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Questions We Can Ask. How fast will it occur – Kinetics How much heat will it give off or absorb – D H (enthalpy) Will it create more or less disorder – D S (entropy) Will it occur at all? - D G (Gibb’s Free Energy). Spontaneous Reactions. - PowerPoint PPT Presentation
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Thermodynamics
Thermodynamics
1. How fast will it occur – Kinetics
2. How much heat will it give off or absorb – H (enthalpy)
3. Will it create more or less disorder – S (entropy)
4. Will it occur at all? - G (Gibb’s Free Energy)
Questions We Can Ask
Thermodynamics
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)
Spontaneous Reactions
Thermodynamics
4. Many spontaneous rxns:
1. Exothermic – Give off heat
2. Increase disorder
Spontaneous Reactions
Thermodynamics
1. Entropy – measure of disorder
2. More disorder, larger the entropy
3. S+ More disorder (shuffling a deck of cards)
4. S- Less disorder (ordering a deck of cards)
Entropy
Thermodynamics
Boltzmann
Thermodynamics
1. States of matter
Solid Liquid Gas Plasma
2. Dissolving - S+
NaOH(s) Na+(aq) + OH-(aq)
Entropy
Thermodynamics
Thermodynamics
3. Decomposing
CaCO3(s) CaO(s) + CO2(g)
4. Increase in # gas molecules
PCl5(g) PCl3(g) + Cl2(g)
Entropy
Thermodynamics
Is S is positive or negative for:1. H2O(l) H2O(g)2. Ag+(aq) + Cl-(aq) AgCl(s)
3. 2Fe2O3(s) 4Fe(s) + 3O2(g)
4. CaO(s) + CO2(g) CaCO3(s)
5. N2(g) + 3H2(g) 2NH3(g)
6. N2(g) + O2(g) 2NO(g)
Entropy
Thermodynamics
1. Energy is conserved
2. Energy cannot be created or destroyed, it only changes form
3. E = q + w
4. Battery in a motorized toy car
Chemical Electrical Mechanical
Work Done
Some waste heat produced
The First Law
Thermodynamics
1. Entropy is not conserved
2. The universe is getting more disordered.
3. Can only create “local order.”
4. Ex: Cleaning locker – You sweat and threw a lot of things away. Universe got more disordered.
The Second Law
Thermodynamics
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
The Third Law
Thermodynamics
First Energy is conserved
Second Entropy is NOT conserved
Third Pure crystal at 0 K has zero entropy
Laws of Thermodynamics
Which of the following pairs has the higher entropy?
1. H2O(s) or H2O(l)2. NaCl(s) or HCl(g)3. HCl(g) or Ar(g)
4. N2(g) at 78 K or N2(g) at 298 K
5. H2(g) or SO2(g)6. 1 mol of HCl(g) or 2 mol HCl(g)
Thermodynamics
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
Hof So
Fe(s) 0 27.2
4.Sor = nSo
prod – mSoreactants
Calculating Entropy
Thermodynamics
1. Calculate So for:
N2(g) + 3H2(g) 2NH3(g)
(Ans: -198.3 J/ mol K, more order)
2. Calculate So for:
Al2O3(s) + 3H2(g) 2Al(s) + 3H2O(g)
(Ans: 180.4 J/ mol K, more disorder)
Calculating Entropy
Thermodynamics
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. G < 0 Rxn will occur
G = 0 Rxn at equilibrium
G > 0 Rxn will NOT occur
4. G NEVER tells you how fast a rxn will occur
Gibbs Free Energy
Thermodynamics
Thermodynamics
Thermodynamics
1. Combustion of Methane
CH4 + O2
G (-)
CO2 + H2O
Reaction position
Gibbs Free Energy
Gor = nGf
oprod – mGf
orxts
1. Will the following rxn occur?
N2(g) + 3H2(g) 2NH3(g)
(Ans: Gor = -33.32 kJ)
2. Will the following rxn occur?
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
(Ans: Gor = -800.7 kJ)
Thermodynamics
G = H – TS
T must be in Kelvin
1. Use H and S to determine if the following reaction occurs spontaneously at 500oC.
N2(g) + 3H2(g) 2NH3(g) (Ans: G = 61 kJ)
2. At what temperature will it become spontaneous?
Gibbs Free Energy
Thermodynamics
3. Use H and S to determine if the following reaction occurs spontaneously at 25 oC?
2SO2(g) + O2(g) 2SO3(g)
(Ans: G = -140.1 kJ)
4. At what temperature will it become non-spontaneous?
Gibbs Free Energy
Thermodynamics
G = H – TSH S G
- + Always spontaneous
- - Spont at low temps
+ + Spont at high temps
+ - Never spontaneous
Gibbs Free Energy
Thermodynamics
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?
Gibbs Free Energy
Thermodynamics
3. A certain rxn is exothermic and occurs at any temperature. What does this reveal?
Gibbs Free Energy
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 G (a reaction does occur)
c. Predict and explain the sign of S
d.Predict and explain the sign of H
e. High or low temperature spontaneity?
Thermodynamics
HIs heat produced or must heat a rxn
or = no
prod – morxts
Smore or less order
Sor = nSo
prod – mSorxts
Overview
Thermodynamics
G
• Tells you whether a rxn will occur spontaneously
• Considers H, S and temperature
G = H – TS
• Does NOT tell you speed
Overview
ThermodynamicsFree Energy and K
G = 0 at equilibriumG = Go + RT lnQ = Go + RT lnK
Go = -RT lnK or K = e-G/RT
Go negative K>1Go zero K=1Go positive K<1
Thermodynamics
Calculate the value of K for the following equation if Go = -33.32 kJ:
N2(g) + 3H2(g) 2NH3(g)
Go = -RT lnKlnK = Go = -33,320 J
-RT (-8.314 J/mol-K)(298K)lnK = 13.4K = e13.4 = 6.6 X 105
Example 3
Thermodynamics
Calculate Go and K for the following reaction at 298 K.
H2(g) + Br2(g) 2HBr(g)
ANS: -109.6 kJ/mol, 1.6 X 1019
Example 4
ThermodynamicsFree Energy and K
Go is at 298 K (25oC)Can calculate G at other temperatures using:
G = Go + RT lnQ
Go = Free energy change at 25oCR = 8.314 J/mol-KT = Temperature in Kelvin (Absolute temperature)Q = Reaction Coefficient
Thermodynamics
Calculate the G 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)
Example 1
Thermodynamics
Q = [NH3]2 = (0.50)2 = 0.0093
[N2][H2]3 (1.0)(3.0)3
Go = -33.32 kJ
G = Go + RT lnQG = -33,320 J + (8.3.14 J/mol-K)(298 K)(ln0.0093)G = - 44.9 kJ/mol
(larger driving force for making NH3)
Thermodynamics
Calculate the G 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)
Example 2
40.a) As4 b) 1 mol H2O(g)
c) 0.5 mol CH4 d) 100 g Na2SO4(aq)
42.a) S – (fewer moles gas)
b) S+ (more moles gas)
c) S- (fewer moles gas)
d) S – (formation of solid)
50.a) 15.9 b) -147.2 c) -207.2 d) -221.5
54. a) H- b) S+ c) G =-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
d) 40.8 kJ
62. a) -795 kJ b) +519 kJ, 3200K
74.a) G becomes more negative, more spon
b) G becomes more positive, less spon
c) G becomes more negative, more spon
76. a) Go = 29.2 kJ b) G = -9.3 kJ
78.a) K = 2 X 10-14 b) K = 1.6 X 1014
c) K = 3.5 X 1024