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CH 302(post - CHAPTER 6a Chemical Equilibrium 1 Basic Concepts Reversible reactions do not go to completion They can occur in either direction

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1 CHAPTER 6a Chemical Equilibrium 2 Basic Concepts ± Reversible reactions do not go to completion. z They can occur in either direction z Symbolically, this is represented as: ² The reactions proceed until the composition of the reaction mixture reaches a state of minimum free energy. () g g g g D d + C c B b + A a 3 Basic Concepts ± Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate. z It is a stage in a chemical reaction when there is no further tendency for the concentrations or the partial pressures of a reaction mixture composition to change.
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4 Basic Concepts ± Chemical equilibria are dynamic equilibria. z Molecules are continually reacting, even though the overall composition of the reaction mixture does not change. z The criteria that identify chemical equilibrium are: 1. The forward reaction and its reverse reaction are both taking place. 2. The forward and reverse reactions are occurring at equal rates – there is no net change. 5 Nitrogen dioxide shown immediately after expanding 2NO 2(g) Æ N 2 O 4(g) NO 2 colorless The color of the reaction mixture never become colorless. The reaction has established equilibrium. 6 Basic Concepts ± Any chemical reaction will establish equilibrium if it is carried out in a closed container. z If the reaction appears to have gone to completion, the equilibrium lies to the right . z This reaction is product favored. () ( ) forward reverse left right reactants products → ←
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7 Basic Concepts ± An example of a reaction that is product favored is the following. ± Under the proper conditions, hydrogen gas and oxygen gas react to form water vapor. ± After equilibrium is established, the amount of reactants remaining are negligible. z This reaction lies far to the right . z The reaction essentially proceeds to completion. 22 2 () forward gg reverse H OH O → + ← 8 Basic Concepts z If the reaction appears to occur only to a slight extent, the reaction lies to the left . z This reaction is reactant favored. ( ) forward reverse left right reactants products → ← 9 Basic Concepts ± An example of a reaction that is reactant favored is the following. ± In a closed container at 25.0 o C, the decomposition of calcium oxide to calcium and oxygen gas occurs. ± After equilibrium is established, the amount of products are negligible. z This reaction lies far to the left . z The reaction essentially does not occur. 2 forward ss g reverse CaO Ca O ++
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10 Characteristics of Chemical Equilibrium ± An example of chemical equilibrium is the reaction between water vapor and carbon monoxide in a closed container at a high temperature. ± If equal moles of water vapor and carbon monoxide are placed in the closed container, the initial concentration of reactants begins to decrease.
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This note was uploaded on 09/20/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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CH 302(post - CHAPTER 6a Chemical Equilibrium 1 Basic Concepts Reversible reactions do not go to completion They can occur in either direction

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