Electrochemistry question PLEASE HELP!!!! (from AAMC FL 1)

[nostra_damus]

If somebody would please please please pretty please explain this to me, I would be so eternally grateful...

What I understand:

Zn(s) + HCl --> H2(g), which means that 1) H+ gets reduced and 2) the reaction is spontaneous. The question asks if a strip of Al(s) is placed in a beaker, will H2(g) evolve?

Then, we look at the table. We see that for Al(s) + Zn2+(aq), there is a solid formed. Which means that Zn2+ is getting reduced (to form the solid). This must also mean that Zn2+ has a higher reduction potential than Al(s). Does this conversely mean that Al(s) has a higher oxidation potential than Zn2+? And thus if you put the Al(s) into the same beaker with the HCl, it *must* reduce the H+ ions because we know that the oxidation potential is definitely higher than the Zn(s)?

Follow up questions:

If we look at the opposite reaction: Zn(s) + Al3+ (aq), nothing is formed. This means that Al3+ needs to be reduced, but it's not. Which means that Zn(s) has a higher reduction potential than Al3+ -- does this say anything about Al(s)???

I'm also a little confused why a metal with a higher oxidation potential will for sure make the HCl reaction spontaneous. There's some link I'm missing...

Also, does a higher oxidation potential mean a more negative E?

---

Members don't see this ad.

 

[NextStepTutor_3]

Hi there, good questions!

First, a quick note on the comparisons that you described. We shouldn't actually say "Zn2+ has a higher reduction potential than Al(s)," for example, because Zn2+ is a cation while Al(s) is a solid metal. The MCAT will never refer to the reduction potential of Al(s), because to reduce Al(s), we'd actually be making it into an anion. Instead, we'd want to say something like "Zn2+ has a higher reduction potential than Al2+" - that way, we're comparing two like species.

Back to the question. Here's how I would break it down:

Al(s) + Zn2+(aq) forms a solid, meaning that Zn2+ is reduced. Zn(s) + Al3+(aq) does not, meaning that Al3+ is not reduced. The conclusions we can draw from this are 1) that Zn2+ has a higher reduction potential than Al3+ and 2) that Al(s) has a higher oxidation potential than Zn(s). These are very similar to the conclusions you described, but note that I've made sure to compare two like things!

So since H2 gas evolves when in the presence of Zn(s), and Al(s) has an even higher oxidation potential than Zn(s), we know that H2 gas must evolve - or in other words, H+ must be reduced - when in the presence of Al(s).

With regard to your question about oxidation potential and E - it depends on which E value we are talking about. A spontaneous overall reaction will have a positive Ecell, while a nonspontaneous overall reaction will have a negative Ecell. Since you're only talking about an oxidation, however, you're referring to a half-reaction. A higher oxidation potential will have a more positive Eoxidation, but the reverse reaction will have a more negative Ereduction.

Hope you find this helpful! Good luck

 

[nostra_damus]

I was confused about the comparison of the two different things. Thank you so much!!!

 

[nostra_damus]

I am confused about the last part of your explanationt hough. A higher oxidation potentail will have a more positive Eoxidation, yes, but why does that mean the reverse reaction have a more negative Ereduction? I thought that positive numbers were those of higher reduction potential?

 

[aldol16]

Does this conversely mean that Al(s) has a higher oxidation potential than Zn2+?

Yes. Reduction potential and oxidation potential are mirror image opposites because redox reactions are usually reversible. Something that is reduced with a delta G of -40 kJ/mol will get oxidized with a delta G of +40 kJ/mol. Oxidation is simply the reverse of reduction.

And thus if you put the Al(s) into the same beaker with the HCl, it *must* reduce the H+ ions because we know that the oxidation potential is definitely higher than the Zn(s)?

Yes. This directly follows from the above. If Zn can reduce H+, then Al definitely can because it's stronger than Zn. Think of these problems like you would those brain teasers: 'if Mary is taller than John and John is taller than Sam, is Sam definitely shorter than Mary?'

If we look at the opposite reaction: Zn(s) + Al3+ (aq), nothing is formed. This means that Al3+ needs to be reduced, but it's not. Which means that Zn(s) has a higher reduction potential than Al3+ -- does this say anything about Al(s)???

No. Zn doesn't get reduced. You can't end up with Zn2-. Zn2+ is the species that gets reduced. So the redox pair Zn2+/Zn has a higher reduction potential than the Al3+/Al pair. This means that the Zn2+/Zn pair will favor the reduced form (Zn) more than the Al3+/Al pair.

Also, does a higher oxidation potential mean a more negative E?

Yes, it means a more negative E(reduction). The lower the reduction potential, the higher the oxidation potential.

I am confused about the last part of your explanationt hough. A higher oxidation potentail will have a more positive Eoxidation, yes, but why does that mean the reverse reaction have a more negative Ereduction? I thought that positive numbers were those of higher reduction potential?

Again, redox reactions are reversible. Take the Zn2+/Zn redox pair. The reduction potential of Zn2+/Zn is -0.762 V. This means that the oxidation potential of Zn/Zn2+ is +0.762 V. Now do you get why a more positive oxidation potential directly implies a more negative reduction potential?