Download Entropy

Thermodynamics
Thermodynamics
Questions We Can Ask
1. How fast will it occur – Kinetics
2. How much heat will it give off or absorb –
DH (enthalpy)
3. Will it create more or less disorder – DS
(entropy)
4. Will it occur at all? - DG (Gibb’s Free
Energy)
Thermodynamics
Spontaneous Reactions
1. Spontaneous Rxns – occur without an
outside source of energy
2. Direction – Rxns are spontaneous in one
direction only (eggs breaking picture)
3. Temperature has an effect
Ice  Water (spont. Above 0oC, reverse
not spontaneous)
Thermodynamics
Spontaneous Reactions
4. Many spontaneous rxns:
1. Exothermic – Give off heat
2. Increase disorder
Thermodynamics
Entropy
1. Entropy – measure of disorder
2. More disorder, larger the entropy
3. DS+ More disorder
(shuffling a deck of cards)
4. DS- Less disorder
(ordering a deck of cards)
Thermodynamics
Boltzmann
Thermodynamics
Entropy
1. States of matter
Solid  Liquid  Gas  Plasma
2. Dissolving - DS+
NaOH(s)  Na+(aq) + OH-(aq)
Thermodynamics
Thermodynamics
Entropy
3. Decomposing
CaCO3(s)  CaO(s) + CO2(g)
4. Increase in # gas molecules
PCl5(g)  PCl3(g) + Cl2(g)
Thermodynamics
Entropy
Is DS is positive or negative for:
1.
2.
3.
4.
5.
6.
H2O(l)  H2O(g)
Ag+(aq) + Cl-(aq)  AgCl(s)
2Fe2O3(s) 4Fe(s) + 3O2(g)
CaO(s) + CO2(g)  CaCO3(s)
N2(g) + 3H2(g)  2NH3(g)
N2(g) + O2(g)  2NO(g)
Thermodynamics
The First Law
1. Energy is conserved
2. Energy cannot be created or destroyed, it only
changes form
3. DE = q + w
4. Battery in a motorized toy car
Chemical  Electrical Mechanical
Work Done
Some waste heat produced
Thermodynamics
The Second Law
1.
2.
3.
4.
Entropy is not conserved
The universe is getting more disordered.
Can only create “local order.”
Ex: Cleaning locker – You sweat and
threw a lot of things away. Universe got
more disordered.
Thermodynamics
The Third Law
1. The entropy of a pure crystal at absolute
zero is zero
2. Theoretical state of perfect order
3. Above 0 K
At 0 K
atoms wiggle
no molec.
motion
Thermodynamics
Laws of Thermodynamics
First
Energy is conserved
Second
Entropy is NOT conserved
Third
Pure crystal at 0 K has zero entropy
Which of the following pairs has the higher
entropy?
1.
2.
3.
4.
5.
6.
H2O(s) or H2O(l)
NaCl(s) or HCl(g)
HCl(g) or Ar(g)
N2(g) at 78 K or N2(g) at 298 K
H2(g) or SO2(g)
1 mol of HCl(g) or 2 mol HCl(g)
Thermodynamics
Calculating Entropy
1. Standard Molar Entropies – at 1 atm and 25oC (298
K)
2. Unit – Joule/ mol K
3. Standard molar entropies of element is not zero
DHof
So
Fe(s) 0
27.2
4. DSor = SnSoprod – S mSoreactants
Thermodynamics
Calculating Entropy
1. Calculate DSo for:
N2(g) + 3H2(g)  2NH3(g)
(Ans: -198.3 J/ mol K, more order)
2. Calculate DSo for:
Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)
(Ans: 180.4 J/ mol K, more disorder)
Thermodynamics
Gibbs Free Energy
1. Josiah Gibbs – 1st Ph.D. in science from a U.S.
University (Yale, 1863)
2. “Free Energy” – Maximum amount of work you
can get from a chemical reaction
3. DG < 0 Rxn will occur
DG = 0 Rxn at equilibrium
DG > 0 Rxn will NOT occur
4. DG NEVER tells you how fast a rxn will occur
Thermodynamics
Thermodynamics
Thermodynamics
Gibbs Free Energy
1. Combustion of Methane
CH4 + O2
DG (-)
CO2 + H2O
Reaction position
DGor = SnDGfoprod – S mDGforxts
1. Will the following rxn occur?
N2(g) + 3H2(g)  2NH3(g)
(Ans: DGor = -33.32 kJ)
2. Will the following rxn occur?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
(Ans: DGor = -800.7 kJ)
Thermodynamics
Gibbs Free Energy
DG = DH – TDS
T must be in Kelvin
1. Use DH and DS to determine if the following reaction
occurs spontaneously at 500oC.
