CH 302 1st Edition Lecture 25Outline of Last Lecture I. Oxidation and ReductionII. Electrochemical CellsIII. Electrochemical Cell PartsIV. Cell Short HandOutline of Current Lecture I. Standard Hydrogen ElectrodeII. Standard Reduction PotentialsIII. Standard Cell PotentialsIV. Voltaic CellsCurrent LectureStandard Hydrogen Electrode- For any half reaction, we can measure the potential (free energy) by comparing it to a standard reaction. The reaction we choose is the following.- Where the concentration of the H+ is 1M and the pressure of the H2 gas is 1 atm. Such an electrode is called a "standard hydrogen electrode" or SHE.- We can then make a cell with any other half reaction (under standard conditions where concentrations are 1M) and measure the potential (voltage).- We can then measure all such combinations and tabulate the data. - Note: for some reactions, we are measuring the oxidation and for others the reduction, but we will always tabulate the reduction potential for the half reaction.Standard Reduction Potentials- Having compared many reactions to the standard hydrogen potential, we can now make a table of reduction potentials for all half-reaction, (or oxidation potentials but we need to pick one and stick to it).- At the top of the table are the reactions that are "easiest" to oxidize and thus the hardest to run as reductions. - At the bottom are the easiest reductions, and thus the most difficult to run as oxidations.- When looking at the table, we need to be careful since everything is written as a reduction. - For example, from this table we can find the substance that is easiest to oxidize. That is from the top of the table. - But the substance that is being oxidized appears as a product in this table. Li(s) is theeasiest to oxidize. - The easiest to reduce is a reactant in the table. F2 gas is the easiest to reduce.Standard Cell Potentials- We can calculate the standard potential for any electrochemical cell from the standard potentials of the two half reactions.- For example, imagine we had the following cell.< Ni | Ni2+ || Fe3+ , Fe2+ | Pt >- We have at the anode side the reaction- VERY IMPORTANT. Note: the potential is simply the energy difference between the two half reactions. - Do not try to multiple the potentials by the number of electrons. - The number of electrons simply relates how many electrons there are per reaction. Howmany Fe3+ will be reduced per Ni atom that is oxidized. - The potential difference (free energy difference) between the two half-reactions is not dependent on the number of electrons.Voltaic Cells- An electrochemical cell in which the chemistry is spontaneous is called a voltaic cell. - This means that the oxidation will occur spontaneously at the anode and the reduction spontaneously at the cathode. - We should note that the notation of something as a voltaic cell is a choice. - Just as we choose what we want to call the "reactants" and "products" in a chemical reaction, our choice of anode and cathode is based upon what chemistry we would like to see occur. - For a voltaic cell our choice ends up being the spontaneous choice.- The standard potential for a voltaic cell is positive. - The standard free energy for a reaction ΔrG0 is related to the standard potential, E0, such that negative free energy (spontaneous) corresponds to positive potential. - The beauty of electrochemistry and electrochemical cells is that we can now directly measure the free energy difference by measuring electrical
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