Chapter 14. The Concept of Equilibrium and the Equilibrium Constant. We have for the most part depicted reactions as going one way.

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1 Chapter 14 The Conept of Equilibrium and the Equilibrium Constant In hapter 1 we dealt with Physial Equilibrium Physial Changes HO 2 (l) HO 2 (g) In hapter 14 we will learn about Chemial Equilibrium. We have for the most part depited reations as going one way. Balloon filled with H 2 and O2 But many reations are reversible. H 2 (g) + O 2 (g) HO 2 (g) flask of N O (g) 2 NO (g) Dinitrogen tetroxide (N2O 4) is a olorless gas. When plaed in a flask it dissoiates into two nitrogen dioxide (NO 2) moleules. The reation will proeed until the amount of NO 2 reahes equilibrium onentrations. Equilibrium is a steady state system. But it is a dynami proess. The reation appears to have stopped at equilibrium. The forward reation proeeds, but it is aneled out by the reverse reation.

2 The Equilibrium Constant 2 a A + b B C + d D CO (g) + H2O (g) CO 2 (g) + H 2 (g) 4 NO 2 (g) + 6 H2O (g) 7 O 2 (g) + 4 NH 3 (g) If K >> 1 reation favors produts If K << 1 reation favors reatants Any number > 10 is onsidered muh greater than 1 Any number < 0.1 is onsidered muh less than 1 K is onstant for a given temperature

3 Homogeneous Equilibria 3 Reating speies are in the SAME phase gas, liquid Reating speies are everything in the pot both reatants and produts General a A (g) b B (g) subsript indiates onentration For a gas at onstant temperature the pressure is diretly related to onentration. Using the Ideal Gas equation gives or p indiates Pressure <-- Partial Pressure of B <-- Partial Pressure of A K not usually equal to K but is related by p where n = moles of produt moles of reatant only when n = 0 does Kp = K Example: H 2 (g) + Br 2 (g) 2 HBr (g) n = = 0

4 Write Expressions for K, and K p if appliable, for the following reversible reations at equilibrium. 4 Example 1: + HF (aq) + H2O (l) HO 3 (aq) + F (aq) HF is a weak aid. 5.5 moles of water per liter. This is large ompared to the other reating speies. Its onentration will not hange muh in omparison to the onentration of the other reating speies so don t inlude the water in the equilibrium expression. Example 2: 2 NO (g) + O 2 (g) 2 N2O (g)

5 Heterogeneous Equilibria 5 Reating speies are in DIFFERENT phases CaCO 3 (s) CaO (s) + CO 2 (g) Conentration of a solid is an intensive property and does not depend on how muh of the substane is present Copper - the molar onentration or density is the same at 20 C whether there is 1 g or 1 ton

6 Multiple Equilibria 6 Overall Reation A + B C + D C + D E + F A + B E + F If a reation an be expressed as the sum of two or more reations the equilibrium onstant for the overall reation is given by the produt of eah individual reation Example HCO (aq) H (aq) + HCO (aq) K ' = 4.2 x 10 HCO (aq) H (aq) + CO (aq) K "= 4.8 x 10 HCO (aq) 2 H (aq) + CO (aq) K =?

7 The Form of K and the Equilibrium Equation 7 1. When the equation for a reversible reation is written in the opposite diretion, the equilibrium onstant beomes the reiproal of the original equilibrium equation. Example N2O 4 (g) 2 NO 2 (g) at 25 C or 2 NO 2 (g) NO 2 4 (g) 2. The value of K depends on how the equation is balaned NO 2 4 (g) 2 NO 2 (g) ½ NO 2 4 (g) NO 2 (g) We an see that

8 Guidelines for Writing Equilibrium Constant Expressions 8 - The onentrations of the reating speies in the ondensed phase are expressed in mol / L; in the gas phase the onentrations an be expressed in mol / L or atm - The onentrations of pure solids, pure liquids (in heterogeneous equilibria) and solvents (in homogeneous equilibria) do not appear in the equilibrium onstant expressions. Examples are the solids CaCO 3 and CaO, page 5 and water, page 4. - K and K are dimensionless quantities p - In quoting the value for the equilibrium onstant the balaned equation and temperature must be speified - If a reation an be expressed as the sum of 2 or more reations, the equilibrium onstant for the overall reation is give by the produt of the individual onstants

