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For a spontaneous reaction, you usually have to look for values of delta H, delta S and T and results in a negative delta G value. With the Gibbs free energy equation, one of the factors for a reaction to be spontaneous is having a negative delta G.
As was said previously, in a spontaneous reaction, delta G is negative as free energy is released in the reaction. Also, DeltaG = -nFE, which means that for delta G to be negative, E must be positive. The standard potential is relative to electron-pulling power of the cathode, which means that if the cathode has a high positive potential, it call pull the electrons more easily, allowing the transfer of electrons to occur.
For all spontaneous reactions delta G is negative. Therefore, using the Delta G = -nFE equation, we find that E has to be positive for Delta G to be negative. This is because F (Faraday's Constant) is a positive number and n (moles of electrons transferred during the reaction) will also be positive. So, since there is a negative sign in front of the equation, E must be positive in order for Delta G to be negative and thus also for the reaction to be spontaneous.
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