4241LNOT11_S11_PP

4241LNOT11_S11_PP - Environmental Geochemistry, GLY...

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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 1 11-1 11-2 LECTURE 11 - Redox Potentials and Equilibria Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Redox, slide 1 here INTRODUCTION Redox, slide 2 here The problem of equilibrium in geochemistry may be approached in several ways. Equilibrium may be treated by a free energy approach ( ) G = ) H - T ) S) or with equilibrium constants. A third method, which applies to many geochemical problems, uses oxidation and reduction reactions. These reactions are often written as half-cells , or two reactions one involving an oxidation step and another involving a reduction step. The half-cell reactions can be combined into a single reaction and, if the appropriate data is available, an equilibrium constant can be calculated. Redox, slide 3 here Oxidation occurs when an atom or ion gives up one or more electrons, so that its formal valance state increases. The symbol e - will be used to represent an electron. Reduction occurs when an atom or ion accepts electrons, so that its formal valance is reduced. These reactions can be combined by adding the equations and eliminating the
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 2 11-3 11-4 11-5 electrons: Redox, slide 4 here The video shows the processes of oxidation and reduction. These reactions may be quite complicated: Redox, slide 5 here Here, one reduction step (Cr 6+ 6 Cr 3+ ), one oxidation step (Fe 2+ 6 Fe 3+ ) and one step involving neither oxidation nor reduction (H + 6 H 2 O) exists. Redox, slide 6 here Measuring the absolute value of the potential for an ion to gain or lose an electron is impossible. Therefore, an arbitrary standard is chosen: Redox, slide 7 here This has an arbitrary standard potential (at standard conditions [H 2 ] = 1 atm. and 25°C) E° = 0.00 volts . The symbol E° specifically denotes standard potential, whereas E would denote a potential at other than standard conditions. Redox, slide 8 here Once a standard is chosen, all other half-cell reactions can be defined in terms of the standard. Example:
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 3 11-6 11-7 11-8 11-9 11-10 Thus, Choice of a plus or minus sign was originally arbitrary, and unfortunately both choices have been used. The literature is divided approximately fifty-fifty. The choice is not important, if it is used consistently. When reading the literature, knowing which convention the author is using is important. Krauskopf (1979) chooses to express any reaction that is more reducing than H (able to reduce H) as negative, and we will use that convention in this course. Redox, slide 9 here
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4241LNOT11_S11_PP - Environmental Geochemistry, GLY...

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