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الموضوع: Standard Reduction Potentials

  1. #1
    عضو ذهبي الصورة الرمزية ماجده
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    Aug 2011
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    افتراضي Standard Reduction Potentials

    As before, we wish to tabulate thermodynamic data in some useable way. We define a standard set of conditions under which all half-reactions can theoretically operate and we tabulate the half-cell potentials for the reactions under standard conditions. Since the potential of an oxidation reaction (loose electron) is merely the negative of the potential for a reduction reaction reaction (gain electron), we choose to tabulate only the reduction half-reactions. Any oxidation half reactions are merely the reverse of the tabulated reduction reaction and the oxidation potential is the negative of the tabulate reduction potential.
    Table D.4 from Petrucci lists many reduction half-reactions that you will need for problems, etc in the text. Herein, I have duplicated only a few in the following table.
    1 O2 + 4H+ + 4e ® 2H2O +1.229
    2 Ag+ + e ® Ag +0.7996
    3 Cu2+ + 2 e ® Cu +0.3419
    4 Fe2+ + 2 e ® Fe –0.447
    5 Zn2+ + 2 e ® Zn –0.7628
    6 2H2O + 2e ® H2 + 2OH –0.83
    7 Na+ + e ® Na –2.71
    A few years ago, a city experienced a number of very serious water main breaks on sections of pipe that had been recently replaced. The City engineer was fired. His only defence was "How was I to know that brass fittings (mostly copper) on cast iron pipes would corrode so fast." Was the city justified in firing him? Can you explain their reasoning for calling him incompetent?
    Let's look at this small reduction potential table and see if we can use it to explain certain experimentally observable properties of matter. First off, I should point out that the more positive reduction potentials refer to more spontaneous reduction reactions.
    The first reaction in this mini-table shows the reduction of oxygen. This is higher in the table than many other things. That makes sense since we know that oxygen is a good oxidizing agent (it makes other things oxidize while it itself is reduced).
    We can see that water can be both oxidized (-1) and reduced (2). It is oxidized to oxygen and reduced to hydrogen. These two reactions, in fact, serve as a set of boundaries for aqueous solutions. Any materials harder to oxidize than water will not oxidize since the water will. Similarly, any materials harder to reduce than water will never reduce (in water). For example, sodium ions will never reduce to sodium metal in the presence of water since the water will reduce first. In fact, sodium metal in water will spontaneously (explosively) oxidize to Na+ ions and the water will reduce to hydrogen gas and leave a basic solution (equation 6). The overall reaction for this process is the sum (#6) – 2×(#7).
    NOTE: when we add half-cells, their potentials are strictly additive. We don't multiply or divide by any factors even though we do so to the stoichiometric coefficients of the equations in order to balance the overall equation. Potentials don't depend on the total amount or size of the cell, just the concentrations. You already know this. If you buy a AA cell or a D cell, their voltages are both 1.5V. However, their capacity to do work is not the same.
    2Na + 2 H2O ® 2Na+ + H2 + 2OH
    Thus, the overall standard cell potential for this reaction is
    Eºcell = Eºred + Eºox = –0.83 + 2.71 (reverse reaction => change sign) = 1.88 V.
    Positive cell potential means spontaneous reaction.
    When pairing up metals, we know that zinc will dissolve in a copper (II) solution. Here we see that the copper (II) (3) is easier to reduce than the zinc (II) (5).

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  2. #2
    عضو ذهبي الصورة الرمزية م.عبد الرحمن
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    افتراضي رد: Standard Reduction Potentials

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  3. #3
    عضو مميز الصورة الرمزية عماد الكردي
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    افتراضي رد: Standard Reduction Potentials

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  4. #4
    عضو مميز الصورة الرمزية قمر بلحاج
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    افتراضي رد: Standard Reduction Potentials

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