CHAPTER 6 / HARVEY A. CHEMICAL EQUILIBRIUM B. THERMODYNAMICS AND EQUILIBRIUM C. MANUPULATING EQUILIBRIUM CONSTANTS

Transcription

1 CHPTER 6 / HRVEY. CHEMICL B. THERMODYNMICS ND C. MNUPULTING CONSTNTS D. CONSTNTS FOR CHEMICL RECTIONS 1. Precipitatin Reactins 2. cid-base Reactins 3. Cmplexatin Reactins 4. Oxidatin-Reductin Reactins E. LE CHÂTELIER'S PRINCIPLE F. LDDER DIGRMS G. SOLVING PROBLEMS H. BUFFER SOLUTIONS I. CTIVITY EFFECTS J. SUGGESTED PROBLEMS 6.1, 6.2, 6.4, 6.5, 6.7, 6.12, 6.14, 6.19 PPENDIX 3 ND 4 1

2 . Gulberg and Waage (1867) reactin is at equilibrium when the rates f the frward and reverse reactins are equal. It is a dynamic state. a bb cc dd rate f = rate r a b f f c d rate r = kr [ C] [ D] rate = k [ ] [ B] eq c d [ C] [ D] k f a b [ ] [ B] k r K = = The expressin btained is the crrect expressin fr the equilibrium cnstant, but the methd f derivatin has n general validity. Because, reactin rates depend n the mechanism f the reactin! Reactin rates depend n the number f clliding species (mlecularity) whereas the equilibrium cnstant expressin depends nly n the stichimetry f the reactin. e.g. S 2 O 2 8 3I 2 SO 2 4 I 3 Rate = k f [S 2 O 8 2 ] [I ] 2

3 Change in cncentratins f reactants and prducts as the reactin prceeds What can be predicted frm knwledge f the equilibrium cnstant? 1) the tendency f the reactin t ccur and in what directin 2) Nt whether it is fast enugh t be feasible in practice ** Even with a large K, a reactin may prceed frm right t left if sufficiently large cncentratins f prducts are initially present 3

4 B. THERMODYNMICS ND a bb cc dd chemical system evlves spntaneusly twards its lwest energy state. Energy f a chemical system changes during a chemical reactin. What determines the final psitin f a reactin? 1) Enthalpy 2) Entrpy and temperature G = H TS (1) t cnstant pressure and temperature G = H T S (2) G change in Gibbs free energy H change in enthalpy (net flw f energy as heat) S change in entrpy T temperature in Kelvins 4

5 H T S G Frward reactin Reverse reactin < 0 > 0 < 0 Spntaneus < 0 < 0, T S < H < 0 Spntaneus < 0 < 0, T S > H > 0 Spntaneus < 0 < 0, T S = H 0 > 0 < 0 >0 > 0 > 0, T S > H < 0 Spntaneus Spntaneus > 0 > 0, T S < H > 0 > 0 > 0, T S = H 0 Spntaneus 5

6 Gibbs free energy is a functin f cncentratins G = G RT ln Q (3) c d [ C] [ D] [ ] a [ B] b Q = (4) G change in Gibb's free energy under standard -state cnditins Standard-state cnditins 298 K 1M slute Pure slid Pure liquid 1 atm partial pressure t equilibrium G = 0 Therefre G = RT ln K (5) 6

7 Thermdynamic Equilibrium Cnstant versus Cncentratin Equilibrium Cnstant Fr very dilute/ ideal slutins (mm), in the absence f interactins between reactants, the equilibrium cnstant can be calculated using cncentratins. Equilibrium cnstant based n cncentratin is smetimes referred t as cncentratin equilibrium cnstant. C c [ C] [ D] eq d eq K = (6) a [ ] eq [ B] b eq Real thermdynamic equilibrium cnstant c d a C ad K = (7) a b a ab a [ ] = (8) a activity activity cefficient ph = lg a H Mlecular slutes have activity cefficients very near unity up t an inic strength f 0.1. ctivity cefficients f pure liquids and slids are equal t ne. 7

