C142A Ch 6 2011 lecture notes

C142A Ch 6 2011 lecture notes - Chapter 6: Chemical...

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Chapter 6: Chemical Equilibrium 6.1 The Equilibrium Condition 6.2 The Equilibrium Constant 6.3 Equilibrium Expressions Involving Pressures 6.4 The Concept of Activity 6.5 Heterogeneous Equilibria 6.6 Applications of the Equilibrium Constant 6.7 Solving Equilibrium Problems 6.8 LeChatelier’s Principle 6.9 OMIT: Equilibria Involving Real Gases
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Molecular Picture of Establishment of Equilibrium CO( g ) + H 2 O( g ) CO 2 ( g ) + H 2 ( g ) INITIAL 7 CO( g ) + 7 H 2 O( g ) + 0 CO 2 ( g ) + 0 H 2 ( g ) AFTER IT STOPS CHANGING 2 CO( g ) + 2 H 2 O( g ) + 5 CO 2 ( g ) + 5 H 2 ( g )
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Chemical Equilibrium Previously, we assumed that a chemical reaction goes to completion as written. H 2 O( g ) + CO ( g ) H 2 (g) + CO 2 ( g ) In general this is not correct. Instead, a stable state of the system in reached, which includes both reactants and products. It is called the equilibrium state, or simply “equilibrium”. H 2 O( g ) + CO ( g ) H 2 (g) + CO 2 ( g ) How far toward completion it goes depends on the specific reaction, and on temperature.
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Concentration vs. Time CO( g ) + H 2 O( g ) CO 2 ( g ) + H 2 ( g ) Ratio depends on temperature
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Characteristics of Chemical Equilibrium States Reaching equilibrium requires reactions to occur. Once reached, they show no macroscopic evidence of further change. Reached through dynamic balance of forward and reverse reaction rates .
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Figure 5.21: The collision rate of gas particles defines the maximum blue-pink reaction rate! Z = Collision rate (of one pink with blues)= # collisions with blues per s = 4 [N blue /V] d 2 ( π RT/M) 1/2 = 4 [blues] d 2 ( π RT/M) 1/2
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Equilibrium arises through dynamic balance between forward and back reactions Forward rate = k 1 [NO 2 ][NO 2 ] = k 1 [NO 2 ] 2 Back rate = k -1 [NO 3 ][NO] The rate constants k 1 and k -1 reflect probabilities that one collision leads to a successful reaction. Reverse: NO 3 (g) + NO(g) 2NO 2 (g) k -1
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Kinetics of Approach to Equilibrium [ ] [ ][ ] [] [ ] 2 12 1 3 3 1 2 1 2 N ON O N O NO NO NO kk k k = = = K At eqbm.: Forward rate = back rate “Equilibrium constant”
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The Equilibrium Constant - Definition Consider the generalized chemical reaction: a A + b B c C + d D A, B, C and D represent chemical species and a, b, c , and d are their stoichiometric coefficients in the balanced chemical equation. At equilibrium, The square brackets indicate the concentrations of the species in equilibrium. K is a constant called the equilibrium constant. K depends only on T , and not on concentrations. [] [ ] cd ab CD K A B = Note: The “units” for K are concentration units raised to some power = c+d–(a+b)
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Reaching Equilibrium on the Macroscopic and Molecular Level N 2 O 4 (g) 2 NO 2 (g) Colorless Brown K = [NO 2 ] 2 / [N 2 O 4 ] Units are mol/L.
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Equilibrium from Different Starting Points CO( g ) + 2 H 2 ( g ) CH 3 OH( g ) [ ] [] [ ] 3 2 2 has same value at equil. for all 3 starting points CH OH K CO H =
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N 2 ( g ) + 3 H 2 ( g ) 2 NH 3 ( g ) Fe K = [NH 3 ] 2 [N 2 ][H 2 ] 3
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Key Stages in the Haber Synthesis of Ammonia Gerhard Ertl, 2007 Nobel Prize in Chemistry
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Small K N 2 (g) + O 2 (g) 2 NO (g) K = 1 x 10 –30 Essentially only reactants at eqbm. (10 15 x products) Intermediate K 2 BrCl (g) Br 2 (g) + Cl 2 (g) K = 5 Comparable amounts of products and reactants at eqbm.
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C142A Ch 6 2011 lecture notes - Chapter 6: Chemical...

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