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 0 o C, 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  S+More disorder (shuffling a deck of cards)  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

3.Decomposing CaCO 3 (s)  CaO(s) + CO 2 (g) 4. Increase in # gas molecules PCl 5 (g)  PCl 3 (g) + Cl 2 (g) Entropy

Thermodynamics Is  S is positive or negative for: 1.H 2 O(l)  H 2 O(g) 2.Ag + (aq) + Cl - (aq)  AgCl(s) 3.2Fe 2 O 3 (s)  4Fe(s) + 3O 2 (g) 4.CaO(s) + CO 2 (g)  CaCO 3 (s) 5.N 2 (g) + 3H 2 (g)  2NH 3 (g) 6.N 2 (g) + O 2 (g)  2NO(g) Entropy

Thermodynamics 1.Energy is conserved 2.Energy cannot be created or destroyed, it only changes form  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 KAt 0 K atoms wiggleno molec. motion The Third Law

Thermodynamics FirstEnergy is conserved SecondEntropy is NOT conserved ThirdPure crystal at 0 K has zero entropy Laws of Thermodynamics

Which of the following pairs has the higher entropy? 1. H 2 O(s) or H 2 O(l) 2. NaCl(s) or HCl(g) 3. HCl(g) or Ar(g) 4. N 2 (g) at 78 K or N 2 (g) at 298 K 5. H 2 (g) or SO 2 (g) 6. 1 mol of HCl(g) or 2 mol HCl(g)

Thermodynamics 1.Standard Molar Entropies – at 1 atm and 25 o C (298 K) 2.Unit – Joule/ mol K 3.Standard molar entropies of element is not zero  H o f  S o Fe(s)  S o r =  nS o prod –  mS o reactants Calculating Entropy

Thermodynamics 1.Calculate  S o for: N 2 (g) + 3H 2 (g)  2NH 3 (g) (Ans: J/ mol K, more order) 2. Calculate  S o for: Al 2 O 3 (s) + 3H 2 (g)  2Al(s) + 3H 2 O(g) (Ans: J/ mol K, more disorder) Calculating Entropy

Thermodynamics 1.Josiah Gibbs – 1 st 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  G < 0Rxn will occur  G = 0Rxn at equilibrium  G > 0Rxn will NOT occur 4.  G NEVER tells you how fast a rxn will occur Gibbs Free Energy

Thermodynamics

1.Combustion of Methane CH 4 + O 2  G (-) CO 2 + H 2 O Reaction position Gibbs Free Energy

 G o r =  n  G f o prod –  m  G f o rxts 1.Will the following rxn occur? N 2 (g) + 3H 2 (g)  2NH 3 (g) (Ans:  G o r = kJ) 2. Will the following rxn occur? CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) (Ans:  G o r = kJ)

Thermodynamics  G =  H – T  S T must be in Kelvin 1.Use  H and  S to determine if the following reaction occurs spontaneously at 500 o C. N 2 (g) + 3H 2 (g)  2NH 3 (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 o C? 2SO 2 (g) + O 2 (g)  2SO 3 (g) (Ans:  G = kJ) 4. At what temperature will it become non- spontaneous? Gibbs Free Energy

Thermodynamics  G =  H – T  S HH SS G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  H Is heat produced or must heat a rxn  o r =  n  o prod –  m  o rxts  S more or less order  S o r =  n  S o prod –  m  S o rxts Overview

Thermodynamics  G Tells you whether a rxn will occur spontaneously Considers  H,  S and temperature  G =  H – T  S Does NOT tell you speed Overview

Thermodynamics Free Energy and K  G = 0 at equilibrium  G =  G o + RT lnQ  =  G o + RT lnK  G o = -RT lnK orK = e -  G/RT  G o negativeK>1  G o zeroK=1  G o positiveK<1

Thermodynamics Calculate the value of K for the following equation if  G o = kJ : N 2 (g) + 3H 2 (g)  2NH 3 (g)  G o = -RT lnK lnK =  G o = -33,320 J -RT ( J/mol-K)(298K) lnK = 13.4 K = e 13.4 = 6.6 X 10 5 Example 3

Thermodynamics Calculate  G o and K for the following reaction at 298 K. H 2 (g) + Br 2 (g)  2HBr(g) ANS: kJ/mol, 1.6 X Example 4

Thermodynamics Free Energy and K  G o is at 298 K (25 o C) Can calculate  G at other temperatures using:  G =  G o + RT lnQ  G o = Free energy change at 25 o C R = J/mol-K T = Temperature in Kelvin (Absolute temperature) Q = Reaction Coefficient

Thermodynamics Calculate the  G at 298 K for a reaction mixture of 1.0 atm of N 2, 3.0 atm of H 2, and 0.50 atm of NH 3. N 2 (g) + 3H 2 (g)  2NH 3 (g) Example 1

Thermodynamics Q = [NH 3 ] 2 =(0.50) 2 = [N 2 ][H 2 ] 3 (1.0)(3.0) 3  G o = kJ  G =  G o + RT lnQ  G = -33,320 J + ( J/mol-K)(298 K)(ln0.0093)  G = kJ/mol (larger driving force for making NH 3 )

Thermodynamics Calculate the  G at 298 K for a reaction mixture of 0.50 atm of N 2, 0.75 atm of H 2, and 2.0 atm of NH 3. (ANS: kJ/mol) Example 2

40.a) As 4 b) 1 mol H 2 O(g) c) 0.5 mol CH 4 d) 100 g Na 2 SO 4 (aq) 42.a)  S – (fewer moles gas) b)  S+ (more moles gas) c)  S- (fewer moles gas) d)  S – (formation of solid) 50.a) 15.9b) c) d) a)  H-b)  S+c)  G =-32.2 kJ (spon) 56.a) kJb) kJ c) kJ d) kJ 58.a) kJb) 69.1 kJ c) kJ d) 40.8 kJ

62. a) -795 kJb) +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)  G o = 29.2 kJb)  G = -9.3 kJ 78.a) K = 2 X b) K = 1.6 X c) K = 3.5 X 10 24