previously in chem104: what determines reaction spontaneity? entropy
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Previously in Chem104: What determines reaction spontaneity? Entropy. Today in Chem104: A recap The 2 nd Law & No Free Lunch What chemists really use- free energy Worksheet: “What’s keeping you alive?”. Hypothesis 1: Spontaneous reactions are exothermic. HCl + NaOH. - PowerPoint PPT PresentationTRANSCRIPT
Previously in Chem104:
•What determines reaction spontaneity?
•Entropy
Today in Chem104:
• A recap
• The 2nd Law & No Free Lunch
• What chemists really use- free energy
• Worksheet: “What’s keeping you alive?”
Hypothesis 1: Spontaneous reactions are exothermic
HCl + NaOH Na+ + Cl- + H2O
got warm so Hrxn <0 YES!!!
Ba(OH)2.8H2O + 2NH4NO3 Ba(NO3)2 + 2NH3+
10H2Ogot COLD so Hrxn> 0 NO!!!
Conclusion 1: Spontaneous reactions can beexothermic or endothemic
Next, we encountered ENTROPY
ENTROPY:the concept An increase in disorder
An increase in energy dispersal
symbol S
ENTROPY:its magnitude
Depends on state of matter, solid< liquid<gas
Depends on temperature
Depends on complexity of molecule/matter
calculated like enthalpy: Srxn = Sprdt - Srgt
Hypothesis 2: Spontaneous reactions have increased entropy
HCl (g) + NH3 (g) NH4Cl(s)
Entropy reagent gases >> Entropy solid prdt, Srxn< 0 NO!!!
Ba(OH)2.8H2O + 2NH4NO3 Ba(NO3)2 + 2NH3+
10H2O
Srxn> 0 YES!!!
Conclusion 2: Spontaneous reactions can havea decrease in entropy!
Srxn = Sprdt - Srgt
Srxn = [150 + 2(70) + 10(192)] - [500 + 2(151)]Srxn = +432 J/K mol
So,J/K mol = 500 2(151) 150 2(70) 10(192)
Hypothesis 3: Reaction Spontaneity depends on entropy AND enthalpy
HCl (g) + NH3 (g) NH4Cl(s)
Heat released goes to surroundings?!!
Srxn = Sprdt - Srgt
Srxn = [94.6] - [187 + 193]Srxn = - 285 J/K mol
So,J/K mol = 187 193 94.6
Hrxn = Hprdt - Hrgt
Hrxn = [-315] - [-92.3 + -46.3]Hrxn = - 176 kJ/mol
Hof,kJ/mol = -92.3 -46.3 -315
ENTROPY DECREASED
ENTHALPY DECREASED
HCl (g) + NH3 (g) NH4Cl(s)
Heat released to surroundings should increase SSURR
And the net entropy change is:Snet = SSURR + SSYS = SUNIVERSE
SUNIVERSE = 591 - 285 J/K mol = 206 J/K mol
THE ENTROPY of UNIVERSE INCREASED
How much? Use this relationship of enthalpy and entropy:SSURR = -Hsys / T
So the addition of 176 kJ/mol heat to surroundings corresponds to:SSURR = - (-176 kJ/mol) / 289 = 0.591 kJ/K molSSURR = 591 J/K mol
This is the fundamental requirement
long version: a spontaneous change is accompanied by an increase in the total entropy of the system and the surroundings STOTAL = SSURR + SSYS
THE ENTROPY of UNIVERSE INCREASED
This is the 2nd Law of Thermodynamics
shorter version: total entropy change must be positive for a spontaneous reactions
shortest version: the entropy of the universe is constantly increasing
This is the fundamental requirement
Paul’s version: No Free Lunch
This means, a spontaneous exothermic reaction is not just creating heat ( or energy). The price is Entropy—more disorder in the Universe .… or a greater distribution of energy
THE ENTROPY of UNIVERSE INCREASED
This is the 2nd Law of Thermodynamics
Paul Grobstein’s version: The 1st Law: You can’t win.The 2nd Law: You can’t break even.The 3rd Law: You can’t leave the game.
Chemists rarely use this:
Why? Using this fundamental relationship:STOTAL = SSURR + SSYS
it’s too hard—impossible, really—to calculate SSURR
THE ENTROPY of UNIVERSE MUST INCREASE
to predict what will happen
So how do they predict what will happen? A new thermodynamic quantity was derived:
Free Energy“wait a minute - how can you have free energy when you can’t have a free lunch?”
Blame it all on Josiah Gibbs.
This is how he figured it: “If the universe obeys:”
SUNIVERSE = STOTAL = SSURR + SSYS
“And we can replace SSURR :” STOTAL = -HSYS / T + SSYS
“It’s all in terms of the system and we can measure it. Yeah!!”
JosiahWillardGibbs
1839-1903
Blame it all on Josiah Gibbs:
“It’s all in terms of the system. Yeah!!” STOTAL = -HSYS / T + SSYS
“Now if we do some algebra:”T x STOTAL = -HSYS + T SSYS
“And change the signs:”- T x STOTAL = HSYS - T SSYS
“We can name the result after me!” -T x STOTAL = GSYS = HSYS - T SSYS
Gibbs Free Energy, GSYS
“Is this supposed to be an improvement?”
Gibbs Free Energy, GSYS GSYS = HSYS - T SSYS
How do I tell when a spontaneous event occurs?
STOTAL > 0 is still the Law
Since -T x STOTAL = GSYS
when STOTAL >0, GSYS < 0Spontaneous reactions & processes occur whenGibbs Free Energy is negative, GSYS < 0
“Interpretation, please!”
STOTAL > 0 is still the Law
if -T x STOTAL = GSYS
Spontaneous reactions & processes occur whenGibbs Free Energy is negative, GSYS < 0
it means: the disorder in the universe
came from the system
The best part?
Grxn is calculated exactly like Hrxn and Srxn
Grxn = Gprdt - Grgt
Let’s do it!