Chapter 14. Chemical Equilibrium

1 GCh14-1 Chapter 14. Chemical EquilibriumWhat we will learn: Concept Equilibrium Equilibrium constant Writing Equilibrium constant expressions Chemical kinetics and Chemical Equilibrium Meaning of Equilibrium constant Factors that affect Chemical equilibriumGCh14-2 Chemical equilibriumA dynamic stage of a Chemical reaction where the concentrations of reactants and products are not changing in time k1 A + B g C + D rate = k1 [A] [B] k-1 C + D g A + B rate = k-1 [C] [D] k-1 A + B D C + D k1 Reaction is proceeding in both directions at the same rate.

2 At the Equilibrium there is no net change in concentrations of reactants and products GCh14-3N2O4 D 2 NO2 Initially only NO2 Initially only N2O4 Initially NO2 and N2O4 GCh14-4 K = Equilibrium constant [A], [B], [C], [D] = Equilibrium concentrationsMagnitude of K K =~ [products] / [reactants] K >> 10 Have mostly products at Equilibrium ; Equilibrium lies to the right ; favors products K << Have mostly reactants at Equilibrium ; Equilibrium lies to the left ; favors reactants K = 1 Roughly equal concentration of reactants and productsEquilibrium constantGCh14-5 Writing Equilibrium constant expressionsHomogeneous equilibria (all species are in the same phase)For gaseous reactionsSince a partial pressure is proportional to a molar concentration, the Equilibrium constant can be expressed in terms of partial pressures.

3 A simple reaction a A (g) D b B (g)GCh14-6 Kp = (PB)b / (PA)a PA V = nA RT PA = nA RT / V PB V = nB RT PB = nB RT / V KP = (nB RT / V)b / (nA RT / V)a KP = (nB / V)b / (nA / V)a (RT)b-a [A] = nA / V [B] = nB / V KP = [B]b / [A]a (RT)b-a KP = Kc (RT)Dn GCh14-7 Relationship between Kc and Kp based on ideal gas law Kp = Kc (RT)Dn Dn = moles gaseous products - moles gaseous reactantsor Dn = b - aR value = L atm / mole KGCh14-8 Problem Consider the following reaction N2 (g) + O2 (g) D 2 NO (g)If the Equilibrium partial pressures of N2, O2, and NO are , atm, atm, and atm, respectively, at 220 C, what is Kp?

4 Data nn2 - moles of N2 = 1 PN2 - partial pressure = atmnO2 - moles of O2 = 1 PO2 - partial pressure = atmnNO - moles of NO = 2 PNO - partial pressure = atm Kp = (PNO)2 / [(PN2)1 (PO2)1] = ( atm)2 / ( )( ) = x 102 GCh14-9 Homogenous Equilibrium in waterFor ionization of acids in water HA (aq) + H2O (l) D A- (aq) + H3O+ (aq) Kc = [H3O+] [A-] / [HA] [H2O] but in 1 L or 1000 g of water, there are mole, therefore the concentration of H2O = mol/L or M .This quantity is much bigger than concentrations of other compounds therefore it is essentially constant [H2O] = constant Kc = Kc [H2O] = [H3O+] [A-] / [HA] Kc = [H3O+] [A-] / [HA] GCh14-10 ProblemLet us consider the dissociation reaction of acetic acid CH3 COOH(aq) + H2O(l) D CH3 COO-(aq) + H3O+(aq)The Equilibrium constant is Kc = [CH3 COO-][H3O+] / [CH3 COOH][H2O]Practically, 1 L of water includes moles, the concentration of water is mol/L.

5 The is a very large amount of concentration (comparing with concentration of the acid) which is not changing during this reaction. Therefore we assume that [H2O] = constant Kc = Kc [H2O] Kc = [CH3 COO-][H3O+] / [CH3 COOH] GCh14-11 ProblemLet us consider the dissociation reaction of HF HF(aq) + H2O(l) D H3O+(aq) + F-(aq)The Equilibrium constant is Kc = [F-][H3O+] / [HF][H2O] [H2O] = constant Kc = Kc [H2O] Kc = [F-][H3O+] / [HF]GCh14-12 ProblemMethanol is manufactured according to the reaction CO(g) + 2H2(g) D CH3OH(g)The Equilibrium constant (Kc) for the reaction is at 220 C. What is the value of Kp at this - moles of CO = 1nH2 - moles of H2 = 2nCH3OH - moles of CH3OH = 1R - constant = L atm / mole KT - temperature = 220 C = 493 K Dn = 1 - 3 = -2 Kp = Kc (RT)Dn = ( ) ( x 493)-2 = x 10-3 GCh14-13 ProblemThe Equilibrium constant Kp for the decomposition of phosphorus pentachloride (PCl5) to phosphorus trichloride (PCl3) and chlorine (Cl2) PCl5 (g) D PCl3 (g) + Cl2 (g)is found to be at 250 C.

