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Spontaneous Processes and EntropySpontaneous Processes and Entropy
• First Law• “Energy can neither be created nor
destroyed“.• The energy of the universe is constant.
• Spontaneous Processes• Processes that occur without outside
intervention.• Spontaneous processes may be fast or
slow.–Many forms of combustion are fast–Conversion of diamond to graphite is
slow
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Entropy (S)Entropy (S)• A measure of the randomness or A measure of the randomness or disorderdisorder. . • The driving force for a spontaneous The driving force for a spontaneous
process is an process is an increaseincrease in the in the entropyentropy of the of the universe.universe.
• Entropy is a thermodynamic function Entropy is a thermodynamic function describing the describing the numbernumber of of arrangements arrangements that are available to a system.that are available to a system.
• Nature proceeds toward the Nature proceeds toward the statesstates that that have the have the highesthighest probabilities of existing. probabilities of existing.
• The most likely is the most random.The most likely is the most random.
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Positional EntropyPositional Entropy• The The probabilityprobability of occurrence of a of occurrence of a
particular state depends on the particular state depends on the numbernumber of of waysways (microstates) in which that (microstates) in which that arrangement can be achieved.arrangement can be achieved.
• Positional entropy increases from solid Positional entropy increases from solid to liquid to gas.to liquid to gas.
SSsolidsolid < S < Sliquidliquid << S << Sgasgas
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• Solid state: molecules are Solid state: molecules are closeclose together together with relatively few positions available.with relatively few positions available.
• Gaseous state: molecules are Gaseous state: molecules are farfar apart, apart, with more positions available.with more positions available.
• Liquid state is Liquid state is closercloser to the to the solidsolid state state than gaseous state.than gaseous state.
SSsolidsolid < S < Sliquidliquid << S << Sgasgas
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Second Law of ThermodynamicsSecond Law of Thermodynamics "In any spontaneous process there is
always an increase in the entropy of the universe"
"The entropy of the universe is increasing"
For a given change to be spontaneous, Suniverse must be positive
Suniv = Ssys + Ssurr
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Temperature and Spontaneity• Entropy changes in the surroundings are primarily determined
by heat flow.• Exothermic reactions in a system at constant temperature
increase the entropy of surroundings.• ∆Ssurr = positive• Endothermic reactions in a system at constant temperature
decrease the entropy of surroundings.• ∆Ssurr = negative• The impact of the transfer of a given quantity of energy as heat
to or from the surroundings will be greater at lower temperatures.
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•∆Ssurr for a reaction under conditions of constant temperature (K) and pressure:
∆Ssurr = -∆H / T
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•Gaseous reactions:•Calculate change in the number of moles of gas, ∆ngas on going from reactants to products.•∆ngas is positive = entropy is positive
•2NaHCO3 (s) → Na2 CO3 (s) + CO2 (g) + H2O(g)•Gaseous products > Gaseous reactants•Change in Entropy = ∆S = positive
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•Another factor that affects the sign of ∆S is degree of complexity of molecules.
H2 (g) → 2H(g)
•Decrease in complexity and increase in number of particles = increase in entropy
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•“At absolute zero the entropy of a perfectly ordered pure crystalline substance is zero.”
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•Since pure crystal is zero all other must be > 0.•Standard Entropy of 1 mol of substance, So is measured at 298 K (25o C) and 1 atm.•Units are J/K mol.•Values published in Appendix 4 of textbook.
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Calculating Entropy Change in a Calculating Entropy Change in a ReactionReaction
Entropy is an extensive property (a function of the number of moles)
Generally, the more complex the molecule, the higher the standard entropy value
0 0 oreaction products reactantsp rS n S n S