capter 9 chem

capter 9 chem - Chemical Equilibrium Chapter 9 Chem 6B...

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1 6B-Equilibrium (Basics) Keq Basics- 1 Chemical Equilibrium Chapter 9 Chem 6B 6B-Equilibrium (Basics) Keq Basics- 2 Introduction We have assumed that when a reaction goes to thermodynamic completion ( G = 0) all starting materials have been converted to products •Many reactions do NOT go to chemical completion, but rather approach equilibrium . – Once achieved, the composition of the reaction will not change; – Quantitatively we can describe this using an Equilibrium Constant ( K or K x ). Begin with an empirical examination of the Haber process
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2 6B-Equilibrium (Basics) Keq Basics- 3 Equilibrium: Empirical Examination Plot [Reactants] vs. t Plot [Product] vs. t Examine reverse reaction: Isotope experiment: 22 3 3 H ( ) N ( ) 2 NH ( ) gg g ⎯⎯→ + ←⎯⎯ 32 2 2 NH() 3 H() N() g + 3 3 3 D ( ) N ( ) 2 ND ( ) 3 H ( ) N ( ) 2 NH ( ) g g + + 6B-Equilibrium (Basics) Keq Basics- 4 Equilibrium: Empirical Conclusions • Equilibrium is established from either direction; • Values obtained for [H 2 ] eq , [N 2 ] eq , and [NH 3 ] eq are different depending on direction, amounts started with, temperature, etc. but a steady state is achieved regardless . • Equilibrium is a dynamic process (isotope experiment). • As equilibrium is reached, the rate of Rxtnts Æ Pdts equals the rate of Pdts Æ Rxtnts
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3 6B-Equilibrium (Basics) Keq Basics- 5 Fundamental Characteristics of Equilibrium States •N o macroscopic evidence of change seen; that is, the ratio of concentrations of products to reactants must remain constant at equilibrium. • Equilibrium is achieved through spontaneous (thermodynamic) processes. • Equilibrium is established independent of direction of approach. • At equilibrium there is a dynamic balance between the forward and reverse processes. 6B-Equilibrium (Basics) Keq Basics- 6 Mass-Action Ratio and the Empirical Equilibrium Constant Consider the reaction: A quantitative description of equilibrium can be achieved by defining the mass-action ratio is defined as follows: The empirical equilibrium constant K c will have units of aA bB cC dD ++ ⎯⎯→ ←⎯⎯ [ ] [ ] [][] constant ce q cd eq eq ab eq eq CD Ko rK AB == [ ] –a–b –a–b () conc or P + + K c ( or K eq ) is the empirical equilibrium constant
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4 6B-Equilibrium (Basics) Keq Basics- 7 Significance of the Law of Mass Action • Allows a unique quantitative value to be derived for the position of an equilibrium using non- thermodynamic parameters. • Value is an inherent property of the reaction at a given temperature ; • Value is independent of starting concentrations of both reactants and products. • Magnitude tells us about the position of equilibrium. – Essentially, three general cases 6B-Equilibrium (Basics) Keq Basics- 8 Magnitude of K : Large K Case 1: Large K ( K > 1000) G –17 kJ; reaction composition lies almost exclusively on the side of products; Amounts of starting materials left at equilibrium essentially negligible.
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This note was uploaded on 09/05/2009 for the course BILD BILD1 taught by Professor Bever during the Spring '08 term at UCSD.

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capter 9 chem - Chemical Equilibrium Chapter 9 Chem 6B...

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