Chapter 18: Entropy, Free Energy, and Equilibrium (2) First Law of Thermodynamics Energy (E)system = (KE) System + (PE) System ∆Energy = ∑ ∆ Energy final- ∑ ∆ Energy initial

Chapter 18: Thermodynamics: Review (2) First Law of Thermodynamics For A Chemical Reaction: ∆Energy= ∑ ∆ Energy products- ∑ ∆ Energy reactants

Chapter 18: Thermodynamics:Review For energy changes in a chemical reaction, not all of the energy is available to do work. Useful Energy = Gross Energy Change - Random/Disorganized Energy

Chapter 18: Chemical Thermodynamics, Enthalpy Enthalpy (H): Term describes Gross Energy Change Enthalpy (H) Enthalpy (H◦) Exothermic: -∆ H reaction Endothermic: + ∆ H reaction ∆H reaction = ∑ ∆ H products- ∑ ∆ H reactants

Chapter 18: Spontaneous Reactions Spontaneous Reactions proceed to formation of large amounts of products without outside intervention. Reaction rates will vary. Fe(s) + O2(g)  2Fe2O3(s)

Chapter 18: Spontaneous Reactions: Probability of Outcomes/Product Formation Spontaneous Reactions have high probability of product formation. Spontaneous reactions somehow proceed from states of low probability to states of high probability. High probability states usually result when energy is dispersed.

Pg 784

Chapter 18: Nonspontaneous Reactions Nonspontaneous reactions do not proceed to formation of large amounts of products without outside intervention. Second law of Thermodynamics: Reactions that are spontaneous in one direction are nonspontaneous in the reverse direction. Fe(s) + O2(g)  2Fe2O3(s)

Pg 784

Chapter 18: Chemical Thermodynamics, Entropy: Molecular Explanation Entropy (S): Random/Disorganized Energy Useful Energy/Gibbs (G) : Energy actually available to do work Useful Energy = Gross Energy Change - Random/Disorganized Energy ∆ G reaction = ∆ H reaction - T∆ S reaction

Chapter 18: Entropy (S) Degree of Random energy/disordered energy is temperature related. Solid liquid gas

Chapter 18: Entropy Entropy increases if disorder increases. NaCl(s)  Na+ + Cl- Ag+ + Cl-  AgCl(s)

ENTROPY & PROBABILITY Probable Event: Can Happen in Many Ways RANDON DISORDER Improbable Event: Can only happen in one or two ways ORDER

Entropy: Microstates Microstates: Different patterns; Different Outcomes Boltzman: Entropy Related to ln of Number of Microstates S = k ln W K = 1.38E-23 Joules/Kelvin

Entropy (S): Spontaneous and Nonspontaneous Reactions Spontaneous reactions somehow proceed from states of low probability to states of high probability. High probability states usually result when energy is dispersed.

Entropy (S): Spontaneous and Nonspontaneous Reactions Entropy is random or disorganized energy. Spontaneous reactions always have an increase in Entropy.

Entropy (S): Spontaneous and Nonspontaneous Reactions ∆S reaction = ∑ ∆ S products- ∑ ∆ S reactants ∆S◦ reaction = ∑ ∆ S◦ products- ∑ ∆ S◦ reactants Spontaneous reactions always have an increase in entropy. + ∆S◦ reaction

Entropy (S): Spontaneous and Nonspontaneous Reactions ∆S◦ reaction = ∑ ∆ S◦ products- ∑ ∆ S◦ reactants Nonspontaneous reactions usually have a decrease in entropy. - ∆S◦ reaction

Entropy (S): Spontaneous and Nonspontaneous Reactions + ∆S◦ reaction Does not mean reaction will be spontaneous! (a) As temperature increases, entropy (S) usually also increases. ∆Greaction = ∆H reaction -T ∆S reaction

Entropy Projections A process that is spontaneous in one direction is not spontaneous in the opposite direction. The direction of a spontaneous process can depend on temperature: Ice turning to water is spontaneous at T > 0C, Water turning to ice is spontaneous at T < 0C.

Entropy (S): Spontaneous and Nonspontaneous Reactions + ∆S◦ reaction Does not mean reaction will be spontaneous! (b) Vibrational Energy: Energy may be absorbed by bonds. (c) Under certain conditions, molecules of reactants may become so disordered that the inherent energy is not available to form products.

Problem Predict the sign of the entropy change of the system for each of the following reactions. (a) 2SO2(g) + O2(g)  2SO3(g) (b) Ba(OH)2(s) BaO(s) + H2O(g)

Problem Using S◦ values from appendix C, calculate ∆S◦ reaction values for each of the following reactions. In each case, account for the sign of ∆S◦ reaction. (a) C2H4(g) + H2(g)  C2H6(g) (b) N2O4(g) 2NO2(g)

Chapter 18: Useful/Free Energy (G) Useful/Free energy is that energy which does work. SPONTANEOUS: - ∆G◦ reaction NONSPONTANEOUS: + ∆G◦ reaction

Chapter 18: Useful Energy and Equilibrium N2(g) + 3H2(g) ↔ 2NH3(g) If start with reactants, must have - ∆G for forward reaction to form products. If start with products, must have - ∆G for reverse reaction to form reactants.

Chapter 18: Useful Energy and Equilibrium, ∆Greaction versus ∆G ◦reaction At equilibrium, rate of forward = rate reverse At equilibrium ∆Greaction is zero. At equilibrium, ∆G ◦reaction is not zero.

Gibbs Free Energy

CALCULATION OF ∆ G REACTION ∆ E = ∑ (n) (E products) -∑ (n) (E reactants) ∆ H =∑ (n) (H products) -∑(n)(H reactants) ∆ S =∑ (n)(S products) -∑(n)(S reactants) ∆ G =∑(n)(G products) -∑(n)(G reactants)

CALCULATION OF ∆ G REACTION ∆Gº =∑ (n)(Gº products) -∑(n)(Gº reactants) ∆G =∑ (n)(G products) -∑(n)(G reactants) ∆G = ∆H - T∆S ∆G º = ∆H º - T∆S º

Chapter 18: Useful energy and Signs Spontaneous: - ∆ H º Nonspontaneous: + ∆ Hº Spontaneous: + ∆ S º Nonspontaneous: - ∆ S º Spontaneous: - ∆ G º Nonspontaneous: + ∆ G º

Problem Using data from Appendix, Calculate ∆ Gº for the following reactions. (a) 2SO2(g) + O2(g)  2SO3(g) (b) NO2(g) + N2O(g)  3NO(g)

Gibbs Free Energy: Temperature ∆ G reaction = ∆ H reaction - T∆ S reaction

Table 18.3

Calculation of Actual ∆ G Why is ∆ G not always the same as ∆ G º ? ∆ G = ∆ G º + RT ln Q R = 8.314 Joules/(K) (mole)

Problem Consider the reaction 2NO2(g)  N2O4(g). (a) Using appendix C, calculate ∆ G º at 298 K. (b) Calculate ∆ G at 298 K if the partial pressures of NO2 and N2O4 are 0.40 atm and 1.60 atm respectively.

CHAPTER 18: Free Energy and Equilibrium Constant

Problem 18.24: Page 830 For the autoionization of pure water at 25 °C, Kw = 1.0E-14. What is ∆ G º for the reaction. H2O(l)  H+(aq) + OH-(aq)

Problem Use data from Appendix C to calculate Keq at 298 K for each of the following reactions: (a) H2(g) + I2(g) ↔ 2HI(g)