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Unit 10 Lesson 5 Spontaneity In Chemical Reactions Videodisk Unit 4
Demos The Purple Cow Hot and Cold Packs In Chemical Reactions
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Spontaneous Processes
Spontaneous processes (physical or chemical) are those that can proceed without any outside intervention. The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B. © 2009, Prentice-Hall, Inc.
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Spontaneous Reactions
A Spontaneous Reaction proceeds without needing a constant input of energy (they usually have an output of energy) Think about iron rusting, it happens “automatically” and releases heat A non spontaneous reaction will not proceed without a constant input of energy Think about water decomposing, it needs a constant supply of electricity
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Spontaneous Processes
Processes that are spontaneous in one direction are nonspontaneous in the reverse direction. © 2009, Prentice-Hall, Inc.
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Entropy (DS) A measure of the disorder or randomness in a system.
The law of disorder states that the natural tendency of systems is to move towards a state of maximum disorder or randomness. It is related to the various modes of motion in molecules. Increasing disorder = + DS Decreasing disorder = - DS
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Examples Increasing entropy examples (positive DS)
Solid Liquid Gas Gas particles are less “orderly” than solids Dissolving of a solid Orderly crystals dissolve into ions in solutions When temperature increases Particles begin to move faster, have more collisions, and more disorder
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Spontaneous Reactions
If a reaction is spontaneous it will occur without the addition of energy. The spontaneity of a reaction is measured by free energy. Negative free energy (DG) is spontaneous. It means there is a “surplus” of energy so the reaction has more than enough to proceed
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Gibb’s Law of Free Energy
DG = DH – TDS + DG = Non-spontaneous - DG = Spontaneous Entropy Free Energy Enthalpy Temp (in K)
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DG = (-411.6 kJ/mol) – (298 K*.0318 kJ/molK)
Example Problem Calculate the free energy for a reaction at 25°C if DHrxn = kJ/mol & DSrxn = 31.8 J/molK. Is the reaction spontaneous? DG = DH - TDS DG = ( kJ/mol) – (298 K*.0318 kJ/molK) DG = kJ/mol Yes, it is spontaneous
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DG = DH - TDS Enthalpy Entropy Spontaneous Rxn? (neg.) Negative (exo)
Positive Yes Positive (endo) Only at high temperatures Negative Only at low temperatures No
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