8 ctivity cefficients f inic slutes Fr inic slutes the extended Debye-Huckel equatin is used t calculate the activity cefficient lg z = α µ µ (9) at 25 C, µ µ = c i z i (10) 2 i z charge f the in α effective diameter f the hydrated in in nanmeters µ inic strength f the slutin It is handy t recgnize the fllwing Type f inic cmpund Inic strength B C 2 B 2 3 C 2 B 2 4 C 3 B 3 6 C 8

9 Relatinship between cncentratin equilibrium cnstant and thermdynamic equilibrium cnstant K = K C c C a d D b B lg K = lg K C lg c C a d D b B pk C = pk lg c C a d D b B pkc = pk c lg C d lg D a lg b lg B pk C = pk 2 2 z C z 0.51 µ ( D z z c d a b B )( ) α α α α µ C D 2 2 B Measure K C at different inic strength and extraplate t zer t btain K. 9

10 C. MNUPULTING CONSTNTS C.1 Reverse reactin's equilibrium cnstant 1 K 2 = K1 C.2 Overall equilibrium cnstant Ni 2 NH 3 Ni(NH 3 ) 2 Ni(NH 3 ) 2 NH 3 Ni(NH 3 ) 2 2 Ni(NH 3 ) 2 2 Ni(NH 3 ) 3 2 K verall = K1K2K3K 4 NH 3 Ni(NH 3 ) 3 2 NH 3 Ni(NH 3 ) 4 2 If reactins ( B) = reactin C, the equilibrium cnstant f reactin C (Kc) is equal t the prduct f the equilibrium cnstants fr reactin and B. C.3 Equilibrium Cnstants are used fr: Calculatin f cncentratins f chemical species at equilibrium under cnditins where the same infrmatin might be difficult t measure r is unbtainable by direct experiment. Predictin f cnditins that will lead t desired results 10

11 D. CONSTNTS FOR CHEMICL RECTIONS D.1 PRECIPITTION RECTIONS Metathesis reactin gno 3 NaCl gcl( s) NaNO3 g ( aq) NO3 ( aq) Na ( aq) Cl ( aq) gcl( s) Na ( aq) NO3 ( aq) Net inic equatin g ( aq) Cl ( aq) gcl( s) Slubility prduct : K sp gcl( s) g ( aq) Cl ( aq) K sp = [ g ][ Cl ] = pk sp =

12 D.2 CID-BSE RECTIONS D.2.1 Brønsted and Lwry definitin cid: prtn dnr Base: prtn acceptr D.2.2 Strng and weak acids and bases Strng acids and bases are cmpletely dissciated in water HCl, HNO 3, HCLO 4, HI, First prtn f H 2 SO 4 NaOH, KOH Weak acids and weak bases Weak acids and weak bases are weak electrlytes, they are partially dissciated in water. CH 3 COOH H 2 O CH 3 COO - H 3 O Cnjugated base f the acid CH 3 COO - H 2 O CH 3 COOH OH - NH 3 H 2 O NH 4 OH - NH 4 H2 O NH 3 H 3 O Cnjugated acid f the base 12

13 D.2.3 cid and base dissciatin cnstants [ CH 3COO ][ H3O ] K a = [ CH 3COOH ] K b [ CH3COOH ][ OH ] = [ CH3COO ] K a Kb = [ H3 O ][ OH ] = K w 13

14 D.2.4 Dissciatin f water H 2 O(l) H 2 O(l) H 3 O (aq) OH - (aq) Water is amphiprtic 14 K w = at 24 C [ K OH ] = w [ H3 O ] ph poh = pk w =14 pk a pkb = pk w =14 D.2.5 Other amphiprtic species HCO 3 - H 2 O H 2 CO 3 (aq) OH - (aq) HCO 3 - (aq) H 2 O(l) CO 3-2 (aq) H 3 O (aq) D.2.6 Plyprtic acids H 2 CO 3 H 3 PO 4 H 2 C 2 O 4 14