N2(g) + 3H2(g)  2NH3(g) (Ans: DG = 61 kJ)
2. At what temperature will it become spontaneous?
Thermodynamics
Gibbs Free Energy
3. Use DH and DS to determine if the following
reaction occurs spontaneously at 25 oC?
2SO2(g) + O2(g)  2SO3(g)
(Ans: DG = -140.1 kJ)
4. At what temperature will it become nonspontaneous?
Thermodynamics
Gibbs Free Energy
DG = DH – TDS
DH
-
DS
+
DG
Always spontaneous
-
-
Spont at low temps
+
+
Spont at high temps
+
-
Never spontaneous
Thermodynamics
Gibbs Free Energy
1. A certain rxn is exothermic and becomes
more ordered. Will the rxn occur at any
temperature?
2. A certain rxn is endothermic and becomes
less ordered. Will it occur?
Thermodynamics
Gibbs Free Energy
3. A certain rxn is exothermic and occurs at
any temperature. What does this reveal?
A solution of sodium chloride is added to a
solution of silver(I)nitrate.
a. Write the net ionic reaction
b. Predict and explain the sign of DG (a reaction
does occur)
c. Predict and explain the sign of DS
d. Predict and explain the sign of DH
e. High or low temperature spontaneity?
Thermodynamics
Overview
DH
Is heat produced or must heat a rxn
DHor = SnDHoprod – S mDHorxts
DS
more or less order
DSor = SnDSoprod – S mDSorxts
Thermodynamics
Overview
DG
• Tells you whether a rxn will occur
spontaneously
• Considers DH, DS and temperature
DG = DH – TDS
• Does NOT tell you speed
Thermodynamics
Free Energy and K
DG = 0 at equilibrium
DG = DGo + RT lnQ
 = DGo + RT lnK
DGo = -RT lnK
DGo negative
DGo zero
DGo positive
K>1
K=1
K<1
or
K = e-DG/RT
Thermodynamics
Example 3
Calculate the value of K for the following equation
if DGo = -33.32 kJ:
N2(g) + 3H2(g)  2NH3(g)
DGo = -RT lnK
lnK = DGo =
-33,320 J
-RT (-8.314 J/mol-K)(298K)
lnK = 13.4
K = e13.4 = 6.6 X 105
Thermodynamics
Example 4
Calculate DGo and K for the following reaction at
298 K.
H2(g) + Br2(g)  2HBr(g)
ANS: -109.6 kJ/mol, 1.6 X 1019
Thermodynamics
Free Energy and K
DGo is at 298 K (25oC)
Can calculate DG at other temperatures using:
DG = DGo + RT lnQ
DGo = Free energy change at 25oC
R = 8.314 J/mol-K
T = Temperature in Kelvin (Absolute temperature)
Q = Reaction Coefficient
Thermodynamics
Example 1
Calculate the DG at 298 K for a reaction mixture of
1.0 atm of N2, 3.0 atm of H2, and 0.50 atm of
NH3.
N2(g) + 3H2(g)  2NH3(g)
Thermodynamics
Q = [NH3]2
=
[N2][H2]3
DGo = -33.32 kJ
(0.50)2
=
(1.0)(3.0)3
0.0093
DG = DGo + RT lnQ
DG = -33,320 J + (8.3.14 J/mol-K)(298 K)(ln0.0093)
DG = - 44.9 kJ/mol
(larger driving force for making NH3)
Thermodynamics
Example 2
Calculate the DG at 298 K for a reaction mixture of
0.50 atm of N2, 0.75 atm of H2, and 2.0 atm of
NH3.
(ANS: -26.0 kJ/mol)
40.a) As4
b) 1 mol H2O(g)
c) 0.5 mol CH4 d) 100 g Na2SO4(aq)
42.a) DS – (fewer moles gas)
b) DS+ (more moles gas)
c) DS- (fewer moles gas)
d) DS – (formation of solid)
50.a) 15.9
b) -147.2 c) -207.2 d) -221.5
54. a) DHb) DS+
c) DG =-32.2 kJ (spon)
56.a) -2116.4 kJ
b) 218.1 kJ c) -895.0 kJ
d) -240.6 kJ
58.a) -190.5 kJ b) 69.1 kJ c) 192.7 kJ
62. a) -795 kJ b) +519 kJ, 3200K
74.a) DG becomes more negative, more spon
b) DG becomes more positive, less spon
c) DG becomes more negative, more spon
76. a) DGo = 29.2 kJ
b) DG = -9.3 kJ
78.a) K = 2 X 10-14
b) K = 1.6 X 1014
c) K = 3.5 X 1024