Download - Control of Chemical Reactions
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Control of Chemical Reactions
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Thermodynamic Control of Reactions
Enthalpy
Bond Energies– Forming stronger bonds favors reactions.– Molecules with strong bonds are more stable.
Entropy
Randomness– Reactions that
increase random- ness are favored.
– Forming gases favors reactions.
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Remember Enthalpy?
Enthalpy of formation for elements in their natural states = 0.
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H2O(g) Gaseous water -242
ΔH , kJ/molof
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The Laws of Thermodynamics
1st Law: Energy is Conserved
2nd Law: Any “spontaneous” process leads to an increase in entropy of the universe.
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Entropy
A measure of randomness. Units of J/K.
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Trends in entropy
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Entropy Change
• First define:
System Surroundings
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Entropy ChangeFor the System
If the system gets more random, DS is positive. (Favors the reaction)
If the system gets more ordered, DS is negative. (Disfavors the reaction)
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Calculating SChemical Reactions
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l)
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Calculating SSpecial Case: Phase Changes
T
HS
T
qS changephaserev
Heat of fusion (melting) of ice is 6000 J/mol. What is the entropy change for melting ice at 0 oC?
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Calculate entropy change for formation of rain:
H2O(g) H2O(l)
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What types of reactions lead to increased entropy?
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Entropy vs. Enthalpy Control of Reactions
Second law of thermodynamics:
Suniverse = Ssystem + Ssurroundings
systemuniverse system
-ΔHΔS = ΔS +
T
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Question: How can rain form?
H2O(g) H2O(l)
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Calculate Suniverse for H2O(l) H2O(g) at:
90 oC
110 oC
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Putting S, H and Temperature Together
Gibb’s Free Energy: G = H - TS
When G is negative, reaction is favored.
When G is positive, reaction is disfavored.
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2 Fe2O3(s) + 3 C(s) 4 Fe(s) + 3 CO2(g)
H = +468 kJS = +561 J/K G = H - TS
What is G at 25 oC and at 1000 oC?
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Enthalpy vs. Entropy Control of Reactions
G = H - TS
At high temperatures:
At low temperatures:
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Temperature Domains and Reaction Favorability
H + - +S -
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2 Fe2O3(s) + 3 C(s) 4 Fe(s) + 3 CO2(g)
H = +468 kJS = +561 J/K
In what temperature range will this reaction be favored?
High or low?
What temperature?
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Free Energy vs. Temperature Curves
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Catalytic Converters
Nitrogen oxides cause smog.
N2(g) + O2(g) 2 NO(g) H = +180 kJ S = +25 J/K
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Free Energy of Formation: Only used at 25 oC
2 BaO(s) + C(s) 2 Ba(s) + CO2(g)