CHAPTER 13 2009-Zumdahl - Pre13.1OverviewandReview a....

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Chapter 13 - CHEMICAL EQUILIBRIUM Pre 13.1 Overview and Review a. Introduction to Chemical Equilibrium i- Equilibrium is  the exact balancing of two opposing processes .   ii- It is a dynamic state, where  both forward and reverse processes continue  with      equal rates     iii-  At equilibrium the  concentrations of all species  in the system  remain  constant.    (may or may not be equal to  each other ) iv- All chemical reactions in a  closed system  are essentially reversible and will  eventually reach a state of equilibrium v- Many chemical processes in nature also occur in reversible manner and established  equilibrium. For example, the formation of stalactite and stalagmite in limestone caves  are the results of reversible reactions. When rain water saturated with dissolved CO 2   seeps through limestone hills (composed mainly of CaCO 3 ), the following reaction  occurs in which some of CaCO 3  (limestone) are dissolved. CaCO 3(s)  + H 2 O ( l )  + CO 2(aq)      Ca 2+ (aq)  + 2 HCO 3 - (aq) ..... (1) The rain water that sieve through the rocks becomes saturated with Ca 2+  and HCO 3 -  ions causing  the   reverse   reaction   to   occur,   depositing   CaCO 3   that   eventually   form   the   stalactites   and  stalagmites Ca 2+ (aq)  + 2HCO 3 - (aq)      CaCO 3(s)  + H 2 O ( l )  + CO 2(g) ..... (2) We may say that the formation of stalactite and stalagmite in limestone caves is the result of the  following reversible reactions: CaCO 3(s)  + H 2 O ( l )  + CO 2(aq)    Ca 2+ (aq)  + 2 HCO 3 - (aq) vi- However,  not all reactions reach equilibrium .  Some reactions go to       completion. The following classes reactions are examples of these types of reactions: 1. Acid-base reactions.(Reactions in which nonelectrolytes are formed) 2. Precipitation reactions. 3. Spontaneous oxidation-reduction reactions (electron transfer reactions).      4.  Reactions producing gases in an open system. 1
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13.1  The Equilibrium Conditions a. All chemical reactions are preceded by molecular collisions, but only those collisions      with proper orientation and sufficient energy to form the transition state complex  would subsequently lead to the formation of products. Consider the following  reversible reaction: Haber Process N 2( g )  + 3H 2( g )    2 NH 3( g ) which consists of a forward and a reverse processes: Forward reaction:  N 2( g )  + 3H 2( g )     2 NH 3( g )   (1) Reverse reaction:  2 NH 3( g )       N 2(g)  +  3H 2(g)     (2) 1. The rate of forward reaction (equation-1) depends on the frequency of collisions, which in turn  depends  on the concentrations of N
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This note was uploaded on 06/20/2011 for the course CHEM 111A taught by Professor Hockings during the Fall '08 term at Washington University in St. Louis.

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CHAPTER 13 2009-Zumdahl - Pre13.1OverviewandReview a....

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