6 The Equilibrium partial pressures of PCl5 and PCl3 are atm and atm, respectively. What is the Equilibrium partial pressure of Cl2 at 250 C. DataPPCL5 - partial pressure = atmPPCL3 - partial pressure = atmKp - Equilibrium constant = Kp = PPCL3 PCl2 / PPCL5 = ( )PCl2 / ( ) PCl2 = ( ) ( ) / ( ) = atmGCh14-14 Heterogeneous equilibria Some reactants or products are in different phasesConcentrations of pure liquids and solids are effectively constants, therefore they do not appear in K expressionsExampleLet us consider the reaction CaO(s) + SO2(g) D CaSO3(s)the Equilibrium constant Kc = [CaSO3] / [CaO] [SO2] GCh14-15 The concentrations of solid compounds are practically constant [CaO] = constant [CaSO3] = constant Kc = Kc [CaO] / [CaSO3] Kc = 1 / [SO2] The value of Kc does not depend on how much CaSO3 and CaO are present at the Equilibrium , therefore Kc = 1 / [SO2] The Equilibrium constant depends practically only on the concentration of the gas substanceGCh14-16 ExampleHeating calcium carbonate has a particular Equilibrium constant CaCO3(s) D CaO(s) + CO2(g) Kc = [CaO][CO2] / [CaCO3]The concentrations of solid compounds (CaCO3 and CaO)

7 Are practically constant [CaO] = constant [CaCO3] = constant Kc = Kc [CaO] / [CaCO3] Kc = [CO2] The value of Kc does not depend on how much CaCO3 and CaO are present at the Equilibrium , and the Equilibrium constant depends practically only on the concentration CO2 GCh14-17 ProblemConsider the following heterogenous Equilibrium CaCO3(s) D CaO(s) + CO2(g)At 800 C the pressure of CO2 is atm. Calculate Kp and Kc for the reationDataPCO2 - partial pressure = atmT - temperature = 800 C = 1073 KR - constant = L atm / mole K Kp = Kc(RT)Dn Dn = 1 Kp = PCO2 = = Kc( ) (1073) Kc = x 10-3 GCh14-18 Problem Consider the following Equilibrium at 295 K: NH4HS (s) D NH3 (g) + H2S (g)The partial pressure of each gas is atm.

8 Calculate Kp and Kc for the reactionDataPNH3 - partial pressure = atmPH2S - partial pressure = atmT - temperature = 295 C = 568 KR - constant = L atm / mole K Kp = Kc(RT)Dn Dn = 2 Kp = PNH3 PH2S = ( ) ( ) = = Kc [ ( ) (568) ] 2 Kc = ( ) / ( ) = x 10-6 GCh14-19 Multiple equilibriaTwo Chemical reactions where products of the first reaction are reactants of the second one A + B D C + D K c C + D D E + F K c A + B D E + F KcFirst reaction Kc = [C] [D] / [A] [B] Seond reaction Kc = [E] [F] / [C] [D] Kc = [E] [F] / [A] [B] Kc Kc = [C] [D] / [A] [B] x [E] [F] / [C] [D] = Kc GCh14-20If a reaction can be expressed as sum of two or more reactions, the Equilibrium constant for the overall reaction is given by the product of the Equilibrium constants of the individual reactions Kc = K c K cExampleThe Equilibrium constants for carbonic acid (H2CO3) dissociation at 25 C H2CO3(aq) D H+(aq) + HCO3-(aq)

9 Kc = [H+] [HCO3-] / [H2CO3] = x 10-7 HCO3-(aq) D H+(aq) + CO32-(aq) Kc = [H+] [CO32-] / [HCO3-] = x 10-11 The overall reaction is the sum of these two reactions H2CO3(aq) D 2H+(aq) + CO32-(aq)GCh14-21and the corresponding Equilibrium constant is given by Kc = [H+]2 [CO32-] / [H2CO3]and the value is Kc = Kc Kc Kc = ( x 10-7 ) ( x 10-11 ) = x 10-17 GCh14-22 Form of K and the Equilibrium equation Two rules of writing Equilibrium constants1. When the equation for a reversible reaction is written in the opposite direction, the Equilibrium constant becomes the reciprocal of the original Equilibrium constant N2O4 (g) D 2 NO2 (g) Kc = [NO2]2 / [N2O4] = x 10-3 However the reverse reaction 2 NO2 (g) D N2O4 (g) Kc = [N2O4] / [NO2]2 = 216 Kc = 1 / Kc GCh14-232.

10 The value of K depends on how the Equilibrium equation is balanced 1/2 N2O4 (g) D NO2 (g) Kc = [NO2] / [N2O4]1/2 = N2O4 (g) D 2 NO2 (g) Kc = [NO2]2 / [N2O4] = x 10-3 and Kc = (Kc)2 If we multiply the first reaction by 2 then the Equilibrium constant will be (Kc)2 GCh14-24 Problem Write the Equilibrium constant (Kc) for each of the following reactions and show how they are related to each other:a) N2 (g) + 3H2 (g) D 2 NH3 (g)b) 1/2 N2 (g) + 3/2H2(g) D NH3 (g)c) 1/3 N2 (g) + H2(g) D 2/3 NH3 (g) Ka = [NH3]2 / [N2] [H2]3 Kb = [NH3] / [N2]1/2 [H2]3/2 Kc = [NH3]2/3 / [N2]1/3 [H2] Ka = Kb2 Ka = Kc3 GCh14-25 Problem Write the Equilibrium constant (Kc) for each of the following reactions and show how they are related to each other: 3 O2 (g) D 2 O3 (g) Kc = [O3]2 / [O2]3 O2 (g) D 2/3 O3 (g) Kc = [O3]2/3 / [O2]The final relation is Kc = (Kc )3 GCh14-26 Summary Writing Equilibrium constant expressions Concentrations of products and reactants are in mol/L.