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Spontaneous Processes
Spontaneous Process
• Proceeds without any assistance from
outside the system.
– Water evaporates at 25oC
– Iron rusts in the presence of O2 and H2O
– NaCl dissolves in water
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Thermodynamics 8.7
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Entropy
Non-Spontaneous Processes
Spontaneous Processes
St
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• Assistance from outside the system is
necessary in order to induce the desired
change.
– Water does not boil at 75oC and 1atm.
– Water does not freeze at 15oC.
A process that is spontaneous in one direction
is non-spontaneous in the other direction.
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Non-Spontaneous Process
H2O(l) Æ H2O(s) at –10oC (spontaneous)
H2O(s) Æ H2O(l) at –10oC (non-spontaneous)
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– Fe2O3(s) + 3 C(s) Æ 2 Fe(s) + 3 CO(g) at 25oC.
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Spontaneous Processes
Spontaneous Processes
Endothermic reactions can be spontaneous
• Nature tends to favors processes that cause a
reduction in energy.
• In an exothermic reaction, the bonds in the
products contain less energy than the bonds in the
reactants. The excess energy is released as heat.
• Evaporation is spontaneous.
H2O(l) Æ H2O(g)
∆Ho = + 40.7 kJ
C3H8(l) Æ C3H8(g)
∆Ho = + 16.7 kJ
• Dissolving soluble compounds is spontaneous.
NaCl(s) Æ Na+(aq) + Cl-(aq) ∆Ho = +3.9 kJ
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Exothermic reactions are often spontaneous
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Entropy (S)
Increases in entropy are favored
• A measure of the disorder of a system.
• Your house gets dirty.
• Making a stir-fry.
• The impossibility of a maintaining a
perfect lawn.
• Spilling a glass of milk.
• Breaking a window.
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• A greater degree of disorder is favorable.
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• A positive value for ∆S is favorable.
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∆S = S(products) – S(reactants)
Entropy is a State Function
1st Law of Thermodynamics
St
2nd Law of Thermodynamics
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The energy contained within the universe is
constant.
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Laws of Thermodynamics
The degree of disorder contained within the
reactants is a constant.
The degree of disorder contained within the
products is a constant.
The route that was taken is not important.
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The entropy of the universe is constantly
increasing.
∆S = S(products) – S(reactants)
∆S > 0 when melting
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∆S > 0 for the following processes:
Sa
• Melting
• Vaporization
• Reactions where the products are in the
same phase as the reactants but contain
more particles than the reactants
• Reactions that produce more random phases
• Making most solutions
• Adding heat
H2O(s)
H2O(l)
Very Organized
Less Organized
Small S
Larger S
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∆S > 0 when products have more
particles
∆S > 0 when vaporizing
4 C3H5N3O9(l) Æ 6 N2(g) + 12 CO2(g) + 10 H2O(g) + O2(g)
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• 4 liquid molecules are converted into 29 gaseous
molecules.
H2O(g)
Relatively Low S
High S
• Changing from liquid to gas increases the
number of potential arrangements.
∆S > 0
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H2O(l)
N
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• Increasing the number of moles increases the
number of potential arrangements.
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∆S > 0 (usually) when making
solutions with solids and/or liquids
∆S < 0 when making solutions with
liquids and gases
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When dissolving a gas in a
liquid the entropy will
decrease.
St
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• A ionic compound is
very organized.
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• A solution containing
the same ions is much
more disordered.
The rapid and chaotic
movements of the gas
particles are greatly
reduced by the molecules
in the solution.
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∆S < 0
∆S > 0 when adding heat
Entropy (S)
Sa
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Calculating ∆S
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s
∆Svaporization
∆Sfusion
∆Sorxn = Σ n So (products) – Σ n So (reactants)
d
liqui
Sum of
d
so l i
Melting point
stoichiometric
coefficients
Boiling point
Temperature
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How can you tell if a process is
spontaneous?
Ex) Calculating ∆S
Ex) Calculate ∆S for the following reaction at 298K.
N2(g) + 2 O2(g) Æ N2O4(g)
Exothermic Reactions are favorable.
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∆H < 0
∆So = Σ n So (products) - Σ n So (reactants)
N
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Systems that create disorder are favorable.
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∆S > 0
The Surroundings
The
system
C
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2) ∆G (Gibbs Free Energy)
The Universe
St
1) ∆Suniverse
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Two methods for determining if a
process is spontaneous
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∆Suniverse
Sa
∆Suniverse = ∆Ssystem + ∆Ssurroundings
If ∆Suniverse > 0, the reaction is spontaneous.
∆Ssurroundings and ∆H
Heat flow links the system to the surroundings.
• Exothermic Reactions
– Ssurr increases (∆Ssurr > 0)
Heat
The
system
• Endothermic Reactions
– Ssurr decreases (∆Ssurr < 0)
Heat
The
system
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∆Suniverse
∆Suniverse = ∆Ssystem + ∆Ssurroundings
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If a process:
Sa
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C
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St
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• increases the entropy of the system (∆Ssys > 0),
• and is exothermic (∆Ssurr > 0),
• it must also be spontaneous (∆Suniverse > 0).
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