Gas Phase Equilibrium Chemical Equilibrium Equilibrium Constant Keq Equilibrium constant expression Relationship between Kp and Kc Heterogeneous Equilibria Meaning of Keq Calculations of Kc Solving Equilibrium Problems USING AN EQUILIBRIUM CONSTANT Information obtained from Kc 1 Predict direction of a reaction Q reaction quotien 2 Obtaining equilibrium concentrations of reactants and products Mary J Bojan Chem 110 1 Equilibrium Equilibrium is achieved when opposing processes proceed at equal rates equilibria previously discussed Eg Mary J Bojan Chem 110 2 Chemical Equilibrium In chemical equilibria forward and reverse reactions occur at equal rates Mary J Bojan N2O4 g 2NO2 g colorless brown Chem 110 3 Achieving equilibrium Initial State reactants only Mary J Bojan Initial state products only Chem 110 4 Equilibrium Constant Equilibrium point of any reaction is characterized by a single number In general Example Mary J Bojan jA kB B pR qS 2A N2O4 Chem 110 2NO 5 EXPRESSION for Keq N2 g 3H2 g 2NH3 g What is the equilibrium constant expression for the Haber process Mary J Bojan Chem 110 6 Equilibrium Constant The ratio of equilibrium concentrations will be constant Keq is the equilibrium constant for the reaction It is a NUMBER Keq the number DOES NOT depend on concentration It s a function of temperature only Notation Keq Kc concentration is expressed in M mol L Keq Kp concentration is expressed using partial pressures in atm Mary J Bojan Chem 110 7 Relationship between Kp and Kc where n nprod nreact When given Keq how do I know if it is Kc or Kp Use context of problem If concentrations are given in M then Keq Kc If concentrations are given in P then Keq Kp Mary J Bojan Chem 110 8 Calculating Kp and Kc N2 g 3H2 g 2NH3 g Equilibrium concentrations of NH3 N2 and H2 were determined at 472 C H2 0 1207M N2 0 0402M NH3 0 00272M Find Kc and Kp Mary J Bojan Chem 110 9 Heterogeneous Equilibria involves reactants and products in more than one phase 3Fe s 4H2O g Fe3O4 s 4H2 g 4 Fe3O4 H2 Keq 4 3 Fe H2O What is Fe Fe3O4 Activities of pure solids and liquids 1 Mary J Bojan Chem 110 10 What is the equilibrium constant expression for these reactions AgCl s C6H6 g 3H2 g CaCO3 s Mary J Bojan Ag aq Cl aq C6H12 g CaO s CO2 g Chem 110 11 Meaning of Keq Reactants Products Is Keq large or small K 1 Mary J Bojan Chem 110 12 Problem Which one of the following has the greatest tendency to proceed as written 1 4NH3 g 3O2 g 2N2 g 6H2O g Kp 1 1022 2 N2 g O2 g 2NO g Kp 5 10 31 3 2HF g F2 g H2 g Kp 1 10 13 4 2NOCl g 2NO g Cl2 g Kp 4 7 10 4 Mary J Bojan Chem 110 13 Is Keq large or small Cu 2 aq 4NH3 aq Cu NH3 4 2 aq Ni 2 aq 6NH3 aq Ni NH3 6 2 aq PbI2 s Pb 2 aq 2I aq Mary J Bojan Chem 110 14 More Relationships H2 g I2 g 2HI g A Keq What is Keq if the reaction is doubled 2H2 g 2 I2 g 4HI g B Keq What is Keq for the reverse reaction 2HI g H2 g I2 g C Keq Mary J Bojan Chem 110 15 At a certain temperature Kc for the following reaction is 16 H2 g I2 g 2HI g At the same temperature what is Kc for this reaction HI g H2 g I2 g 1 1 16 2 4 3 1 4 4 16 5 there is not enough information to answer this question Mary J Bojan Chem 110 16 General Approach to Equilibrium Constant Problems 1 Write the balanced reaction 2 Write the general form for Keq 3 Set up a data table may need algebraic unknowns initial conditions changes in concentrations equilibrium concentrations 4 Substitute equilibrium concentrations into the expression for Keq and solve Mary J Bojan Chem 110 17 Calculating Keq 1 00 mole of SO2 g and 1 00 mole of O2 g are added to a 1 00 L container and react until equilibrium is achieved At equilibrium the container has 0 919 moles of SO3 g Find Kc at 1000K Mary J Bojan Chem 110 18 Calculating Keq A one L container holds 224g of Fe and 5 00 mole of H2O l It is heated to 1000K and reaches equilibrium 56g of Fe are left unreacted What is Kc at 1000K 3Fe s 4H2O g Fe3O4 s 4H2 g initial change final Mary J Bojan Chem 110 19 Predicting the direction of a reaction aA bB cC dD Reaction quotient Q c d C D Q a b A B Note the concentrations used are NOT equilibrium concentrations Mary J Bojan Chem 110 20 Predicting the Direction of a Reaction When Q Kc system IS at equilibrium When Q Kc reaction moves to right produces product When Q Kc reaction moves to left produces reactant Mary J Bojan Chem 110 21 H2 g I2 g 2HI g Kc H2 I2 1 25 10 3 HI 2 If we put 0 1 mole of HI in a 1L container what will happen 1 reaction shifts to right 2 reaction shifts to left 3 no change occurs If the initial concentrations of all three gases in the vessel are 0 1 mol L what will happen Mary J Bojan Chem 110 22 Using Keq to obtain equilibrium concentrations of reactants and products Initially IBr I2 Br2 0 0500 M What are the final concentrations of reactants and products Kc 2 50 10 3 2 IBr g Mary J Bojan Br2 g I2 g Chem 110 23 Haber process Industrial process used to make ammonia N2 g 3H2 g Mary J Bojan 2NH3 g heat Chem 110 24 Factors that affect equilibrium What happens to a system at equilibrium when it is disturbed by changing concentration volume pressure temperature Answer Le Chatelier s Principle If a system at equilibrium is disturbed the system will shift its equilibrium position to minimize the effect of the disturbance Mary J Bojan Chem 110 25 Le Chatelier s Principle To illustrate this principle use the following chemical system in a closed 1L container N2 g 3H2 g Hrxn 92 4kJ mol 2NH3 g heat T 700 C Initial conditions N2 3 00 M H2 4 50 M NH3 0 H2 1 50 M NH3 1 41 M At equilibrium N2 2 00M What is Kc Mary J Bojan Chem 110 26 Use LeChatelier s principle to answer the following questions 1 What will happen to NH3 if 3 0 moles of H2 are added to the system at equilibrium Increase the amount of a reactant or product 2 What will happen if the volume decreases from 1 0L to 0 5L Increase P by decreasing volume 4 What will happen if 5 0 moles of inert gas is added to the system Increase pressure by adding an inert gas 6 What will happen if the …
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