15 H 2 CO 3 (aq) H 2 O(l) HCO 3 - (aq) H 3 O (aq) HCO 3 - (aq) H 2 O(l) CO 3-2 (aq) H 3 O (aq) [ HCO3 ][ H3O ] 7 K a 1 = = [ H 2CO3] pk a1 = K a 2 [ CO 3 ][ H3O ] 11 = 4.69 [ HCO3 ] 2 = 10 pk a2 = K f K f K a1 a2 a3 15

16 D.3 COMPLEXTION RECTIONS D.3.1 Definitins G.N. Lewis definitin f acids and bases cid: electrn pair acceptr Base: electrn pair dnr in the frmatin f a cvalent bnd. Cmplexatin generally refers t reactins where a ligand (a Lewis base) dnates an electrn pair t a metal in (a Lewis acid) t frm a cvalent bnd between. D.3.2 Frmatin cnstants (K f ) and dissciatin cnstants (K d ) Stepwise frmatin cnstants ( K i ) Cumulative r verall frmatin cnstants ( β ) β i = K 1 K 2 K3... K i i 16

17 D.4 OXIDTION-REDUCTION RECTIONS D.4.1 Definitins Reactin in which electrns are transferred frm ne reactant t anther. The reducing reagent is xidized (xidatin state increases) The xidizing reagent is reduced (xidatin state decreases) 5 C 2 O -2 4 (aq) 2 MnO - 4 (aq) 16 H (aq) 10 CO 2 (g) 2 Mn 2 8H 2 O(l) Oxalate is xidized [xidatin state f C changes frm 3 t 4] Permanganate is reduced (xidatin state f manganese changes frm 7 t 2) Oxalate is the reducing agent Permaganate is the xidizing agent 17

18 D.4.2 Equilibrium Cnstant fr Redx reactins? Because electrchemical ptentials are easily measured fr redx reactins, they are cnveniently used t express equilibrium and t calculate equilibrium cnstants. Free energy fr mving a charge under the influence f a ptential E is given by: G = E Q (1) Q = nf (2) G = nfe (3) n number f mles f electrns per mle f reatant F Faraday's cnstant (96,485 C.ml 1 ) Negative sign???? G = nfe (4) G = G RT ln Q (5) Substituting (3) and (4) in (5), yields (6) nfe = nfe RT ln Q (6) Divide (6) by (nf) RT E = E ln Q (7) nf 1 1 R = JK ml T = ln Q = lgQ RT = V = 59.16mV F 18

19 E = E lgq (8) n Standard-State Electrchemical Ptential fr the Reactin ( E ) t equlibrium E = 0, therefre E RT = lg K (9) nf Therefre, if the standard-state ptential f a reactin is knwn, the equilibrium cnstant f the reactin can be calculated. Standard-state ptentials fr chemical reactin can be calculated using available standard state-ptentials f the xidatin and reductin halfreactins. E reac = E red E x E red standard-state reductin ptential f the reduced reactant E x standard-state reductin ptential f the xidized reactant 19

20 What are Standard-State Reductin Ptentials? (ppendix 3D) Standard-state reductin ptential f chemical species relative t the reductin ptential f the hydrnium in, which by cnventin is set t V. They express the tendency f chemical species t be reduced relative t the hydrnium in. 2 H 3 O (aq) 2 e - 2 H 2 O(l) H 2 (g) E H O / 3 H 2 = 0.000V If E > 0.000, species has a greater tendency t be reduced than H Strng xidizing agents have large psitive E (dixygen, permanganate) If E < 0, species has lesser tendency t be reduced than hydrgen and a greater tendency t be xidized. Strng reducing agents have large negative E (Zinc, Sdium) 2H ( aq) 2e H 2 ( g) E = V 2 Zn 2e Zn( s) E = V O2 ( g) 4H 4e 2H 2O E = V 3 2 Fe e Fe E = V I3 2e 3I E = V 20

21 Calculate the equilibrium cnstant f the fllwing reactin 3 2 2Fe 3I 2Fe I3 1) Standard-State reactin ptential 2) Equilibrium cnstant 21