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