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In a simple process we can think about how the Entropy of a system changes
As Boltzmann demonstrated, increasing the number of microstates increases
the entropy
Factors that can cause an increase in the number of microstates include.
T
Temperature
t
Increasing the temperature
The number of particles
Increasing the number of particles is like increasing the number of cards in a
deck
Volume
Increasing the Volume gives molecules greater freedom to move around and
increases the number of possible microstates
What happens as the Temperature of a system is lowered?
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The Third Law Of Thermodynamics
At 0K, all the units in the lattice have no thermal energy, no motion, 1
Microstate
S = k lnW W = 1 lnW = 0
So S = 0
What happens to Entropy as we continue to heat
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19.4 Entropy Changes in Chemical Reactions
We can measure ΔH of a reaction by using calorimetry, however there is
no such easy method for determining ΔS for a reaction
Absolute values for Entropies, S, can be obtained
Standard Molar Entropy So is defined for pure
substances at 1 atm pressure and 298K
The Entropy change for a reaction can be calculated from the Standard
Molar Entropies
ΔSo = Σ nSo (products) - Σ mSo (reactants)
Sample Exercise 19.5
Entropy Changes in the Surroundings
The Entropy change of the surroundings depends on the how much heat is
absorbed or given off by the system
For a constant pressure reaction (usual) then qp (heat exchanged at
constant pressure) = ΔH so the entropy change of the surroundings can be
written as
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Because Souniv is positive (increases) for any spontaneous reaction we can
put together the equations for calculating ΔSsys with ΔSsurr to predict
whether a reaction will be spontaneous
For the reaction CO(g) + 2H2(g) → CH3OH (l) at 298K
Prediction of Spontaneity depends on
The sign of ΔH and ΔS
And the magnitude of ΔH, ΔS and the temperature (in KELVIN)
A spontaneous reaction can be exothermic or endothermic
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19.5
Gibbs Free Energy
From the calculation previous using ΔSouniv = ΔSosys + ΔSosurr the
spontaneity of a reaction was seen to involve enthalpy H and Entropy S
J W Gibbs came up with a new state function G, that connected entropy
and enthalpy to predict whether a reaction occurring at a constant
temperature would be spontaneous
From the definition of ΔSuniv, we can relate the state function G to
spontaneity
ΔSuniv = ΔSsys + ΔSsurr
= ΔSsys + (ΔHsys/T)
Multiply both sides by –T gives
-TΔSuniv = ΔHsys - TΔSsys = ΔGsys
ΔGsys = - TΔSuniv at constant temperature and pressure
So the value of ΔG should predict whether a reaction will be spontaneous
In any spontaneous process occurring at constant
temperature and pressure the free energy always decreases
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This can be explained in the diagram for the formation of ammonia
The expression for Q is the same as the Equilibrium Constant except that the
reactants and products need not be in equilibrium
Q < K reaction goes towards products, (Q > K vice versa)
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