Chapter 12_May11 - CHAPTER 12(15) Chemical Equilibrium 1...

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1 CHAPTER 12(15) Chemical Equilibrium Chemical Equilibrium
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2 CONTENTS 12.1 The Concept of Equilibrium 12.2 The Equilibrium Constant 12.3 Heterogeneous Equilibria 12.4 Calculating Equilibrium Constants 12.5 Application of Equilibrium Constants 12.6 Le Châtelier’s Principle
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3 Learning Outcomes Student should be able to use concept of equilibrium to calculate equilibrium constant and equilibrium concentrations. Able to apply Le Chaterlier’s Principle to predict the direction of a reaction hence apply it in commercial sense in increasing yield, reduce cost etc. Differentiate homogenous and heterogeneous equilibria, reaction quotient and Equilibrium constant
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4 12.1 The Concept of Equilibrium Consider Colorless frozen N 2 O 4 . At room temperature, it decomposes to  brown NO 2 . N 2 O 4 (g)   2NO 2 (g) At some time, the color stop changing and  we have a mixture of N 2 O 4  and NO 2 . the concentration of all reactants and products no longer change with time. N 2 O 4 (g) 2 NO 2 (g)
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5 12.1 The Concept of Equilibrium N 2 O 4 (g) 2 NO 2 (g) Chemical equilibrium is the point at which the concentrations of all species are constant. (concentrations of reactants and products cease to change with time) Chemical equilibrium occurs opposing reactions are proceeding at equal rate. Rate forward = Rate reverse
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6 N 2 O 4 (g) - colorless NO 2 (g) - reddish brown Concentration (color) remains constant at equilibrium
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7 time
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8 12.1 The Concept of Equilibrium Consider a simple reaction: A(g) B(g) We can write rate expressions for each reaction: Forward reaction: A B Rate = k f [A] k f = rate constant (forward reaction) Reverse reaction: B A Rate = k r [B] k r = rate constant (reverse reaction)
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9 12.1 The Concept of Equilibrium For gaseous substances we can use the ideal gas equation to convert between concentration and pressure: PV=nRT so M(Molarity) = (n/V) =(P/RT) For substances A and B: [A] = (P A /RT) and [B] = (P B /RT) Rate fwd = k f ( P A /RT) and Rate rev = k r (P B /RT)
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10 12.1 The Concept of Equilibrium Place some pure compound A into a closed container. As A reacts to form B, the partial pressure of A will decrease and the partial pressure of B will increase. Expect forward reaction rate to slow and reverse reaction rate to increase. Eventually, we get to equilibrium where forward and reverse rates are equal.
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11 Haber Process N 2 (g) + 3H 2 (g) 2NH 3 (g) The reaction is carried out under conditions of high pressure and high temperature. Equilibrium can be established either by starting with N 2 and H 2 or by starting only with NH 3 .
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12.1 The Concept of Equilibrium At Equilibrium: Rearrange: a constant This mixture is called an equilibrium mixture . This is an example of
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Chapter 12_May11 - CHAPTER 12(15) Chemical Equilibrium 1...

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