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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.

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Previously in Chem104: What determines reaction spontaneity? Entropy

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Previously in chem104 what determines reaction spontaneity entropy

  • 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?”


Previously in chem104 what determines reaction spontaneity entropy

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


Previously in chem104 what determines reaction spontaneity entropy

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


Previously in chem104 what determines reaction spontaneity entropy

Hypothesis 2: Spontaneous reactions have

increased entropy

HCl (g) + NH3 (g)

NH4Cl(s)

So,J/K mol = 500 2(151) 150 2(70) 10(192)

DSrxn = DSprdt - DSrgt

Ba(OH)2.8H2O + 2NH4NO3

Ba(NO3)2 + 2NH3+ 10H2O

DSrxn = [150 + 2(70) + 10(192)] - [500 + 2(151)]

DSrxn = +432 J/K mol

DSrxn> 0  YES!!!

Entropy reagent gases >> Entropy solid prdt, DSrxn< 0  NO!!!

Conclusion 2: Spontaneous reactions can have

a decrease in entropy!


Previously in chem104 what determines reaction spontaneity 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]

DSrxn = - 285 J/K mol

ENTROPY DECREASED

DHof,kJ/mol = -92.3 -46.3 -315

DHrxn = DHprdt - DHrgt

DHrxn = [-315] - [-92.3 + -46.3]

DHrxn = - 176 kJ/mol

ENTHALPY DECREASED

Heat released goes to surroundings?!!


Previously in chem104 what determines reaction spontaneity entropy

Heat released to surroundings should increase DSSURR

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

HCl (g) + NH3 (g)

NH4Cl(s)

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


Previously in chem104 what determines reaction spontaneity entropy

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: the entropy of the universe is constantly increasing


Previously in chem104 what determines reaction spontaneity entropy

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.


Previously in chem104 what determines reaction spontaneity entropy

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?”


Previously in chem104 what determines reaction spontaneity entropy

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!!”


Previously in chem104 what determines reaction spontaneity entropy

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 xDSTOTAL = -DHSYS + T DSSYS

“And change the signs:”

- T xDSTOTAL = DHSYS - T DSSYS

“We can name the result after me!”

-T xDSTOTAL = DGSYS = DHSYS - T DSSYS

Gibbs Free Energy, DGSYS


Previously in chem104 what determines reaction spontaneity entropy

“Is this supposed to be an improvement?”

Gibbs Free Energy, DGSYSDGSYS = DHSYS - T DSSYS

How do I tell when a spontaneous event occurs?

DSTOTAL > 0 is still the Law

Since -T xDSTOTAL = DGSYS

when DSTOTAL >0, DGSYS < 0

Spontaneous reactions & processes occur when

Gibbs Free Energy is negative, DGSYS < 0


Previously in chem104 what determines reaction spontaneity entropy

“Interpretation, please!”

DSTOTAL > 0 is still the Law

if-T xDSTOTAL = DGSYS

it means: the disorder

in the universe

came from

the system

Spontaneous reactions & processes occur when

Gibbs Free Energy is negative, DGSYS < 0


Previously in chem104 what determines reaction spontaneity entropy

The best part?

DGrxn is calculated exactly like DHrxn and DSrxn

DGrxn = DGprdt - DGrgt

Let’s do it!


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