Previously in
Chem104:
Today in Chem104:
•What determines
reaction
spontaneity?
• A recap
•Entropy
• 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 DHrxn <0  YES!!!
Ba(OH)2.8H2O + 2NH4NO3
Ba(NO3)2 + 2NH3+ 10H2O
got COLD so DHrxn> 0  NO!!!
Conclusion 1: Spontaneous reactions can be
exothermic or endothemic
Next, we encountered ENTROPY
ENTROPY:
the concept
An increase in disorder
An increase in energy dispersal
symbol S
calculated like enthalpy: DSrxn = DSprdt - DSrgt
ENTROPY:
its magnitude
Depends on state of matter, solid< liquid<gas
Depends on temperature
Depends on complexity of molecule/matter
Hypothesis 2: Spontaneous reactions have
increased entropy
Ba(OH)2.8H2O + 2NH4NO3
Ba(NO3)2 + 2NH3+ 10H2O
So,J/K mol = 500
2(151)
150
2(70)
10(192)
DSrxn = DSprdt - DSrgt
DSrxn = [150 + 2(70) + 10(192)] - [500 + 2(151)]
DSrxn = +432 J/K mol
DSrxn> 0  YES!!!
HCl (g) + NH3 (g)
NH4Cl(s)
Entropy reagent gases >> Entropy solid prdt, DSrxn< 0  NO!!!
Conclusion 2: Spontaneous reactions can have
a decrease in entropy!
Hypothesis 3: Reaction Spontaneity depends on
entropy AND enthalpy
HCl (g) + NH3 (g)
NH4Cl(s)
So,J/K mol = 187
193
94.6
DSrxn = DSprdt - DSrgt
DSrxn = [94.6] - [187 + 193] ENTROPY DECREASED
DSrxn = - 285 J/K mol
DHof,kJ/mol = -92.3
-46.3
-315
DHrxn = DHprdt - DHrgt
DHrxn = [-315] - [-92.3 + -46.3]
ENTHALPY DECREASED
DHrxn = - 176 kJ/mol
Heat released goes to surroundings?!!
Heat released to surroundings should increase DSSUR
How much?
Use this relationship of enthalpy and entropy:
DSSURR = -DHsys / T
So the addition of 176 kJ/mol heat to surroundings
corresponds to:
DSSURR = - (-176 kJ/mol) / 289 = 0.591 kJ/K mol
DSSURR = 591 J/K mol
And the net entropy change is:
DSnet = DSSURR + DSSYS = DSUNIVERSE
DSUNIVERSE = 591 - 285 J/K mol = 206 J/K mol
THE ENTROPY of UNIVERSE INCREASED
HCl (g) + NH3 (g)
NH4Cl(s)
This is the fundamental requirement
THE ENTROPY of UNIVERSE INCREASED
This is the 2nd Law of Thermodynamics
long version:
a spontaneous change is accompanied by
an increase in the total entropy of the system and the
surroundings
DSTOTAL = DSSURR + DSSYS
shorter version:
total entropy change must be positive for
a spontaneous reactions
shortest version:
increasing
the entropy of the universe is constantly
This is the fundamental requirement
THE ENTROPY of UNIVERSE INCREASED
This is the 2nd Law of Thermodynamics
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
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:
THE ENTROPY of UNIVERSE MUST INCREASE
to predict what will happen
Why? Using this fundamental relationship:
DSTOTAL = DSSURR + DSSYS
it’s too hard—impossible, really—to calculate DSSURR
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.
Josiah
Willard
Gibbs
1839-1903
This is how he figured it:
“If the universe obeys:”
DSUNIVERSE = DSTOTAL = DSSURR + DSSYS
“And we can replace DSSURR :”
DSTOTAL = -DHSYS / T + DSSYS
“It’s all in terms of the system and we can measure it.
Yeah!!”
Blame it all on Josiah Gibbs:
“It’s all in terms of the system. Yeah!!”
DSTOTAL = -DHSYS / T + DSSYS
“Now if we do some algebra:”
T x DSTOTAL = -DHSYS + T DSSYS
“And change the signs:”
- T x DSTOTAL = DHSYS - T DSSYS
“We can name the result after me!”
-T x DSTOTAL = DGSYS = DHSYS - T DSSYS
Gibbs Free Energy, DGSYS
“Is this supposed to be an improvement?”
Gibbs Free Energy, DGSYS
DGSYS = DHSYS - T DSSYS
How do I tell when a spontaneous event occurs?
DSTOTAL > 0 is still the Law
Since
when
-T x DSTOTAL = DGSYS
DSTOTAL >0,
DGSYS < 0
Spontaneous reactions & processes occur when
Gibbs Free Energy is negative,
DGSYS < 0
“Interpretation, please!”
DSTOTAL > 0 is still the Law
if
-T x DSTOTAL = DGSYS
it means: the disorder
in the universe
came from
the system
Spontaneous reactions & processes occur when
Gibbs Free Energy is negative,
DGSYS < 0
The best part?
DGrxn is calculated exactly like DHrxn and DSrxn
DGrxn = DGprdt - DGrgt
Let’s do it!