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Standard potential and K

Posted: Thu Feb 04, 2016 10:31 am
by Rachael_1H
If the standard potential of cathode > standard potential of anode, why does the reaction favor products? And what does this have to do with K>1? From looking at a list of reactions, how would you know which have a value of K>1?

Re: Standard potential and K

Posted: Thu Feb 04, 2016 11:33 am
by Sabina Fridman 1D
K is the ratio at equilibrium of [products]/[reactants] so by seeing the ratios of the concentrations of the reaction's products and reactants you would know which equation has a K>1 (pretty much if there are higher concentrations of the products than reactants/ the reaction favors the forward direction.)

In terms of standard potentials, Ecell= Ecathode-Eanode. So if the cathode potential is greater than anode, the E of the cell would be a positive value, if E is a positive value the reaction is spontaneous, and favors the forward direction (favors products being made.)

Re: Standard potential and K

Posted: Thu Feb 04, 2016 11:53 am
by Noah Reid 4C
To get the Standard Potential of a cell, we subtract the Standard Potential of the cathode minus the Standard Potential of the anode. This will usually give us a positive Standard Potential of a cell, which we then plug into the equation Delta G = -nFE. If the Standard potential of the Cell is positive, it will give a negative value for Delta G signifying that the reaction is spontaneous. We get a positive value for E of the cell when we subtract, (E of Cathode - E of Anode), and when the E of Cathode > E of Anode. If this is not the case we will get a negative value which in turn will multiply with the negative sign in the equation Delta G = -nF, showing that the battery is not spontaneous, and therefore not a useful battery. The Nernst Equation is E = Standard E - (RT/nF)lnQ. At equilibrium Delta G= 0, E=0, and Q=K. Which changes the equation to
0 = Standard E - (RT/nF)lnK. Allowing us to solve for Standard Potential of the Cell. Now if K > 1 then the value of Standard Potential of the Cell will be positive, which you can then plug back into Delta G Standard = -nF(Standard Potential of the Cell). If K<1 then the Standard Potential of the Cell is negative. This shows that if K>1 the reaction will be spontaneous, and if K<1 it will not be spontaneous. You can then look at a list of reactions and given K determine whether the Standard Potential is positive or negative. Or given the Standard Potentials determine whether K >1 or K<1. Which will also tell you whether it is spontaneous or not.