# Chemical Equilibrium Quantitative Aspects Part 1 Green/Damji – Chapter 7.2 Chang - Chapter 14 Copyright © The McGraw-Hill Companies, Inc. Permission required.

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Chemical Equilibrium Quantitative Aspects Part 1 Green/Damji – Chapter 7.2 Chang - Chapter 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

A + B C + D 14.1 Chemical Equilibrium – Part 2a Assume this reaction is first order with respect to each species… Forward rate = k f [A][B]Reverse rate = k r [C][D] At equilibrium the rates are equal so k f [A][B]= k r [C][D] rearranged to k f =[C][D] k r [A][B] since both k f and k r are constants (at a given temp) their ratio must also be a constant… called the equilibrium constant and symbolized K c (or K eq )

K c = [C] c [D] d [A] a [B] b 14.1 Equilibrium Constant Expression (aka Law of Mass Action) aA + bB cC + dD “expression” indicates you only put in symbols… no actual values

If K >> 1 If K << 1 Lie to the rightFavor products Lie to the leftFavor reactants Equilibrium Will 14.1 When actual numbers are put into the equations, a value for the Kc can be determined. That value can reveal information about the reaction.

Derive the equilibrium constant expression for the following homogenous equilibrium: N 2 O 4 (g) 2NO 2 (g) K c = 14.2 = 4.63 x 10 -3

Example: Ex 7.2 # 1 The equilibrium constant for a reaction that occurs totally in the gas phase is given below. What is the chemical equation for this equilibrium? Kc = [CO 2 ] [CF 4 ] [COF 2 ] 2 A.CO 2 (g) + CF 4 (g) ⇌ COF 2 (g) B. CO 2 (g) + CF 4 (g) ⇌ 2 COF 2 (g) C.2 COF 2 (g) ⇌ CO 2 (g) + CF 4 (g) D.COF 2 (g) ⇌ CO 2 (g) + CF 4 (g)

Ex: Ex 7.2 # 8e Balance the equation below and write the Kc expression. NO (g) + Cl 2 (g) ⇌ NOCl (g) Kc =

Ex: Ex 7.2 # 8f Balance the equation below and write the Kc expression. NH 3 (g) + O 2 (g) ⇌ H 2 O (g) + O 2 (g) Kc =

Ex: Ex 7.2 # 8h Balance the equation below and write the Kc expression. CH 3 OH (l) + CH 3 COOH (l) ⇌ CH 3 COOCH 3 (l) + H 2 O (l) Kc =

CaCO 3 (s) CaO (s) + CO 2 (g) K c = ‘ [ ][ ] [ ] [CaCO 3 (s)] = constant [CaO (s)] = constant K c = [ ] = K c x ‘ [ ] 14.2 Derive the equilibrium constant expression for the following heterogeneous equilibrium:

If the concentration of a substance does not change, then that substance is not included in the expression for the equilibrium constant. 14.2 For instance… all solids pure liquids especially water … when it is a liquid involved in an aqueous solution Note: if water is a gas it must be included AND if water is a liquid with mixed with other liquids (not an aqueous situation)

CH 3 COOH (aq) + H 2 O (l) CH 3 COO - (aq) + H 3 O + (aq) K c = ‘ [ ][ ] [H 2 O (l)] = constant, so K c = [ ][ ] [ ] =K c [H 2 O] ‘ Units for an equilibrium constant vary by situation. Sometimes they are not even included. 14.2

Ex: Ex 7.2 # 8g Balance the equation below and write the Kc expression. CH 3 NH 2 (aq) + H 2 O (l) ⇌ CH 3 NH 3 + (aq) + OH – (aq) Kc =

N 2 O 4 (g) 2NO 2 (g) = 4.63 x 10 -3 K = [NO 2 ] 2 [N 2 O 4 ] 2NO 2 (g) N 2 O 4 (g) K = [ ] ‘ = 1 K = _______ When the equation for a reversible reaction is written in the opposite direction, the equilibrium constant becomes the reciprocal of the original equilibrium constant. 14.2

A + B C + D C + D E + F A + B E + F K c = ‘ [C][D] [A][B] K c = ‘ ‘ [E][F] [C][D] [E][F] [A][B] K c = KcKc ‘ KcKc ‘‘ KcKc KcKc ‘‘ KcKc ‘ x If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions. 14.2

H 2 O (g) + CO (g) + ⇌ H 2 (g) + CO 2 (g) H 2 O (g) + C (s) ⇌ H 2 (g) + CO (g) K c = ‘ [H 2 ][CO 2 ] [H 2 O][CO] K c = ‘ [H 2 ][CO] [H 2 O][C] [H 2 ] 2 [CO 2 ] [H 2 O] 2 K c = KcKc ‘ KcKc ‘‘ KcKc KcKc ‘‘ KcKc ‘ x What? 14.2 2 H 2 O (g) + C (s) + ⇌ 2 H 2 (g) + CO 2 (g) ------------------------------------------------------

SUMMARY: Writing Equilibrium Constant Expressions 1.The concentrations of the reacting species in the condensed phase are expressed in M. In the gaseous phase, the concentrations can be expressed in M or in atm. 2.The concentrations of pure solids and pure liquids often do not appear in the equilibrium constant expressions. 3.The equilibrium constant is a dimensionless quantity (by convention in the US… IB will occasionally ask for these) 14.2

Writing Equilibrium Constant Expressions 4.In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature. 5.If a reaction can be expressed as a sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions. 14.2

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