Chemical Equilibrium: Ch Dynamic Equilibrium. Dynamic Equilibrium. Three Approaches to Equilibrium The Equilibrium Constant Expression

Transcription

1 Chemical Equilibrium: Ch Dynamic Equilibrium 15- The Equilibrium Constant Expression 15- Relationships Involving Equilibrium Constants 15-4 The Magnitude of an Equilibrium Constant 15-5 The Reaction Quotient, Q: Predicting the Direction of a Net Change 15-6 Altering Equilibrium Conditions: 15-7 Equilibrium Calculations: Some Illustrative Examples 15-1 Dynamic Equilibrium Equilibrium two opposing processes taking place at equal rates. Prepared Equilibrium Dynamic Equilibrium H O(l) NaCl(s) H O CO(g) + H (g) H O(g) NaCl(aq) I (H O) I (H O) I (CCl 4 ) I (CCl 4 ) I (H O) I (CCl 4 ) Reversible reactions: Both forward and reverse reactions take place 15- The Equilibrium Constant Expression Methanol synthesis is a reversible reaction. CO(g) + H (g) CO(g) + H (g) Three Approaches to Equilibrium H (g) + ICl(g) I (g) + HCl(g) ICl CO(g) + H (g) H As is Ammonia production (Haber-Bosch process) I N (g) + H (g) NH (g)

2 The Equilibrium Constant: A kinetic perspective H (g) + ICl(g) I (g) + HCl(g) Mechanism: Step 1 H (g) + ICl(g) HI(g) + HCl(g) d[p] k[h ][ICl] dt At Equilibrium [H ][ICl] [HI][HCl] Briggs Raucher reaction:oscillating reactions Step HI(g) + ICl(g) I (g) + HCl(g) k [HI][ICl] k - [I][HCl] step1 step k1 [ H ][ ICl] k [ I ][ HCl] [ HI] k [ HCl] k [ ICl] So, 1 [ I][ HCl] [ H ][ ICl] k1k k k 1 K c The Equilibrium Constant and Activities; a Activity Thermodynamic concept introduced by Lewis. Dimensionless ratio referred to a chosen reference state. a [ B] γ B B 0 cb coefficient of activity and c B0 is a standard reference state 1 mol L -1 γ B accounts for non-ideal behaviour in solutions and gases. γ B [B] can be considered an effective or active concentration. General Expressions a A + b B. g G + h H. [G] g [H] h. Equilibrium constant K c [A]a [B] b. Thermodynamic Equilibrium constant K eq (a G )g (a H ) h. (a A ) a (a B ) b. [G] g [H] h. ( γ G ) g (γ H ) h. [A] a [B] b. (γ A ) a (γ B ) b. (c o ) (a+b )-(g+h ) 1 under ideal conditions 1 mol L Relationships Involving the Equilibrium Constant When chemical equations (with K 1 and K ) are added, the overall equilibrium constant is K 1 K Reversing an equation causes inversion of K. Multiplying all coefficients by a common factor raises the equilibrium constant to the corresponding power. Dividing the coefficients by a common factor causes the equilibrium constant to be taken to that root. Combining Equilibrium Constant Expressions N O(g) + ½O NO(g) K c? N (g) + ½O N O(g) K c() [NO] K c [N O][O ] ½ [NO] [N ][O ] [N ][O ] ½ [N O] [N O] [N ][O ] ½ [NO] N (g) + O NO(g) K c() [N ][O ] 1 K c() K c()

