Download Chemical Thermodynamics

Chemical Thermodynamics
Second Law of Thermodynamics
Unlike energy, entropy is NOT conserved:
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Spontaneous Processes
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Entropy and the Second Law of Thermodynamics
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The Molecular Interpretation of Entropy
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Entropy Changes in Chemical Reactions
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Gibbs Free Energy
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Free Energy and Temperature
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Free Energy and the Equilibrium Constant
The Universe wants to become more and more disordered, it wants its
total entropy to increase continually!
Reversible process ΔSuniv = ΔSsys + ΔSsurr = 0
Irreversible process ΔSuniv = ΔSsys + ΔSsurr > 0
Spontaneous Processes
First Law of Thermodynamics stated:
Energy can not be created
nor destroyed, it can be
converted from one form
to another or transferred
from a system to
surroundings (or vice
versa)
Entropy
ΔS = Sfinal – Sinitial
units J /K
S = k ln W
So, if increasing the temperature increases the number of microstates
could we decrease the temperature to a point where all motion stops and
we have a single microstate?
The Third Law Of Thermodynamics
The Entropy of a pure, crystalline substance at absolute zero (0 K) is zero
Standard Free Energy
The standard free energy change ΔGo is defined as the change in the free
energy when reactants in their standard states are converted to products
in their standard states:
ΔGo = Σ n ΔG o (products) - Σ m ΔG o (reactants)
Conventions used in establishing
Standard free energies
Solid
Pure solid
Liquid
Pure liquid
Gas
1 bar pressure
solution
1M
∆Gof in standard state = 0
elements
ΔGo
At 0K, all the units in the lattice have no thermal energy, no motion, 1
Microstate
Because
is the difference in Gibbs free energy between products in their
standard states and reactants in their standard states, it is a fixed quantity for a
given reaction at a given temperature
ΔG also depends on the composition of reaction mixture and varies and might even
change sign as the reaction proceeds.
S = k lnW W = 1 lnW = 0
Therefore: S = 0
What happens to Entropy as we continue to heat
Melting the solid
overcomes the forces
holding the particles
together, this gives them
a greater degree of
freedom
Calculate the standard free energy change ΔG o for this reaction at 25 oC
and 500 oC. What do the calculated values tell you?
Given that ΔH o = -92.38 kJ and ΔSo = -198.4 J/K
Equilibrium (isothermal)
N2(g) +
ΔGo = ΔHo - TΔSo
3H2 (g)
2NH3 (g)
ΔS = qrev /T
ΔSfus = ΔHfus /T
Why is the Entropy of phase changes following the order
Ssolid < Sliquid < Sgas
Entropy Changes in Chemical Reactions
Increasing the temperature from 25 oC to 500 oC changes the sign of ΔGo , indicating
that a mixture of N2, H2 and NH3 at 1 bar pressure will spontaneously form more NH3
at 25 oC and spontaneously form more N2 and H2 at 500 oC
Free Energy and Temperature