Chapter 14 Chemical Equilibrium Georgia Gwinnett College Chem 1212K Fall 2013 B Shepler Chapter 14 1 Some Reactions Proceed to Completion Proceed to Completion All of the reactants are converted to products Many reactions do proceed essentially to completion Example Combustion of propane Many other reactions do not proceed to completion Only some of the reactants are converted to products We are left with a mixture of reactants and products Example Vaporization of water below its boiling point vapor pressure Chapter 14 2 Dynamic Equilibrium Reaction rates depend on the concentration of the reacting species If we start with all reactants we start to form products As products are formed the reverse reaction starts to occur More products Reverse reaction gets faster Less reactants Forward reaction gets slower Dynamic Equilibrium is achieved when the rate of forward and reverse reactions are equal No net change in concentrations of reactants or products However the forward and reverse reactions continue to occur Chapter 14 3 Equilibrium Constant The equilibrium constant is a way to quantify the amount of reactants and products that are present at equilibrium Equilibrium constant is the ratio of the concentrations of products to reactants AT EQUILIBRIUM C c D d K a b A B Example write the equilibrium constant expression for C2 H 6O g O 2 g CO 2 g H 2O g Chapter 14 4 Connection Between Kinetics and Equilibrium Write the equilibrium constant expression for the above reactions If I tell you the above reaction is an elementary step Write the rate law for the forward reaction Write the rate law for the reverse reaction If we are at equilibrium then the rates of the forward and reverse reactions are equal Rearrange Note the equilibrium constant expression can be written for any reaction not just elementary steps Chapter 14 5 Equilibrium Constant aA bB cC dD c d C D Kc a b A B Small K would be significantly less than 1 At Eq more reactants than products K 1 x 10 30 Large K would be significantly greater than 1 At Eq more products than reactants K 1x10380 Intermediate K would be 1 Significant amount of both reactants and products at Eq K 5 Chapter 14 6 Pressures or Concentrations The equilibrium expression and equilibrium constant for a gas phase reaction can be expressed using partial pressures rather than concentrations Chapter 14 7 Examples Consider the following reaction CO g 2 H2 g CH3OH g An equilibrium mixture of this reaction at a certain temperature was found to have CO 0 105 M H2 0 114 M and CH3OH 0 185 M What is the value of the equilibrium constant Kc at this temperature At 775 K the reaction below has an equilibrium constant K 0 0584 If N2 0 120 M and H2 0 140 M at equilibrium what is the equilibrium concentration of NH3 N2 g 3 H2 g 2 NH3 g Chapter 14 8 Heterogeneous Equilibria Reactions involving solids or liquids Concentration moles Volume This is a constant for pure liquids and solids It is the density Pure liquids and solids DO NOT APPEAR in the equilibrium expression they are given a value of 1 CaCO3 s CaO s CO2 g Amount of CO2 Does not depend On relative amounts Of the two solids 9 Relationships Between K s of Related Reactions 1 Forward and Reverse Reactions 1 2 H2 g O2 g 2 H2O g 2 2 H2O g 2 H2 g O2 g 10 Relationships Between K s of Related Reactions 2 When the coefficients in a balanced chemical equation are all multiplied by a constant factor the corresponding equilibrium constant is raised to a power equal to that factor 2 H2 g O2 g 2 H2O g 4 H2 g 2 O2 g 4 H2O g 11 Relationships Between K s of Related Reactions 3 When chemical equations are added to give a new equation their equilibrium constants are multiplied to give the equilibrium constant associated with the new equation 2 BrCl g Br2 g Cl2 g PBr2 PCl2 PBrCl 2 Br2 g I2 g 2 IBr g PIBr 2 o K 0 051 25 C 2 PBr2 PI2 K1 0 45 25oC 2 BrCl g I2 g 2 IBr g Cl2 g 12 The Reaction Quotient When a reaction is not at equilibrium the ratio of the product and reactant concentrations is called the reaction quotient Qc aA bB cC dD Q is calculated with the same equation as K If the reaction is at equilibrium we call the ratio K If the reaction is not a equilibrium we call the ratio Q At equilibrium Q K 13 The Reaction Quotient The reaction quotient tells us in which direction the reaction will go 14 Example The equilibrium constant Kp for the reaction P4 g 2 P2 g is 1 39 at 400oC Suppose that 2 75 atm of P4 g and 1 08 atm of P2 g are mixed in a closed 25 0 L container at 400oC Compute Q and state the direction in which the reaction proceeds to attain equilibrium 15 Calculations Involving Equilibrium Constants ICE Tables A 2B C 3D I C E 16 Example Calculate Equilibrium Constant When Given Initial Concentrations and one equilibrium concentration Consider the equilibrium 4 NO2 g 2 N2O g 3 O2 g The three gases are introduced into a container at partial pressures of 3 6 atm for NO2 5 1 atm for N2O and 8 0 atm for O2 and react to reach equilibrium at a fixed temperature The equilibrium partial pressure of the NO2 is measured to be 2 4 atm Calculate the equilibrium constant of the reaction at this temperature NOTE Don t think about the different types of equilibrium calculations as labeled by the book as being different types of calculations They are all just equilibrium calculations They are all ice table problems 17 Example Consider the following reaction PbCl2 s Pb2 aq 2Cl aq Kc 1 7 x 10 5 In a reaction mixture at equilibrium the concentration of Pb 2 is 0 024 M What is the concentration of Cl Chapter 14 18 Example Assume that the reaction for the formation of gaseous hydrogen fluoride from hydrogen and fluorine has an equilibrium constant of 1 15 x 102 at a certain temperature In a particular experiment at this temperature 3 000 moles of each component was added to a 1 500 liter flask Calculate the equilibrium concentrations of all species 19 Change this examle Consider the following reaction Br2 g Cl2 g 2BrCl g Kp 0 010 A reaction mixture initially contains a Br2 partial pressure of 1 5 atm and a Cl2 partial pressure of 1 3 atm at 150 K Calculate the equilibrium partial pressure of BrCl Chapter 14 20 Simplifying Assumption when K is small At a particular temperature K 2 0 x 10 6 mol L for the reaction 2CO2 g 2CO g O2 g If 2 0 mol CO2 is initially placed into a 5 0 L vessel calculate the equilibrium concentrations of all species 21 Le Ch telier s Principle Le Ch telier s …