Review Unit 7 (Chp 5,8,19): Thermodynamics (∆Ho, ∆So, ∆Go, K) Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Review Unit 7 (Chp 5,8,19): Thermodynamics (∆Ho, ∆So, ∆Go, K) John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc.

Changes in Internal Energy Energy is transferred between the system and surroundings, as either heat (q) or work (w). E = q + w E = ? E = (–) + (+) Surroundings E = + System q in (+) q out (–) E = q + w w on (+) w by (–)

+ = Energy (E) Enthalpy (H) (kJ) Entropy (S) (J/K) Free Energy (G) (kJ) ΔH = ΔE + PΔV internal work by energy system (KE + PE) (–w) (disorder) microstates –T∆Suniv as: ΔHsys & ΔSsys at a T dispersal of matter & energy at T max work done by favorable rxn ΔE = q + w PΔV = –w (at constant P) K > 1 means –∆Gsys & +∆Suniv ∆Suniv = + ΔH = q (heat) ΔS = ΔH T ΔG = ΔH – TΔS

Big Idea #5: Thermodynamics Bonds break and form to lower free energy (∆G). Systems are driven by a decrease in DG (–∆G) by: a decrease in enthalpy (–∆H), or an increase in entropy (+∆S), or both.

Chp. 5,8: Calculate ∆H (4 Ways) 1) Bond Energies Hrxn = (BEreactants)  (BEproducts) 2) Hess’s Law Hoverall = Hrxn1 + Hrxn2 + Hrxn3 … 3) Standard Heats of Formation (Hf ) H = nHf(products) – nHf(reactants) 4) Calorimetry (lab) q = mc∆T (surroundings or thermometer) –q = ∆H ∆H/mol = kJ/mol (molar enthalpy) (NOT given) (+ broken) (– formed) (NOT given)     (given) (given)

Entropy (S) (Molecular Scale) S : dispersal of matter & energy at T S(s) < S(l) < S(aq) < S(g) (s) + (l)  (aq) +∆S (dispersal) gas solid T more microstates Temperature  Volume  Particle mixing  Particle number  Particle size V H2O(g) H2O(g) So = So(products) – So(reactants) (given)

Thermodynamically Favorable Chemical and physical processes are driven by: decrease in enthalpy (–∆Hsys) increase in entropy (+∆Ssys) causes (+∆Ssurr) (+) (+) Suniv = Ssystem + Ssurroundings > 0 Thermodynamically Favorable: (defined as) increasing entropy of the universe (∆Suniv > 0) ∆Suniv > 0 (+Entropy of the Universe)

(∆Suniv) ↔ (∆Gsys) Hsystem For all thermodynamically favorable reactions: Suniverse = Ssystem + Ssurroundings > 0 (Boltzmann) Hsystem T Suniverse = Ssystem + (Clausius) multiplying each term by T: –TSuniverse = –TSsystem + Hsystem rearrange terms: –TSuniverse = Hsystem – TSsystem Gsystem = Hsystem – TSsystem (Gibbs free energy equation)

(∆Suniv) & (∆Gsys) –TSuniv = Hsys – TSsys Gsys = Hsys – TSsys (Gibbs free energy equation) Gibbs defined TDSuniv as the change in free energy of a system (Gsys) or G. Free Energy (G) is more useful than Suniv b/c all terms focus on the system. If –Gsys , then +Suniverse . Therefore… –G is thermodynamically favorable. “Bonds break & form to lower free energy (∆G).”

Standard Free Energy (∆Go) and Temperature (T) (on equation sheet) (consists of 2 terms) DG = DH – TS free energy (kJ/mol) enthalpy term (kJ/mol) entropy term (J/mol∙K) units convert to kJ!!! max energy used for work energy transferred as heat energy dispersed as disorder The temperature dependence of free energy comes from the entropy term (–TS).

Standard Free Energy (∆Go) and Temperature (T) DG = DH  TS Thermodynamic Favorability ∆Go = (∆Ho) ∆So – T( ) ( ) –T( ) (high T) – (low T) + (fav. at high T) (unfav. at low T) + + = ( ) – T ( ) + + – (unfav. at ALL T) + = ( ) – T( ) + (fav. at ALL T) – = ( ) – T( ) – + (high T) + (low T) – ( ) –T( ) (unfav. at high T) (fav. at low T) – – = ( ) – T ( ) – –

Calculating ∆Go (4 ways) Standard free energies of formation, Gf : Gibbs Free Energy equation: From K value (next few slides) From voltage, Eo (next Unit) DG = SG(products) – SG(reactants) f (given equation) DG = DH – TS (given equation) (may need to calc. ∆Ho & ∆So first) (given equation) (given equation)

Free Energy (∆G) & Equilibrium (K) G = –RT ln K (on equation sheet) If G in kJ, then R in kJ……… R = 8.314 J∙mol–1∙K–1 = 0.008314 kJ∙mol–1∙K–1 –∆Go RT = ln K –∆Go RT Solved for K : (NOT on equation sheet) K = e^

Free Energy (∆G) & Equilibrium (K) G = –RT ln K ∆Go = –RT(ln K) K @ Equilibrium – + = –RT ( ) > 1 product favored (favorable forward) + – = –RT ( ) < 1 reactant favored (unfavorable forward)