Copyright 001 by Harcourt, Inc. All rights reserved.! Ch 16. Chemical Equilibria N O 4 (g) NO (g) The concept of equilibrium and K Writing equilibrium constant expressions Relationship between kinetics and equilibrium What does the equilibrium constant tell us? o redicting direction of reaction o Calculating equilibrium concentrations Factors that affect chemical equilibrium equilibrium Start with NO Start with N O 4 Start with NO & N O 4 Equilibrium is a state in which there are observable changes as time goes by. Chemical equilibrium is achieved when: the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant [NO] When equilibrium is established at a given temp, K has a constant value, [NO4] regardless of starting quantities. Copyright 001 by Harcourt, Inc. All rights reserved.! Law of Mass Action aa + bb cc + dd Κ [C]c [D] d [A] a [B] b K>1: Rxt is product-favored at equil K<1: Rxt is reactant-favored at equil Homogeneous Equilibria aa(aq) + bb(aq) cc(aq) + dd(aq) K c c d [C] [D] a b [C] M moles/l [A] [B] Solvent (H O) concentration does t appear; it does t change appreciably K has units, even if there are a different number of terms in the numerator and deminator aa(g) + bb(g) cc(g) + dd(g) K c c d [C] [D] a b [A] [B] OR ( ) ( ) K p ( ( c C a A) d D b B) ( C ) c partial pressure in atm K p K c only if total moles of reactants total moles of products Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!
Copyright 001 by Harcourt, Inc. All rights reserved.! K p K c (RT) Δn where Δn n product gas n reactant gas Ex: N + H NH Δn -4 - [NH ] M NH K c [N][H] ( M N )( M H ) since V nrt or RT n V Heterogeneous Equilibria CaCO (s) CaO(s) + CO (g) K CO K C [CO ]? K p K c Terms for solids or liquids do t appear in K. The concentration of a solid or liquid is an intrinsic quality (like density). NH RT N H RT RT ( )( ) 1 4 N NH H 1 RT RT K p( RT ) or K p Kc( RT ) ( )( ) N NH RT H ( ) (you are given this equation) At a given temp, both reactions produce same amt of CO, as long as some of both are present Copyright 001 by Harcourt, Inc. All rights reserved.! Heterogeneous Equilibria Relations between equilibrium constants Ag(s) + H + (aq) + NO - (aq) Ag + (aq) + NO (g) + H O Solutes appear as molarities; gases as partial pressures in atm + [Ag ] NO K + [H ] [NO ] K is defined for the reaction with specified states; gases enter as partial pressure in atm (even though they could have been expressed as molarity), solutes as molarity; pure solids, liquids, and solvents do t appear. HI(g) H (g)+i (g) K 0.016 H (g)+ I (g) HI(g) K' 1/K 1/0.016 6 4HI(g) H (g) + I (g) K'' K (0.016).6E-4 Rule of multiple equilibria: If Eq 1 + Eq Eq, then K 1 K K A + B C K 1 C D K A + B D K K 1 K Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!
Copyright 001 by Harcourt, Inc. All rights reserved.! Chemical Kinetics and Chemical Equilibrium For a reaction mechanism with a single elementary step: k f A + B AB k r rate f k f [A][B] rate r k r [AB ] Chemical Kinetics and Chemical Equilibrium For a multi-step reaction mechanism: K B B A + B K AB At equilibrium, rate f rate r k f [A][B] k r [AB ] A + B AB K c K K In general, for aa + bb cc+ dd Rule of Multiple Equilibria k f k r K c [AB ] [A][B] Since k changes with temperature, K c also changes with temperature [C] c [D] d K [A] a [B] b Regardless of whether the reaction is a single- or multistep mechanism Copyright 001 by Harcourt, Inc. All rights reserved.! What does K tell us? Reaction Quotient, Q, for concentrations at any point during the reaction HI(g) H (g) + I (g) redicting the direction of reaction Calculating equilibrium concentrations [HI] At any point, [H ][I ] [H][I ] Q [HI] At equilibrium Q K [H][I ] K [HI] Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!
