How To Write A Chemical Equilibrium

Transcription

1 Chemical Equilibrium Simplificatins/assumptins we've been wrking under: 1. s g t cmpletin ("") 2. Only the frward reactin is invlved 3. s stp when cmpleted Nne f these things are strictly true! What have we learned frm kinetics? Ptential Energy Prgress 1 Chemical Equilibrium Reasnable t write ALL reactins as reversible. Requirements? Generic Equilibrium: aa + bb k 1 k -1 cc + dd At equilibrium rate frward = rate reverse k 1 (A A ) a (A B ) b = k -1 (A C ) c (A D ) d A A = activity f cmpund (r element) A 2 1

2 Chemical Equilibrium Mving all the cnstants t ne side Activity is a measure f effective reactivity pprtunity t get invlved in the prcess f interest. A A = A [A] (fr slutins) r A A = A p A (fr gases) In equilibrium cnsideratins, we typically y relate (rati) all cnditins t a reference state, c (1 M fr slutins, 1 atm fr gases) K c c C C A A c c a D D B b B c d 3 Equilibrium Cnstant Expressin Generic Equilibrium Cnstant Expressins ( kind f A): K c = [C] c [D] d = [prducts] x K p = (p C ) c (p D ) d x = p prducts [A] a [B] b [reactants] t (p A ) a (p B ) b p y reactants NOTE: K is dimensinless! IMPORTANT! CRITICAL! ESSENTIAL! CRUCIAL! 1. In rder fr a system (reactin) t be at equilibrium, the K eq expressin MUST be satisfied!!! 2. If the K eq expressin is satisfied, the system is in equilibrium! We can use this t predict the extent f a reactin and/r the directin a reactin is likely t prceed. 4 2

3 Equilibrium Cnstant Expressin Expressing K eq in varius cncentratins: Gases Describe K in terms f partial pressures WHY? aa(g) + bb(g) cc(g) + dd(g) K p = (p C ) c (p D ) d (p A ) a (p B ) b Slutins Describe K in terms f mlar cncentratins (M) Often nted at K c 5 Linking Kinetics and Thermdynamics Relatinship between tendencies and activatin parameters Thermdynamics and kinetics must cperate fr a reactin t prceed tward prducts K k 1? k -1 Diagram G rxn Tendency >1 Energy E Prgress 6 3

4 Linking Kinetics and Thermdynamics K k 1? k -1 Diagram G rxn Tendency <1 Energy Prgress =1 Energy Prgress 7 Glden Rules f Equilibria 1. K eq is dimensinless and cnstant at a given temperature. 2. When writing equilibrium cnstant expressins, mit slids, pure liquids, and slvents. WHY? 3. Always use smallest integer cefficients when balancing equatins and writing K eq. H 2 + I 2 2 HI ½H 2 + ½I 2 Eq.: [HI] = 1.58 M, [H 2 ] = M, [I 2 ] = M, calculate K c. 8 4

5 Glden Rules f Equilibria 4. When a balanced reactin is reversed, K reverse = 1/K frward H 2 + I 2 2 HI vs 2 HI H 2 + I 2 5. When adding reactins, K eq fr the net reactin is the prduct f the K eq 's fr the individual reactins. Example: Calculate the K p fr the gas phase reactin: H(g) + Br(g) HBr(g) H 2 (g) + Br 2 (g) 2 HBr(g) K 1 = 7.9 x Br 2 (g) 2 Br(g) K 2 = 4.8 x H 2 (g) 2 H(g) K 3 = 2.2 x Glden Rules f Equilibria 6. At equilibrium, the cncentratins f reactants and prducts MUST satisfy the equilibrium cnstant expressin!! Example: Acetic acid is a weak acid that dissciates via the equilibrium shwn belw. We disslve 1.00 mle f acetic acid in 1.00 L f water. After allwing the slutin t cme t equilibrium, the [H 3 O + ] is measured t be M. If the equilibrium cnstant fr this prcess is 1.8 x 10-5, what is the rati f dissciated t undissciated acid? CH 3 COOH(aq) + H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) 10 5

6 Using K eq t Predict Directin" f When given a set f cnditins (cncentratins) ask: 1. Is the system already at equilibrium? 2. If yes, yu're dne! If n, what des the system need t d t reach equilibrium? Accmplish (1) by calculating Qutient (Q): K with nnequilibrium values. Cmpare Q with K eq Q = K eq Q > K eq Q < K eq System is in equilibrium System t prduct "heavy" System t reactant "heavy" Accmplish (2) by "mapping" the reactin and ding a little math. 11 Using K eq t Predict Directin" f Example: N 2 + O 2 2 NO At 2000K the K c fr the frmatin f NO is 4.0 x Yu have a cntainer in which, at 2000 K, the cncentratin f N 2 is 0.50 M, O 2 is 0.25 M, and NO is 4.2 x 10-3 M. Is the system at equilibrium? If nt, which directin must the reactin prceed t apprach equilibrium? Calculate the equilibrium cncentratins f N 2, O 2, and NO. 12 6

7 Le Chatelier's Principle and Bullying s Le Chatelier's Principle Start with system in equilibrium, what happens if we perturb things? 1. Change Cncentratins: 2. Change Pressure: 3. Change Temperature: 13 7