Electrochemistry - Reduction Potentials.

In this entry, I shall begin talking about standard reduction potentials. Hopefully, it will clear some doubts and instill some understanding.

In essence, this term actually is about a REDUCTION process that is occuring at STANDARD condition and we are trying to find out how FAVOURABLE the process will be.

But how did this come about? The following is a simple description to how standard reduction potentials are obtained.

Reduction Potential

Are you curious to why is the standard reduction potential in your data are all written with the reversible arrow? In simple terms (as compared to the actual explanation), the explanation is as follows:

Let's assume we observed that A is reduced to A^- in the atmosphere. But how do we determine that A indeed likes to be reduced when we compare it to another substance B?

The easiest way would be to put A into a solution of A^- and when an equilibrium between A and A^- is reached, and if there is more A^-, we would see that the remaining A being increasing positive since some of it has to lose electrons so that the other A particles can be reduced.

This steps is repeated to see if substance B likes to reduce and its the same for all the other substances.

But, how are we going to compare the ease to reduce between A and B? This can be answered by connecting an A/A^- half-cell to B/B^- half-cell and attached a voltmeter between them and allow both to reach equilibrium.

At equilibrium, the electrode of the half-cell which has a more positive charge will pull electrons from the other electrode, thus at the other half-cell would be made to prefer oxidation.

Therefore, the direction of electron flow will indicate to us which substance would favour reduction more and rendering (or forcing) the other to favour oxidation.

However, we can't measure the electron flow, we actually measure the potential difference. Which implies the cell that prefers reduction more than the other (at equilibrium) would have it electrode gain a positive charge.

While, the half-cell that prefers reduction less (than the other cell thus is made to allow oxidation to occur such that it can supply electrons to the other cell) will gain a negative charge hence, we have a measurable potential difference.

However, we are many different elements in the periodic table and obviously there will be an overwhelming number of comparisons to make and it will make such comparison tedious. Thus, all the elements are compared to the H⁺/H2 system. Essentially, their reduction is compared with respect to the reduction of H⁺. What we have done, is to arbituary set the reduction potential of H⁺ as zero.

When a substance ease of reduction is greater than H₊, its reduction potential will be positive. Converse is true for substances who prefers to remained in the oxidised form, thus metal/cation half-cell usually have a negative reduction potential (taken with respect to the H⁺/H₂).

Concentration and reduction potentials

In addition, if you recall Le Chatelier's principle, concentration changes causes equilibrium position to change and that will result in your reduction potential to differ from one concentration to another (How concentration will affect the value of reduction potential is beyond the scoop of your syllabus, you may refer to the Nernst equation to find out more). Hence, some further standardisation is required, therefore standard conditions is set.

Standard Condition

In electrochemistry, like in chemical energetics, refers to standard conditions as the following. Temperature of 298 K, pressure of 1 atm, concentration of any aqueous solution to be 1 M.

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Article written by Kwok YL 2007.

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