical Reactions and Entropy Change
If the number of product gas molecules equals the number
of reactant gas molecules then entropy usually decreases
when the atoms of a particular element go from being:
1.
2.
part of a gaseous compound to being part of a
solid compound.
part of different groups to being grouped together.
text pages 120-121
2.4
Entropy Change versus
Enthalpy Change
Natural Thermodynamic Drives
Two thermodynamic “drives” influence an equilibrium’s
position:
1.
2.
the drive toward decreasing enthalpy, i.e. in the
exothermic direction.
the drive toward increasing entropy or disorder.
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2.4
Entropy Change versus
Enthalpy Change
Enthalpy, Entropy, and Equilibrium
If the drive towards decreasing entropy and the drive towards
increasing entropy are:
1.
2.
3.
opposing then the equilibrium position will be central,
with a reasonable amount of reactants and products.
both towards reactants then the equilibrium position
will be far to the left, i.e. almost no products.
both towards products then the equilibrium position will
be far to the right, i.e. the rx. goes almost to completion.
text pages 122-124
2.4
Entropy Change versus
Enthalpy Change
Enthalpy, Entropy, and Spontaneity
A spontaneous process is one that happens “on its own.”
Chemical systems move spontaneously toward equilibrium.
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