3 Gases: The Equilibrium Constant, K P Mixtures of gases are solutions just as liquids are. Define K P, based on partial pressure (or activities) of gasses, rather than concentration SO (g) + O (g) SO (g) (a K P SO ) (aso ) (a O ) Gases: The Equilibrium Constant, K C SO (g) + O (g) SO (g) (a K P SO ) (P SO ) (a SO ) (a O ) (P SO ) (P O ) ([SO ] ) ([SO ] ) ([O ] ) ;P X n_ x [X] P SO P SO P O a SO a SO a SO K P (P SO ) (P SO ) (P O ) [SO ] [SO ] [O ] K C Where 1 bar Gases: The Equilibrium Constant, K C SO (g) + O (g) SO (g) K c [SO ] [SO ] [O ] P SO P SO P O P SO P P SO O K c K P () K P K c () -1 In general: K P K c () Δn Heterogeneous Reactions: Pure Liquids and Solids Equilibrium constant expressions do not contain concentration terms for solid or liquid phases of a single component (that is, pure solids or liquids). C(s) + H O(g) [CO][H K c ] [H O] CO(g) + H (g) P CO P H () 1 P HO 15-4 The Significance of the Magnitude of the Equilibrium Constant. Significant quantities of both reactants and products are likely to be present at equilibrium only if ~10-10 < K < ~ The Significance of the Magnitude of the Equilibrium Constant. Does a large equilibrium constant ensure that a reaction will take place if only reactants are present? No!! The rate can be so low that no product is formed Thermodynamic control: concentration of products and reactants depends on relative stability of the species. Kinetic control: concentration of products and reactants depends rates of reaction (i.e. k and Ea)

4 15-5 The Reaction Quotient, Q: Predicting the Direction of Net Change. CO(g) + H (g) Equilibrium can be approached various ways. What changes in concentration do we expect as equilibrium is approached from the conditions prepared at some time, t? The reaction quotient helps us predict: [G] Q c tg [H] th [A]ta [B] tb At equilibrium Q c K c Three Approaches to Equilibrium CO(g) + H (g) H CO CH OH Reaction Quotient 15-6 Altering Equilibrium Conditions: When an equilibrium system is subjected to a change in temperature, pressure, or the concentration of a reacting species, the system responds by attaining a new equilibrium that partially offsets the impact of the change. What happens if we add some extra SO to the reaction system at equilibrium? SO (g) + O (g) SO (g) [SO Q ] [SO ] [O K c ] K c.8 10 at 1000K Q > K c Effect of Changes in P or Adding a gaseous reactant or product changes P gas. Equivalent to a change in concentration Adding an inert gas changes the total pressure, but relative partial pressures are unchanged. Changing the volume of the system causes a change in the equilibrium position. K c [SO ] [SO ] [O ] n SO n SO n O n SO n n SO O

5 Effect of Change in olume g n H h K c [G] g [H] h n G [A]a [B] b (a+b)-(g+h) a a n A n B n g h G n H a a n A n B -Δn When the volume of an equilibrium mixture of gases is reduced, a net change occurs in the direction that produces fewer moles of gas. When the volume is increased, a net change occurs in the direction that produces more moles of gas. Effect of Temperature on Equilibrium SO (g) + O (g) SO (g) ΔH kj/mol Raising the temperature of an equilibrium mixture shifts the equilibrium condition in the direction of the endothermic reaction. Lowering the temperature causes a shift in the direction of the exothermic reaction. What does this behaviour say about the temperature dependence of compared to? Effect of a Catalyst on Equilibrium A catalyst changes the reaction mechanism to one with a lower activation energy. A catalyst has no effect on the condition of equilibrium: the relative thermodynamic stability of reactants and products in unchanged. A catalyst does affect the rate at which equilibrium is attained Equilibrium Calculations: Some Illustrative Examples. Five numerical examples are given in the text that illustrate ideas that have been presented in this chapter. Refer to the comments which describe the methodology. These will help in subsequent chapters. Exercise your understanding by working through the examples with a pencil and paper. Synthesis of Ammonia Haber-Bosch process: N (g) + H (g) NH (g) Problem Set: Ch15 7,9,1,1,,7,6,47,51,55,56,69,79,8 The optimum conditions are only for the equilibrium position and do not take into account the rate at which equilibrium is attained.