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March 06, 2013
some N2 (g) + 3H2 (g) ↔ 2NH3 (g) ∆H= ‐46.3 kJ/mol
Kc at 375 C is 1.2
What conditions could be used to try and maximize production of NH3
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Why Equilibrium
What is the opposite of equilibrium
We will explore the following questions
Why equilibrium at all
Why are products are reactants favoured in equilibrium systems
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The Second Law of Thermodynamics states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." ex a battery will power a device until the potential energy is in the battery is used up
your car engine will run until the car runs out of gas Both of these systems will work spontaneously until the potential energy that runs them
is reduced to zero.
What does spontaneous mean
These systems will not spontaneously reverse themselves and will stay they way they are until energy is put back into the process.
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For each of these processes, predict if Entropy increases or decreases.
1. 2H2(g) + O2(g) ㌯ 2H2O(g) 2. 2SO3(g) ㌯ 2SO2(g) + O2(g) 3. Ag+(aq) + Cl­(aq) ㌯ AgCl(s) 4. Cl2(g) ㌯ 2Cl(g) 5. H2O(l) ㌯ H2O(g) 6. CaCO3(s) + 180 KJ ㌯ CaO(s) + CO2(g) 7. I2(s) ㌯ I2(aq)
8. 4Fe(s) + 3O2(g) ㌯ 2Fe2O3(s) 5
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Nature prefers processes that lead to
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What types of changes increase entropy
Is entropy temperature dependent
Entropy unit
Looking at the entropy unit what is entropy really all about
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Statistical Entropy The idea of microstates
Molecules can be distributed in a number of different ways at any given instant of time.
If you could photograph these molecules at a given instant in time you could see their positions.Each picture would show a different distribution 'each distribution is a microstate
A microstate is one out of all the possible possible distributions at any given time for a molecule
Any change that increases the number of microstates for molecules increases entropy
Microstates for coloured squares moving between two halves of a container.
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How would any of these changes affect the number of available microstates for a population of molecules
Increase temperature
Change from a liquid to a gas
Decrease pressure or increase volume on a gas 9
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Consider both Enthalpy and Entropy and determine if each reaction will
a) go to completion
b) not occur or
c) go to equilibrium
H2O(l) ► H2O(g) DH = 150 KJ
CaCO3(s) + 180 KJ ► CaO(s) + CO2(g) I2(s) ► I2(aq) + 608 KJ 4Fe(s) + 3O2(g) ► 2Fe2O3(s) ΔH = 1570 KJ Cl2(g) ► 2Cl(g) DH = +26.8 KJ Ag+(aq) + Cl­(aq) ► AgCl(s) + 86.2 KJ
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The interplay between entropy and enthalpy can be summarized in the formula
ΔG = ΔH ­ TΔS
or
ΔGo = ΔHo ­ TΔSo
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H
S
­
+
+
­
­
­
+
+
G
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Calculate H and S for the following reaction and decide in which direction each of these factors will drive the reaction.
N2(g) + 3 H2(g) 2 NH3(g)
Compound Hfo(kJ/mol) S°(J/mol­K)
N2(g) 0 191.61
H2(g) 0 130.68
NH3(g) ­46.11 192.45
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feb15.notebook
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Calculate H and S for the following reaction and decide in which direction each of these factors will drive the reaction.
N2(g) + 3 H2(g) 2 NH3(g)
Compound Hfo(kJ/mol) S°(J/mol­K)
N2(g) 0 191.61
H2(g) 0 130.68
NH3(g) ­46.11 192.45
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Use the values of H and S calculated in Practice Problem 5 to predict whether the following reaction is spontaneous at 25C:
N2(g) + 3 H2(g) 2 NH3(g)
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Predict whether the following reaction is still spontaneous at 500C:
2 NH3(g)
N2(g) + 3 H2(g) 18
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Relationship of Gibbs free energy to Equilibrium
What is the meaning of Q
What is the meaning of K
What is the meaning of deltaG
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N2(g) + 3 H2(g) 2 NH3(g)
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ΔG = ΔGo + RT ln Q
y = b + m x
At equilibrium deltaG is 0 and Q equals K
0 = ΔGo + RT ln K
rearranging
ΔGo = ­ RT ln K
this is the most significant formula in chemistry
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