Copyright 001 by Harcourt, Inc. All rights reserved.! redicting direction of reaction Compare the actual reaction quotient, Q, to that required at equilibrium, K. redicting direction of reaction HI (g) H (g) + I (g) K p 0.016 Suppose HI 0.100 atm, H or I ; which way will rxt proceed? H I 0 Q 0 < K system shifts to right 0.10 HI Suppose HI H I 0.100 atm If Q < K, forward reaction occurs to make Q larger. If Q K, system is already at equilibrium, so thing happens. If Q > K, reverse reaction occurs to make Q smaller. H I 0.10 Q 1 > K 0.10 HI system shifts to left Copyright 001 by Harcourt, Inc. All rights reserved.! Calculate Equilibrium Concentrations HI(g) H (g) + I (g) K p 0.016 Calculate Equilibrium Concentrations N O 4 (g) NO (g) K p 11 Start with pure HI at 0.100 atm. Find equil pressures of all species. HI H I I 0.100 0.000 0.000 C -x +x +x E 0.100-x x x To avoid confusion, do NOT include coefficients when listing species present Avoid x/ Start with pure N O 4 at 1.00 atm. Find equilibrium pressures N O 4 NO I 1.00 0.00 C -x +x E 1.00 - x x x (0.100 x) 0.016 ; x 0.1 ; x 0.010 0.100 x 0.100 x 0.080 atm ; H I x HI 0.010 atm 4x /(1.00 - x) 11; x 0.78 (by quadratic formula) NO x 1.56 atm ; N 1 x 0. atm O4 Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!
Copyright 001 by Harcourt, Inc. All rights reserved.! Effect of changes in conditions on equilibrium position Changes in Concentration Adding or removing a gaseous species Increase in pressure Change in temperature N (g) + H (g) Equilibrium shifts left to offset stress NH (g) add NH Add catalyst Le Châtelier s principle: If a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the system will, if possible, shift so as to partially counteract the change. After equilibrium is reestablished, there is more NH than before When equil composition is perturbed by adding or removing a reactant or product, reaction tends to occur in the direction that restores the value of Q to that of K Time net increase in [NH ] Copyright 001 by Harcourt, Inc. All rights reserved.! Changes in Concentration Start at equilibrium in 1.00 L flask with [iso] 1.5 M and [butane] 0.50 M. Now add 1.50 mol butane. What are new equil concentrations? Change in ressure [iso] K c.50 [butane] N O 4 (g) NO (g) K c [NO ] 0.0059 at 98 K [N O 4 ] butane I 0.50 M + 1.50 M 1.5 M C - x + x E.00 x 1.5 + x [iso] 1.5 + x K C.50 [butane].00 x isobutane Solve x 1.07 M New equil concs: [butane].00 x 0.9 M, [isobutane] 1.5 + x. M Initial amt of butane includes the added dose before equilibration Q: What is the effect of an increase by reducing the volume (at constant T)? A: Concentrations of both NO and N O 4 increase, but b/c there are more moles of gas on the product side, the numerator of K is affected more than deminator. Reaction shifts LEFT to consume NO and produce N O 4. Q: How does the equilibrium mixture respond to an increase in caused by adding an inert gas, such as He or Ne? A: No change since inert gas does t affect the concentration or the partial pressures of reactant or product. Collisions with inert gas are nproductive. Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!
Copyright 001 by Harcourt, Inc. All rights reserved.! Change in Temperature Changes K Temperature Effects on K Energy + N O 4 (g) NO (g) ΔH o 57 kj/mol colorless brown A + B C + D + heat T shifts reaction At 7K, K c 7.7 10-4 E a (forward) 0 o C, 7K 5 o C, 98K What happens to K c when T? At 98K, K c 59 10-4 E a (reverse) For endothermic rxts, T results in K eq For exothermic rxts, T results in K eq For an exothermic reaction, E a (reverse) > E a (forward); so k r depends more strongly on T than k f. When T is raised, k r increases more than k f ; therefore K will decrease and reactants will be more favored. Copyright 001 by Harcourt, Inc. All rights reserved.! Catalyst Effect on K? Le Châtelier s rinciple uncatalyzed Copyright 001 by Harcourt, Inc. All rights reserved.! catalyzed Catalyst lowers E a for both forward and reverse reactions. Adding a Catalyst does t change K (K only depends on T and difference in energy between reactants and products) does t change equilibrium composition but system will reach equilibrium sooner Change Copyright 001 by Harcourt, Inc. All rights reserved.! Δ Equilibrium Composition? Change K eq? Concentration yes ressure by Δ n reactant/product (see below for ΔV) ressure (Δ n inert gas )* Volume (+ pressure) yes yes if n reactant n product Temperature yes yes Catalyst * Addition of inert gas does t change concentration or partial pressure of reacting gases.