August 06, 2020, 03:39:23 AM
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Topic: Silver, copper, zinc ion electrolysis; could there be a mistake in my book?  (Read 236 times)

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Offline smoky

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https://imgur.com/wNzyAyL

The matter at hand concerns a solution in an electrolytic cell of Ag+, Cu2+ and Zn2+. Standard reduction potentials to Ag, Cu and Zn are listed as 0.80, 0.34 and -0.76 respectively.

Next is it noted that when a voltage gradually increasing from zero is applied in the direction contrary to the galvanic cell potential, Ag would plate out first, then Cu, and only then Zn.

Later in the book it is said that for electrolysis, we should take the additive inverse of the standard reduction potentials to arrive at the standard oxidation potentials, of which the most positive one is then most likely to occur. However, I presume that they mean the one with the most positive oxidation potential is the one most likely to occur as a reduction (under electrolytic conditions)? Because if the one with the most positive oxidation potential oxidates, that's the same as the one with the least positive reduction potential oxidating (since the two are each other's additive inverse), which is the same as what happens in a regular galvanic cell. During electrolysis, the opposite should happen?

Since the reduction potential of Ag+ to Ag is highest, silver should plate spontaneously, and therefore when a current is applied in the opposite direction, silver should be the last to be plated... at least that's the way it seems to me right now. As a matter of fact, I'm having trouble understanding how any silver would plate at all, since during electrolysis silver should become the electron donor and Ag+ concentration would therefore only increase?

I'm not sure what's going on here, it's my first introduction to the matter... Any help, please?
« Last Edit: February 18, 2020, 12:53:33 AM by smoky »

Offline Borek

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Definitely silver will plate out first.
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Offline Enthalpy

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Hi Smoky,

if you try to think like "the more negative is the least positive but it's smaller because its magnitude is bigger, but here it's a reduction so it's reversed, at the cathode that is sometimes negative because it depends on the current direction which is opposed to the usual generator convention because it's a power receptor, and electrons go the inverse direction to current", your chances of making it right are 50.001%.

Only software can work like that, not humans - and I know several pieces of software bugged exactly there. No to forget that conventions can vary over time and place.

Silver is the most difficult to oxidise, so it will reduce first. Add some sort of sign mock reasoning if you have to, but find the correct result based on that knwoledge, not on successive signs. If you happen to have made an odd number of mistakes, add one more.

The same holds for data tables. Check some known examples like lithium and gold, and read your desired data like "same sign as gold" and "same sign as zinc", not as one sign among a ****load.

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