Exam 1 Monday and Tuesday and Wednesday AM For PC this week due Feb 16 do the following EOCs 2 4 8 12 14 16 20 24 26 28 32 38 44 48 53 62 64 1 Ch 16 Principles of Chemical Reactivity Equilibria Chemical reactions are reversible Chemical reactions proceed spontaneously in the direction that leads toward equilibrium In a closed system a state of equilibrium between reactants and products is achieved eventually and that outside forces P T adding or removing reactants or products can affect the equilibrium 2 16 1 Chemical Equilibrium A Review Chemical equilibrium exists when 2 opposing reactions occur simultaneously at the same rate dynamic equilibrium molecules are continually reacting even though the overall composition of the reaction mixture does not change 3 Reversible reactions do not go to completion They can occur in either direction Symbolically this is represented as cC dD a A g b B g g g 4 One of the fundamental ideas of chemical equilibrium is that equilibrium can be established from either the forward or reverse direction 2SO2 O2 2SO3 5 16 2 The Equilibrium Constant and Reaction Quotient For a simple one step mechanism reversible reaction such as A g B g C g D g The rates of the forward and reverse reactions can be represented as Rate f k f A B which represents the forward rate Rate r k r C D which represents the reverse rate 6 A g B g C g D g When a system is at equilibrium Ratef Rater Substitute for the rate relationsh ip to give k f A B k r C D which rearranges to k f C D k r A B 7 A g B g C g D g Because the ratio of two constants is a constant we can define a new constant as follows kf Kc and kr C D Kc A B 8 Similarly for the general reaction a A g b B g c C g d D g we can define a constant C D Kc a b A B c d products reactants This expression is valid for all reactions 9 The Equilibrium Constant Kc is the equilibrium constant Kc is defined for a reversible reaction at a given temperature as the product of the equilibrium concentrations in M of the products each raised to a power equal to its stoichiometric coefficient in the balanced equation divided by the product of the equilibrium concentrations in M of the reactants each raised to a power equal to its stoichiometric coefficient in the balanced equation 10 Writing Equilibrium Constant Expressions In an equilibrium constant expression all concentrations are reported as equilibrium values Product concentrations appear in the numerator and reactant concentrations appear in the denominator Each concentration is raised to the power of its stoichiometric balancing coefficient Values of K are dimensionless The value of the constant K is particular to the given reaction at a specific temperature 11 Write equilibrium constant expressions for the following reactions at 500oC All reactants and products are gases at 500oC PCl5 PCl3 Cl2 H 2 I 2 2 HI 12 N2 3H2 2NH3 2NH3 N2 3H2 4 NH 3 5 O 2 4 NO 6 H 2 O 13 The Equilibrium Constant Equilibrium constants are dimensionless because they actually involve a thermodynamic quantity called activity Activities are directly related to molarity The magnitude of Kc is a measure of the extent to which reaction occurs For any balanced chemical equation the value of Kc is 1 Constant at a given T 2 Changed if the T changes 3 Does not depend on the initial concentrations 14 Reactions Involving Solids In general the concentrations of any solid reactants and products are not included in the equilibrium constant expression S s O2 g SO2 g Reactions in Solution The concentration of any liquid reactants and products are not included in the equilibrium constant expression NH3 aq H2O l NH4 aq OH aq 15 When writing equilibrium constant expression we can ignore solids and liquids because they have an activity of 1 CuCl42 aq Cu2 aq 4Cl aq Kc 4H3O aq 2Cl aq MnO2 s Mn2 aq 6H2O l Cl2 aq Kc 16 Write equilibrium constant expressions for NiO s CO g Ni s CO2 g 2Fe3 aq 3 S2 aq Fe2S3 s TlCl3 s TlCl s Cl2 g 17 Reactions Involving Gases Kc and KP Kc equilibrium expression for concentration so units in M KP equilibrium expression for gases so units for P For gas phase reactions the equilibrium constants can be expressed in partial pressures rather than concentrations For gases the pressure is proportional to the concentration We can see this by looking at the ideal gas law PV nRT P nRT V n V M P MRT and M P RT 18 For convenience we may express the amount of a gas in terms of its partial pressure rather than its concentration To derive this relationship we must solve the ideal gas equation PV nRT n P RT V n Because has the units mol L V P RT Thus at constant T the partial pressure of a gas is directly proportional to its concentration 19 Consider this system at equilibrium at 5000C 4 HCl O 2 Cl 2 H O 2 g 2 g g 2 g PHCl PO HCl O 2 Kc and K p 2 2 2 2 Cl2 H 2O PCl PH O 4 4 2 2 2 20 Kc P P P PHCl PHCl 4 PO2 RT RT 2 P 2 P Cl2 RT 4 O2 2 H 2O Cl 2 RT 2 H 2O 1 5 RT 1 4 RT 1 so for this reaction K c K p RT 1 K c K p RT or K p K c RT 1 L atm Must use R 0 0821 mol K 21 Relationship Between Kp and Kc From the previous slide we can see that the relationship between Kp and Kc is K p K c RT n or K c K p RT n n of moles of gaseous products of moles of gaseous reactants When is Kc Kp What kind of R should you use 22 The Meaning of the Equilibrium Constant K N2 3H2 2NH3 Kc 3 6x 108 25 C If Kc 1 product favored reaction so Kc large product small reactant x small reactant The conc of products are greater than the conc of reactants at eq N2 O2 2 NO Kc 1 x 10 30 If Kc 1 reactant favored reaction so Kc small product large reactant x large reactant The conc of reactants are greater than the conc of products at eq 23 24 Fig 16 4 p 727 The equilibrium constant Kc is a constant at a specific T that does not mean that the equilibrium concentration of the reactants and products are the same just the Kc No matter what combinations of reactant and product concentrations we start with the resulting equilibrium concentration at a certain T for the reversible reaction would always give the same value of Kc 25