# 24 - Friday Lecture 24 Announcements 1 In case you were...

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Friday October 21, 2011 Lecture 24 Announcements 1. In case you were sick: make-up exam SUNDAY 10/23 from 1 - 3:30PM in Biotech rooms. Contact prof. The Wednesday lecture: 1. The concentration cell; 2. How concentrations of reactants and products influence equilibrium; 3. The influence of the electrical potential difference across a membrane on the movement of ions across that membrane; 4. Intro to redox reactions Today's lecture p. 179 For any chemical rxn we can write G = G o ' + RTln(products)/(reactants) , and this holds true for redox rxns. From this equation, you can see where the concentrations come into the G calculation for redox rxns. The special treatment of redox rxns is the relationship between G and the measured voltage: G = -n F E , where F is Faraday’s constant, and n is the number of electrons in the rxn as written. Redox reactions have the property that since voltages and charges are involved, we can directly measure E, hence G o ' for each half reaction. We show this with the following equation: G o ' = - {(n F E o ' ) rxn1 + (n F E o ' ) rxn2 } = -n F E o ' The first term, (n F E o ' ) rxn1 , corresponds to the half-reaction of the reduction , AS WRITTEN (with the value of E o ' read directly from the table of standard reduction potentials). The second term, (n F E o ' ) rxn2 , corresponds to the half-reaction of the oxidation AS WRITTEN (with the value of E o ' for the reverse rxn of that from the table of standard reduction potentials, because this table expresses all half rxns as reductions, but one half rxn MUST be an oxidation). The equation as written must have one half-reaction that is a reduction as written, and one half-reaction that is an oxidation as written, because electrons must leave one molecule, and enter another molecule.

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See pp 516 in the text. This is a clear treatment of the calculation for a redox reaction. Again we note the contrast between electron transfers in biochemistry and in electrical circuits. In biochemistry, electrons are initially located on the reduced molecule, and then transfer to the molecule that is (initially) more oxidized. But not all molecules are suitable for such reactions!!
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24 - Friday Lecture 24 Announcements 1 In case you were...

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