REDOX Questions! (Help!)

[manohman]

1) does low red potential always imply mean high ox potential? If you want something that oxidizes easily, its more right to say that you want somthing with a low ionization energy rather than just a low reduction potential (according to berkley review).

But the way i figured it was if it has a low red potential, then it should have a high oxidation potential. for example, if its reduction potential was -5 then its ox potential would be +5. However, red potentials can be positive so i guess this doesnt hold true?


2) So does excess voltage only cause side reactions when all the original more favorable redox products are formed? (once the metal has been completely reduced). And does this side reaction occur because the voltage can be applied to other species? (so in a pure NaCl molten solution for example this wouldnt occur- or basically in any non aequous solution?)

The question reads:
"What problem can occur when a voltage greater than 1.86 volts is applied to an aqeous metal halide solution over a long duration of time?" It electric potential for the metal halide reaction is like 1.3V (given) So here we have an excess voltage.

The answer key says that Electrolysis of water to H2 gas and O2 gas can occur once the metal is completely reduced.

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[mehc012]

1) does low red potential always imply mean high ox potential? If you want something that oxidizes easily, its more right to say that you want somthing with a low ionization energy rather than just a low reduction potential (according to berkley review).

But the way i figured it was if it has a low red potential, then it should have a high oxidation potential. for example, if its reduction potential was -5 then its ox potential would be +5. However, red potentials can be positive so i guess this doesnt hold true?


2) So does excess voltage only cause side reactions when all the original more favorable redox products are formed? (once the metal has been completely reduced). And does this side reaction occur because the voltage can be applied to other species? (so in a pure NaCl molten solution for example this wouldnt occur- or basically in any non aequous solution?)

The question reads:
"What problem can occur when a voltage greater than 1.86 volts is applied to an aqeous metal halide solution over a long duration of time?" It electric potential for the metal halide reaction is like 1.3V (given) So here we have an excess voltage.

The answer key says that Electrolysis of water to H2 gas and O2 gas can occur once the metal is completely reduced.

For 1)
The difference between a reaction's oxidation potential and reduction potential is sign. So in your example, something with a reduction potential of -5 would have an ox potential of +5, yes...and just continue this logic into your next example: something with a positive reduction potential simply has a negative ox potential.

As for 2), I don't know if it gave you any additional information (table of red potentials, etc), but it's just like any other reaction - side reactions can and do occur. However, it's all a question of relative reactivity. If you have highly reactive species in high concentration, that reaction will tend to occur predominately. When the concentration approaches zero, however, other reactions will dominate. In this instance, the electrolysis of water is less favorable than the metal halide reaction...but once the concentration dips, the metal halide reaction is less likely/favorable than water electrolysis.

Voltage can be applied to any species, but the more favorable reaction is more likely to occur. However, a reaction for which the reactants aren't present cannot occur, period. I can't think of any other way to respond to your final question. I'm not even sure if that's what you are asking, as it doesn't make sense to me, but hey, I tried.

 

[manohman]

For 1)
The difference between a reaction's oxidation potential and reduction potential is sign. So in your example, something with a reduction potential of -5 would have an ox potential of +5, yes...and just continue this logic into your next example: something with a positive reduction potential simply has a negative ox potential.

As for 2), I don't know if it gave you any additional information (table of red potentials, etc), but it's just like any other reaction - side reactions can and do occur. However, it's all a question of relative reactivity. If you have highly reactive species in high concentration, that reaction will tend to occur predominately. When the concentration approaches zero, however, other reactions will dominate. In this instance, the electrolysis of water is less favorable than the metal halide reaction...but once the concentration dips, the metal halide reaction is less likely/favorable than water electrolysis.

Voltage can be applied to any species, but the more favorable reaction is more likely to occur. However, a reaction for which the reactants aren't present cannot occur, period. I can't think of any other way to respond to your final question. I'm not even sure if that's what you are asking, as it doesn't make sense to me, but hey, I tried.

#1) So a good reducing agent would have a very low reducing potential then right? (very low reducing potential --> means its more negative --> means higher oxidation potential). And it would also have a low ionization energy, correct? So its easier to oxidize.

2) okay that clears it up a lot. Sorry I switched the order of the question (the bolded final question was the question from the book that spurred the side reaciton question).

