Wednesday, February 26, 2014

Aqueous reduction reactions


Next in a series of blog posts about one of my favorite subjects, electrochemistry.

The hydrogen evolution reaction is one of the most famous chemical reactions there is. On a water planet like ours, it is the most important reduction reaction.

So, if something is oxidized in water:
            M = M++ + 2e-

Then there are basically only four choices for the reduction reaction.
If oxygen is present, then the reduction reaction is always:
½ O2 +2e- = O-2
In acidic water, the reaction is 2H+ + ½ O2 + 2e- = H2O
In basic water, the reaction is H2O + 1/2O2 +2e- = 2OH-

Note that in acidic water, the reduction reaction of O2 neutralizes the acid.
While in basic water, the reduction of O2 makes it more basic.
Reduction of O2 to O- always moves the pH of the solution up.

But that’s only if oxygen is present.
When oxygen is absent H+ must be reduced in an aqueous system.
This happens at far lower potentials—it takes a lot more energy to make 2H+ take two electrons than  1/2 O2 to take two electrons. (in fact—you can think of oxygen as grabby when it comes to electrons. It’s why breathing feels so good.)

In an acidic environment: 2H+ + 2e- = H2(gas) which bubbles out.
In a basic (or neutral) environment: 2H2O + 2e- = H2 + 2OH

Note—again in both of these reactions, the water evolves to a higher pH—less acidic, more basic.

Questions to consider: Does this mean that the change in pH of the oceans over geologic time is a fundamental marker indicating whether the surface Earth is becoming more or less oxidized?
So: if oceans are increasing in acidity—that means that the Earth is reducing (so ocean rxn is to oxidize). Or if oceans are decreasing in acidity—that means that the earth is oxidizing with time?  

As you consider this question, remember that CO2 is also a big player in the ocean-climate story, adjusting the pH and water chemistry.

In an upcoming post, I examine the hydrogen evolution reactions in some more detail.

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