CHAPTER 13 Chemical Equilibrium 건국대학교 생명공학과 김성현

Copyright © Houghton Mifflin Company. All rights reserved.13–2 A + B ⇄ C + D Chemical equilibrium: the state where the concentrations of all reactants (A, B) and products (C, D) remain constant with time. * In stoichiometric calculations, we usually assume reactions proceed to completion. [C], [D] >> [A], [B] Otherwise, we have to consider equilibrium constant into consideration.

Copyright © Houghton Mifflin Company. All rights reserved.13– The Equilibrium Condition Equilibrium is dynamic, not static. No apparent change in concentration Ex) H 2 O(g) + CO(g) ⇄ H 2 (g) + CO 2 (g)

Copyright © Houghton Mifflin Company. All rights reserved.13–4 Figure 13.4 As the Concentrations of the Reactants Decrease, the Forward Reaction Slows Down At equilibrium, the forward reaction rate equals reverse reaction rate! H 2 O(g) + CO(g)  H 2 (g) + CO 2 (g) v f H 2 O(g) + CO(g)  H 2 (g) + CO 2 (g) v r v f = v r

Copyright © Houghton Mifflin Company. All rights reserved.13–5 Figure 13.5 A Concentration Profile Fritz Haber process of ammonia production N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) Usually this reaction is very slow under ambient conditions. Fritz Haber developed an efficient way to synthesize using high pressure and catalysts.

Copyright © Houghton Mifflin Company. All rights reserved.13– The Equilibrium Constant Low of mass action jA + kB ⇄ lC + mD :Equilibrium constant [A] is equilibrium concentration of species A, usually expressed in molarity (mol/L). * C A is analytical concentration of species A in mol/L. K has no unit. K always has the same value at a given temperature.

Copyright © Houghton Mifflin Company. All rights reserved.13–7 lC + mD ⇄ jA + kB njA + nkB ⇄ nlC + nmD Equilibrium position: each set of equilibrium concentrations There are infinite number of equilibrium positions but only one equilibrium constant at a given temperature.

Copyright © Houghton Mifflin Company. All rights reserved.13–8

Copyright © Houghton Mifflin Company. All rights reserved.13– Equilibrium Expressions Involving Pressures PV = nRT or P = (n/V)RT = CRT C = n/V : molar concentration of the gas N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) Equilibrium constant in terms of Partial pressure Exercise 13.4

Copyright © Houghton Mifflin Company. All rights reserved.13–10 Relationship between K and K p For jA + kB ⇄ lC + mD  n = (l + m) – (j + k) Exercise 13.5

Copyright © Houghton Mifflin Company. All rights reserved.13– Heterogeneous Equilibria Involve more than one phase Ex: CaCO 3 (s) ⇄ CaO(s) + CO 2 (g) Note! Position of equilibrium does not depend on the amount of pure solids or liquids present.  If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not included in the equilibrium expression.

Copyright © Houghton Mifflin Company. All rights reserved.13–12 Examples 2H 2 O(l) ⇄ 2H 2 (g) + O 2 (g) K = [H 2 ] 2 [O 2 ], K p = (P H2 2 )(P O2 ) 2H 2 O(g) ⇄ 2H 2 (g) + O 2 (g) Exercise 13.6

Copyright © Houghton Mifflin Company. All rights reserved.13– Applications of the Equilibrium Constant It is possible to predict the tendency of the reaction to occur, not the speed. Example Assume K = 16 = AB DC

Copyright © Houghton Mifflin Company. All rights reserved.13–14 Two Types of Molecules are Mixed Together in the Following Amounts

Copyright © Houghton Mifflin Company. All rights reserved.13–15 Conditions of Equilibrium Reactions  x = 8

Copyright © Houghton Mifflin Company. All rights reserved.13–16 Equilibrium Mixture

Copyright © Houghton Mifflin Company. All rights reserved.13–17 Extent of a reaction Large K: equilibrium lies to the right (products) Small K: equilibrium to the left (reactants) The size of K and the time required to reach equilibrium are not directly related. Determined by thermodynamic factor (  E) Depends on the reaction rate (E a )

Copyright © Houghton Mifflin Company. All rights reserved.13–18 Reaction Quotient (Q) The expression for Q is exactly the same as that for K. Q can be obtained at any instant of the reaction, but K is only calculated at equilibrium  1) Q = K : equilibrium 2) Q > K : The system shifts to the left, consuming products and forming reactants 3) Q < K: The system shifts to the right, consuming reactants and forming products Ex 13.7 jA + kB ⇄ lC + mD

Copyright © Houghton Mifflin Company. All rights reserved.13–19 Figure 13.8 a-c (a) The Initial Equilibrium Mixture of N 2, H 2, and NH 3 (b) Addition of N2. (c.) The New Equilibrium Position for the System Containing More N 2 (due to Less H 2, and More NH 3 than in (a)

Copyright © Houghton Mifflin Company. All rights reserved.13–20 Figure 13.9 a-c (a) A Mixture of NH 3 (g), N 2 (g), and H 2 (g) at Equilibrium (b) The Volume is Suddenly Decreased (c) The New Equilibrium Position for the System Containing More NH 3 and Less N 2 and H 2

Copyright © Houghton Mifflin Company. All rights reserved.13–21 Anhydrous Ammonia is Injected into the Solid to Act as a Fertilizer

Copyright © Houghton Mifflin Company. All rights reserved.13–22 The Seven Sisters Chalk Cliffs in East Sussex, England

Copyright © Houghton Mifflin Company. All rights reserved.13–23 Hydrated Copper (II) Sulfate on the Left. Water Applied to Anhydrous Copper (II) Sulfate, on the Right, Forms the Hydrated Compound

Copyright © Houghton Mifflin Company. All rights reserved.13–24 Photo 13.4 Apollo II Lunar Landing

Copyright © Houghton Mifflin Company. All rights reserved.13–25 Photo 13.5 a-b LeChatelier's Principle II

Copyright © Houghton Mifflin Company. All rights reserved.13–26 Table 13.1 Results of Three Experiments for the Reaction N 2 (g) + 3H 2 (g) -- 2NH 3 (g)

Copyright © Houghton Mifflin Company. All rights reserved.13–27 Table 13.2 The Percent by Mass of NH 3 at Equilibrium in a Mixture of N 2, H 2, and NH 3 as a Function of Temperature and Total Pressure

Copyright © Houghton Mifflin Company. All rights reserved.13–28 Table 13.3 Observed Value of K for the Ammonia Synthesis Reaction as a Function of Temperature

Copyright © Houghton Mifflin Company. All rights reserved.13–29 Table 13.4 Shifts in the Equilibrium Position for the Reaction 58 kJ + N 2 O 4 (g) -- 2NO 2 (g)