So in essence, until the primary reaction(s) are exhausted, the side reactions wont occur with any appreciable frequency, but if the species exist they are occuring? Thanks man!

 

[mehc012]

#1) So a good reducing agent would have a very low reducing potential then right? (very low reducing potential --> means its more negative --> means higher oxidation potential). And it would also have a low ionization energy, correct? So its easier to oxidize.

Yes. It's usually clearer to say 'large negative' vs 'very low'. Your phrasing is technically correct, but lends itself to the sort of confusion seen in the first post. I haven't specifically looked into the ionization energy bit...it makes intuitive sense at first, but remember that ionization energy describes a very specific situation (removing an electron from the neutral atom in its gaseous state), so while its trends are useful and possibly relevant to this, they may not be exact, as there are a lot of ways to oxidize an atom, the atom is often participating in bonding, and may or may not start neutral, have a 'zero' oxidation number, etc.

2) okay that clears it up a lot. Sorry I switched the order of the question (the bolded final question was the question from the book that spurred the side reaciton question).
So in essence, until the primary reaction(s) are exhausted, the side reactions wont occur with any appreciable frequency, but if the species exist they are occuring? Thanks man!

That is true in this case, where one of the reactions is clearly more favorable than the other. If the two were of similar favorability, you could very well see 'side reactions' earlier.

Think of it just like any other reaction setup - the reactants have a certain amount of energy associated with them. For a normal, non electrochemical cell situation, that would probably be the internal energy associated with their temperature. The reactants sort of indiscriminately fling themselves around until they happen to collide the right way as to react and begin forming the product. The more energy there is, the more often they collide with things, and the more speed they collide with (colliding with high speed allows higher energy reactions to occur). If there are side reactions, the higher energy can make those more likely as well, both by increasing the frequency of collisions and by making the higher-energy side reactions possible. How common the side reactions are depends on the concentration of reactants for the side reaction, the temperature, etc...

It's the same with an electrochemical cell, only the energy associated with driving reactions is the voltage, and it is somewhat more discriminate as it acts on charged particles.

 

[Teleologist]

1) does low red potential always imply mean high ox potential? If you want something that oxidizes easily, its more right to say that you want somthing with a low ionization energy rather than just a low reduction potential (according to berkley review).

NO, because for one thing, you never specified the reducer. Something might have a low reduction potential with regard to a particular reducer, but that same something might have a great reduction potential with regard to another reducer.

Similarly, a basketball player may be a "crappy" player due to a lack of team chemistry. But once you put him on a team where everyone gets along, he's all of a sudden good again.

Voltage can be applied to any species, but the more favorable reaction is more likely to occur.

Favorable? Thermodynamically or kinetically favorable? Trying to generalize which reaction pathway will be taken is very, very hard. Not only do thermodynamically have to be considered but so do kinetics.

 

[mehc012]

NO, because for one thing, you never specified the reducer. Something might have a low reduction potential with regard to a particular reducer, but that same something might have a great reduction potential with regard to another reducer.

Similarly, a basketball player may be a "crappy" player due to a lack of team chemistry. But once you put him on a team where everyone gets along, he's all of a sudden good again.

OK, that's just not true. We're talking standard reduction potentials, you straight-up just reverse the sign to get standard oxidation potentials.

Favorable? Thermodynamically or kinetically favorable? Trying to generalize which reaction pathway will be taken is very, very hard. Not only do thermodynamically have to be considered but so do kinetics.

You're letting something sit with excess energy for a loooong period of time, this question was all about thermodynamics. Sorry I didn't specify, but it really wasn't relevant here. This was not a kinetic vs thermo product question. You are making this all more complicated than it needs to be. Heck, even my explanation which involved the word 'favorable' was more complex than needed, and it was purposefully vague as it was not intended to draw specific conclusions, but rather to show, generally, which circumstances increase the chances of side reactions. Excess energy will increase the chances of side reactions, as will a high concentration of side-reactants as compared to intended reactants.

I've seen you a lot on this forum and you tend to jump in with super snotty, condescending comments without actually providing useful explanations, so this will be the only response you get on this thread.

 

[Teleologist]

OK, that's just not true. We're talking standard reduction potentials, you straight-up just reverse the sign to get standard oxidation potentials.

Reversing the sign gets you the std. Ox. Potential for the conjugate oxidizer. Let's get the basics straight, please.