9 Relationship between Chemial Kinetis and Chemial Equilibrium 9 K is a onstant at a given temperature regardless of the equilibrium onentrations of the reating speies. rate forward = rate reverse at equilibrium k k and K is always equal to forward reverse K hanges with temperature In terms of hemial kinetis, the equilibrium onstant of a reation an be expressed as a ratio of the rate onstants of the forward and reverse reations.

10 Prediting the Diretion of a Reation 10 H 2 (g) + I 2 (g) 2 HI (g) K = 54.3 at 430 C Suppose moles of H 2 and moles of I moles of HI are plaed into a 1.00 L ontainer at 430 C o = initial onentration 111 is larger than 54.3 so the reation proeeds to reatants Q is the Reation Quotient Q < K reation moves forward Q > K reation moves in reverse Q = K reation at equilibrium

11 Calulating Equilibrium Conentrations 11 is-stilbene trans-stilbene K = 24.0 at 200 C Suppose the initial onentration of only is-stilbene is mol / L From stoihiometry one mole of is-stilbene makes one mole of trans-stilbene. Let x be the equilibrium onentration of trans-stilbene is-stilbene trans-stilbene Initial onentration(m) Change (M) -x + x Equilibrium Con. ( x) +x x = 24.0 ( x) = (24.0)(0.850) x 24.0 x + x = (24.0)(0.850) 25.0 x = 20.4 x = 20.4 / 25.0 = M trans-stilbene M M = 0.034M is-stilbene Most onverts to trans-stilbene

12 Approah to Solving Equilibrium Constant Problems Express equilibrium onentration of all speies in terms of the initial onentrations and a single unknown x. This is the hange. 2. Write the equilibrium onstant expression in terms of equilibrium onentrations. Knowing the value of K solve for x. 3. Having solved for x alulate the equilibrium onentrations for all speies

13 14.43 The equilibrium onstant K for the reation 13 H 2 (g) + Br 2 (g) 2 HBr (g) 6 is 2.18 x 10 at 730 C. Starting with 3.20 moles of HBr in a 12.0 liter vessel alulate the onentration of H 2, Br 2 and HBr at equilibrium.

14 Fators that Affet Chemial Equilibrium 14 Le Ch telier s Priniple Equilibrium will adjust to redue stress stress = hange in onentration, volume, pressure, or temperature Change in onentration CO (g) + 2 H 2 (g) CH3OH (g) Inrease CO - to relieve stress the reation shifts toward produts Inrease H 2 - reation shifts toward produts Inrease CH3OH - reation shifts toward reatants Changes in Volume and Pressure Example CO (g) + 2 H (g) CH OH (g) 2 3 inrease pressure or redue Volume reation favors produts Example N O (g) 2 NO (g) derease P or redue V reation favors reatants In general inrease pressure (derease in volume) derease moles of gas. derease pressure (inrease vol.) inrease moles of gas.

15 Change in Temperature 15 H > 0 absorbs heat H = 58.0 J Endothermi heat + N2O 4 (g) 2 NO 2 (g) H < 0 releases heat H = 58.0 J Exothermi 2 NO 2 (g) heat + N2O 4 (g) A temperature inrease and the Endothermi reation is favored. A temperature derease and the Exothermi reation is favored. Demonstration: Three ontainers of N O one eah at 0 C, RT, and warmed. 2 N2O 4 is olorless, N2O is brown. The ooled ontainer beomes lighter, the RT is brown and the warmed beomes darker. Effet of a Catalyst on Equilibrium - None A atalyst lowers the ativation energy (E a) it does not alter the equilibrium position or value. Of the 4 types of stress only Temperature hanges the Value of the equilibrium onstant. The others hange the onentration of the reation speies