Advanced Thermodynamics. Unit 10 - Chapter 18 Enthalpy, Entropy, And Free Energy

Transcription

1 Advanced Thermodynamics Unit 1 - Chapter 18 Enthalpy, Entropy, And Free Energy

2 Definition of Enthalpy Def n: Property of a substance that can be used to determine the heat absorbed or released in a chemical or physical change. Mathematical Def n: H = U + PV = (q+w) + PV U = internal energy = (KE s + PE s) particles H = U + P V = q (if a constant pressure) q = heat gained or lost (+ if endo) w = work done on or by system (+ if done on) Do Exercise 18.1, page 759 and #23, page 79

3 Exchanges of Heat and Work with the Surroundings Copyright Houghton Mifflin Company. All rights reserved 18-2

4 Reaction of Zinc Metal with Hydrochloric Acid at Constant Pressure Copyright Houghton Mifflin Company. All rights reserved 18-3

5 More on Enthalpy Can t know H absolutely; can only do H Can only compare H of products & reactants relative to each other Always H, never just H Only H f are tabulated Remember def n of formation reaction? Standards (necessary for relative quantity) H f º = for element in reference form at STC H f = for the H + (aq) ion at STC Don t try to find CaCl 2(aq) in any of the tables; instead, look for 1 Ca +2 and 2 Cl -

6 Calculating H H generic = H f (prod) - H f (react) Rules for manipulating an eq n and its H H reverse = - H forward H double batch = 2 H single batch H multistep = H step 1 + H step 2 + H step 3

7 More on Enthaply In H, the indicates a standard H Measured at STC = 25 C, 1 atm, 1 M H is a state function: depends only on initial and final composition, is independent of the process that caused changed One-step process has same H as three-step The Hess Law thing, (prod) - (react), works for any state function

8 First Law of Thermodynamics U = q + w The energy of the Universe is constant Universe = System + Surroundings Whatever system gains, surroundings lose Law of Conservation of Energy

9 Definition of Entropy Def n: a measure of the disorder or randomness in a system Mathematical def n: S = k ln W K = Boltzmann s constant = R 6.22 x 1 23 W = work function = the number of possible arrangements Compare W for a pinochle deck (24 cards) to W for a poker deck (52 cards)

10 An Illustration of W Copyright Houghton Mifflin Company. All rights reserved 18-7

11 Entropy Qualitatively Which of the following is higher in Entropy? Intact beaker or Broken beaker Solute and solvent or Solution Ice or Water or Steam Small molecule or Large molecule (1) 1 atom molec. or (1) 1 atom molec Hot water or Cold water Expanded gas or Compressed gas Concentrated sol n or Dilute sol n Do Exercise 18.2, page 769

12 More on Entropy Unlike enthalpy (H) and free energy (G), entropy can be known absolutely We will use both S and S The Third Law of Thermodynamics gives us our absolute standard S = for a perfect crystal at O K S = for the H + (aq) ion Plain S is tabulated, no need to use S f Though we will see S f, too

13 Standard Entropies, S T18_1 Formula Hydrogen H + (aq) H 2 (g) Sodium Na + (aq) Na(s) NaCl(s) NaHCO 3 (s) Na 2 CO 3 (s) Calcium Ca 2+ (aq) Ca(s) CaO(s) CaCO 3 (s) Carbon C(graphite) C(diamond) CO(g) CO 2 (g) HCO 3 (aq) CH 4 (g) C 2 H 4 (g) C 2 H 6 (g) C 6 H 6 (l) HCHO(g) CH 3 OH(l) CS 2 (g) S º, S º, J/(mol K) Formula J/(mol K) Formula Carbon (continued) CS 2 (l) 151. HCN(g) 21.7 HCN(l) CCl 4 (g) 39.7 CCl 4 (l) CH 3 CHO(g) 266 C 2 H 5 OH(l) 161 Silicon Si(s) SiO 2 (s) SiF 4 (g) Lead Pb(s) PbO(s) PbS(s) Nitrogen N 2 (g) NH 3 (g) NO(g) NO 2 (g) HNO 3 (aq) Oxygen O 2 (g) O 3 (g) OH (aq) H 2 O(g) H 2 O(l) Sulfur S 2 (g) S(rhombic) S(monoclinic) SO 2 (g) H 2 S(g) Fluorine F (aq) F 2 (g) HF(g) Chlorine Cl (aq) Cl 2 (g) HCl(g) Bromine Br (aq) Br 2 (l) Iodine I (aq) I 2 (s) Silver Ag + (aq) Ag(s) AgF(s) AgCl(s) AgBr(s) AgI(s) S º, J/(mol K) Copyright Houghton Mifflin Company. All rights reserved Table 18.1

