MadSci Network: Chemistry
Query:

Re: why use H2O in calculating electrolytic cells and not electrochem cells?

Date: Tue Feb 22 17:30:48 2005
Posted By: Ves Childs, Staff, inventor, electrochemistry, 3M retired
Area of science: Chemistry
ID: 1106038645.Ch
Message:

I have restated the problem you posed using the lead-acid storage battery as an example:

  1. Why are we concerned about the electrochemical (oxidation and reduction) reactions of water when we are charging a storage battery?
  2. Why are we not concerned about the electrochemical (oxidation and reduction) reactions of water when we are discharging that battery?
In a fully discharged lead-acid battery the electrodes are lead metal coated with lead oxide and the electrolyte is sulfuric acid in water.

When a lead-acid storage battery is charged electrons are forced into a first electrode to convert (reduce) lead oxide to lead metal and at a second electrode electrons are removed to convert (oxidize) lead dioxide to lead dioxide.

To get the charging done in a reasonable tine you must use a terminal voltage that is higher than the reversible potential. The penalty you pay for this is that some of the current goes to make hydrogen and oxygen from electrolysis of water. [Not too many years ago enough water was consumed that it had to be replaced regularly. Modern technologies have led to sealed lead-acid batteries.]

When a lead-acid battery is discharged the reverse reactions, at the first electrode the oxidation of lead metal to lead oxide, and at the second electrode the reduction of lead dioxide to lead oxide, do not provide enough potential to discharge the water in that system.

[Note that in the charging case the first electrode was a cathode, and in the discharging case the same first electrode is an anode.]

These are very limited answers in that they only address the inefficiencies in the charging process. These answers do not recognize that water, or something very much like water, plays a vital role in transporting the oxide moieties from one electrode to another in the charging and discharging processes.

For the lead-acid battery case your last question: “Is it simply not likely that water will be [reacting] at [the] anode/cathode so we don’t check the values?” must be answered with a no. Water is clearly involved in both processes and an electrochemist would be concerned with it until convinced there was no reason to be so concerned.

I chose the lead-acid storage battery because it is very familiar and seemed to fit into the parameters of the problem you posed. I could have chosen examples from my own experience in industrial electrolysis, fuels cells, and analytical chemistry.

Even better, you might have offered some specific examples from your reading.


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