4241LNOT04_S11_PP

4241LNOT04_S11_PP - Environmental Geochemistry GLY...

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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 1 4-1 4-2 4-3 4-4 LECTURE 4 - CHEMICAL EQUILIBRIUM Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Chemical Equilibrium, slide 1 here Whenever two substances are brought together the potential for a reaction between them exists; whether the reaction takes place depends on several factors. We may define a chemical reaction as one or more substances, known as reactants, combining chemically to form one or more different substances, known as products . Chemical Equilibrium, slide 2 here Examples: Given that a reaction might occur, a chemist is interested in two questions: Chemical Equilibrium, slide 3 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 2 4-5 4-6 A. How far can the reaction proceed? For example, the combination of a strong acid and a strong base (equation 4-2) will produce an almost complete reaction so that the amount of products will greatly outweigh the amount of reactants. On the other hand, a reaction like produces almost no Ag + or Cl - ions; that is, AgCl is insoluble. Most reactions lie somewhere between the extreme cases. The property in question here is thermodynamic, and given the proper data we can calculate the exact conditions that will exist when equilibrium is attained. Chemical Equilibrium, slide 4 here B. How fast does the reaction proceed? Some reactions are possible but may occur very slowly. A mixture of hydrogen and oxygen gases in the proper proportions will react explosively to form water vapor if a source of energy, such as a spark, initiates the reaction. If no such energy source exists, the reactants may coexist for years with very little product formation. The speed of a reaction is not a thermodynamic property but instead is a kinetic property. Chemical Equilibrium, slide 5 here LAW OF MASS ACTION In the nineteenth century, two Norwegian chemists, Cato Maximilian Guldberg & Peter Waage (1864), determined the key to the numerical handling
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 3 4-7 4-8 4-9 of chemical equilibrium. They discovered that when the driving forces of the forward and back reactions become equal, equilibrium has been achieved. Chemical Equilibrium, slide 6 here Forward reaction: Chemical Equilibrium, slide 7 here Back reaction: Driving force of the forward reaction = k f [NaCl] Driving force of the back reaction = k b [Na + ][Cl - ] Chemical Equilibrium, slide 8 here At equilibrium: k f [NaCl] = k b [Na + ][Cl - ] so the equation may be rewritten: K is called the equilibrium constant for the equation as written . This type of formulation is known as the " Law of Mass Action ." (The term concentration should really be substituted for mass).
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 4 4-10 Chemical Equilibrium, slide 9 here The Law of Mass Action may be described as, "The rate of a reaction is directly proportional to the concentration of each reacting substance." (Krauskopf, 1979, p.2).
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4241LNOT04_S11_PP - Environmental Geochemistry GLY...

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