Why do some metals dissolve in water under a EMF?

[Topher925]

I'm trying to wrap my mind around the concept of metals dissolving in water under different electrical potentials. For example, platinum is know to dissolve in liquid water when subjected to 0.65-1.1V. Why does this happen? I'm guessing it has something to due with water being so polar but why would a metal not dissolve with no potential but then dissolve when a potential is applied?

 

[Mech_Engineer]

Is the process similar in principle to http://en.wikipedia.org/wiki/Galvanic_corrosion" ?

 

[Borek]

If the applied potential is high enough it can strip electrons from the metal oxidizing it. You may think about it this way - high potential means that there is less electrons on the surface, that means metal atoms are easier to be detached as ions.

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[Topher925]

Is the process similar in principle to http://en.wikipedia.org/wiki/Galvanic_corrosion" ?

I don't think so. Corrosion implies an oxidization reaction, the phenomenon I am referring to is insoluble metals becoming soluble via a voltage potential.

If the applied potential is high enough it can strip electrons from the metal oxidizing it. You may think about it this way - high potential means that there is less electrons on the surface, that means metal atoms are easier to be detached as ions.

But when Pt is dissolved in water, there is no OR (not that I know of anyway). I don't really understand how a potential alone could cause electrons to be removed as that would imply current flow, would it not?

 

[Phrak]

I'm not sure this helps, but the subject you are eluding to is called Oxidation-Reduction potentials, or Redox potentials, etc.

Since the general question about how any metal will desolve ionically in water should answer your question, you might also look at "half cell potentials" to find an answer regarding a mechanism.

"But when Pt is dissolved in water, there is no OR (not that I know of anyway). I don't really understand how a potential alone could cause electrons to be removed as that would imply current flow, would it not?"

It does imply a current flow. "Half cell potentials" help explain, how batteries work--or at least quantify the voltage obtained.

 

[Borek]

I don't think so. Corrosion implies an oxidization reaction, the phenomenon I am referring to is insoluble metals becoming soluble via a voltage potential.

But when Pt is dissolved in water, there is no OR (not that I know of anyway).

To some extent there is no difference between both cases - here and here metal is being oxidized and there exist an associated potential. Just the "source of this potential" is different. Two basic possibilities here - reduction half reaction can be "source of potential" (of O₂ or some other oxidizing agent) or applied voltage can be "source of potential" (note: I am using quotes, as this is rather unusual use of terminology).

I don't really understand how a potential alone could cause electrons to be removed as that would imply current flow, would it not?

First of all - there is a current flow. During Pt oxidation electrons are produced:

Pt <-> Pt²⁺ + 2e^-

they have to flow somewhere for the reaction to occur.

Second, think about what happens on the macroscopical level when there is potential applied. Presence of potential difference means presence of electric field. In the electric field charges are shifted. If you put pice of metal in the electric field there will be a little bit less electrons on one end (and here metal is more easily oxidized - ions can be removed) and a little bit more electrons on the other end (and here metal is more easily reduced - any ion coming into contact will take excess electrons from the surface and will stick to it). In both cases there is a current flowing between metal and solution.

This is heaviliy simplified, but I find such pictures quite effective when trying to understand what is going on.

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[Topher925]

Thanks for the replies, I think I'm starting to understand. So a OR actually is occurring as the Pt atoms are losing electrons and becoming ions, due to the EMF. This then allows the Pt²⁺ to become soluble since it now has a positive charge. The electrons that are removed from the Pt would just flow into the solution (or whatever else is in contact) then, correct?

So why is it that there is only a range of voltage at which this can occur, like in this example 0.65v-1.1v? Is it because at least 0.65v is required to cause the OR of Pt and anything above 1.1v would form an oxide film (like Pt₃O₄, PtO₂)?