14 More on Entropy Also a State Function Calculate with S = S(prod) - S(react) Notice the differences (missing s and f ) Can calculate with S = k ln W, but difficult Rules for manipulating eq ns and their S s are the same as for H (change sign, double, and add) Do Exercise 18.5, page 771

15 The Second Law Perhaps the most important law in nature The total entropy of universe always increases in a spontaneous process Mathematically, H - T S < for a spontaneous process This is the basis for Gibbs Free Energy Natural systems prefer low enthalpy and high entropy

16 Definition of Gibbs Free Energy Def n: Quantity used to determine if a process is spontaneous Spontaneous: occurs by itself, though activation energy is needed to initiate the process Mathematical def n: G = H - T S G < for a spontaneous process (Third Law) G > for a non-spontaneous process G = for a process at equilibrium Do #43, page 791

17 Free Energy Qualitatively G = H - T S < for a spont process Four possible combinations of H and S H - and S > guaranteed spont H + and S > guaranteed non-spont H - and S > spont at low temp H + and S > spont at high temp Do #63, 65 on page 792

18 More on Gibbs Free Energy Also a State Function, so Hess Law works Rules for manipulating are the same as for H and S (change sign, double, add) Two ways to calculate G generic = G f (prod) - G f (react) G = H - T S Do Exercise 18.6, 18.7, 18.8 on pages 772-5

19 Finding non-standard G G = G + RT ln Q Q = conc. Quotient = [prod] x [react] y R = 8.31 J/(mol K) Corrects for non-std T s, [ ] s, P s We won t use eq n for this, though At equilibrium, G = and Q = K eq, so G = - RT ln K eq K eq = e - G /RT Do Exercise 18.11, on page 782-3

20 G and Work It s called free energy because it s the energy available to do work Much energy wasted on entropy (heat, expansion, ) Ex: car engine Free Energy = Total Energy - Waste Energy G = -w max G = the max work that can be done by a system Do #49 on page 791

21 Equilbrium Processes For an equilibrium process, G = This means = H - T S, so H = T S Also, S = H/T, a handy way to determine S in the lab Phase changes (boiling and melting) are examples of equilbrium processes Do Example 18.1, page 764

22 Standard Free Energies of Formation (at 25º C) T18_2A Formula Hydrogen H + (aq) H 2 (g) Sodium Na + (aq) Na(s) NaCl(s) NaHCO 3 (s) Na 2 CO 3 (s) Calcium Ca 2+ (aq) Ca(s) CaO(s) CaCO 3 (s) Carbon C(graphite) C(diamond) CO(g) CO 2 (g) HCO 3 (aq) CH 4 (g) Gº f, kj/mol Formula C 2 H 4 (g) C 2 H 6 (g) C 6 H 6 (l) HCHO(g) CH 3 OH(l) CS 2 (g) CS 2 (l) HCN(g) HCN(l) CCl 4 (g) CCl 4 (l) CH 3 CHO(g) C 2 H 5 OH(l) Silicon Si(s) SiO 2 (s) SiF 4 (g) Gº f, kj/mol Carbon (continued) Copyright Houghton Mifflin Company. All rights reserved Table 18.2A

23 Standard Free Energies of Formation (at 25º C) T18_2B Formula Lead Pb(s) PbO(s) PbS(s) Nitrogen N 2 (g) NH 3 (g) NO(g) NO 2 (g) HNO 3 (aq) Oxygen O 2 (g) O 3 (g) OH (aq) H 2 O(g) H 2 O(l) Sulfur S 2 (g) S(rhombic) S(monoclinic) SO 2 (g) H 2 S(g) Gº f, kj/mol Formula Fluorine F (aq) F 2 (g) HF(g) Chlorine Cl (aq) Cl 2 (g) HCl(g) Bromine Br (aq) Br 2 (l) Iodine I (aq) I 2 (s) Silver Ag + (aq) Ag(s) AgF(s) AgCl(s) AgBr(s) AgI(s) Gº f, kj/mol