Ch 16. Chemical Equilibria. Law of Mass Action. Writing Equil Constant Expressions Homogeneous Equilibria. 2NO 2 (g) N 2 O 4 (g)

Transcription

1 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 When equilibrium is established at a given temp, [NO] K has a constant value, regardless of starting quantities. [N O ] 4 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) D cc(aq) + dd(aq) K c c d [C] [D] a b [C] M moles/l [A] [B] aa(g) + bb(g) D cc(g) + dd(g) 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 K c c d [C] [D] a b [A] [B] OR ( ) ( ) K p ( ( c C a A) d D b B) K p K c only if total moles of reactants total moles of products ( C ) c partial pressure in atm Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!

2 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 D NH Δn -4 - [NH ] K c [N][H ] NH RT N H RT RT M NH ( M )( M ( )( ) 1 4 N NH N H H ) 1 RT RT K p( RT ) or K p Kc( RT ) since V nrt or ( )( ) N NH RT H ( ) RT n V (you are given this equation) Heterogeneous Equilibria CaCO (s) D 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). 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 Ag(s) + H + (aq) + NO - (aq) D 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. Relations between equilibrium constants HI(g) D H (g)+i (g) K H (g)+ I (g) D HI(g) K' 1/K 1/ HI(g) D 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 D C K 1 C D D K A + B D D K K 1 K Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!

3 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 ] At equilibrium, rate f rate r k f [A][B] k r [AB ] k f k r K c [AB ] [A][B] Since k changes with temperature, K c also changes with temperature Chemical Kinetics and Chemical Equilibrium For a multi-step reaction mechanism: K B B A + B K AB A + B AB K c K K In general, for aa + bb cc+ dd [C] c [D] d K [A] a [B] b Rule of Multiple Equilibria 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 redicting the direction of reaction HI(g) D H (g) + I (g) 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.!

4 Copyright 001 by Harcourt, Inc. All rights reserved.! redicting direction of reaction Compare the actual reaction quotient, Q, to that required at equilibrium, K. 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. redicting direction of reaction HI (g) D H (g) + I (g) K p Suppose HI 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 atm H I 0.10 Q 1 > K system shifts to left 0.10 HI Copyright 001 by Harcourt, Inc. All rights reserved.! Calculate Equilibrium Concentrations Start with pure HI at atm. Find equil pressures of all species. HI H I I C -x +x +x E x x x x (0.100 x) ; x 0.1 ; x x HI(g) D H (g) + I (g) K p x atm ; H I x HI To avoid confusion, do NOT include coefficients when listing species present Avoid x/ atm Calculate Equilibrium Concentrations N O 4 (g) D NO (g) K p 11 Start with pure N O 4 at.1.00 atm. Find equilibrium pressures N O 4 NO I C -x +x E x x 4x /( 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.!

5 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) D 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 net increase in [NH ] Time 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? butane I M 1.5 M C - x + x E.00 x x [iso] x K C.50 [butane].00 x isobutane Solve x 1.07 M New equil concs: [butane].00 x 0.9 M, [isobutane] x. M [iso] K c [butane].5 Initial amt of butane includes the added dose Change in ressure N O 4 (g) D NO (g) K c [NO ] at 98 K [N O 4 ] What is the effect of an increase by reducing the volume (at constant T)? 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. How does the equilibrium mixture respond to an increase in caused by adding an inert gas, such as He or Ne? 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.!

6 Copyright 001 by Harcourt, Inc. All rights reserved.! Change in Temperature Changes K Energy + N O 4 (g) D NO (g) ΔH o 57 kj/mol colorless brown At 7K, K c What happens to K c when T? Temperature Effects on K A + B D C + D + heat T shifts reaction E a (forward) 0 o C, 7K 5 o C, 98K At 98K, K c E a (reverse) For endothermic rxts, T results in K For exothermic rxts, T results in K 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? uncatalyzed 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 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) Le Châtelier s rinciple 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. Copyright 001 by Harcourt, Inc. All rights reserved.! Copyright 001 by Harcourt, Inc. All rights reserved.!