[1stplace]

Determine the ultimate fate of a small piece of vanadium metal placed in contact with a 1 M [H+] solution in the presence of 1 atm of Hydrogen gas at 25C.

I know that this can be solved with the frost diagram method but I have 2 concerns.....In the half reactions, there are 2 ways that the vanadium metal can go: to V(2+) and to V2O5 directly. So, I don't know how to graph that.

My second concern is that is there a quicker way of solving this?

THanks a lot

[Structuralist]

Take in care the electromotive force of the 2 possible cells, the first one with V(+2) and the second with V2O5 formed.
Which one is bigger?

[1stplace]

Right Right , Thank you very much

I have another question....The second part of the question asks for the most stable vanadium species in a pH=4 solution...I know that I can use the Nernst equation to find the potentials for all these reactions and calculate which is the most stable species but do I have to consider the effects of the pH on the 2H+ + 2e- => H2 equation? Since all these vanadium reactions proceed with 2H+ + 2e- => H2, I have to consider that too, right?

Ex. E3 turns into -0.136 V in a pH=4 solution and since the 2H+ + 2e- => H2's potential turns to -0.237 V, the overall reaction of E3 turns to be -0.136 + 0.237 = 0.101 V. Is this correct?

[Structuralist]

You have to use the Nernst equation because the concentration of the H+ will be 10^-4.
Do you think that the H+ concentration will influence the 2H+ + 2e- => H2 equation?
Use the Nernst equation that's enough in this case

[1stplace]

Okay , thanks a lot

Can you help me with the last part of this question? (there are many parts)

Determine the exact pH range, under which a 1 molar solution of VO2(+) would be stable if all other conditions are standard. Hint: the range is from acidic to slightly basic. Show your work.

For this, I am not sure how to approach this problem. Can you help me?

Thanks a lot

[Structuralist]

in this case you should use the Nernst's equation were you have to find two limit concentrations of H+ that make up the conditions needed
After that you will calculate the pH for the two H+ values, therefore you will find the pH range.

[1stplace]

Um....I don't really know how to set that up, can you give me an example?

Thanks a lot

[Structuralist]

As you can see there are several outcomes when you put V in a cell.
The cell will have specific emf for each reaction.
Calculate the range of the emf when VO2+ will be most stable. After that consider that the concentration of H+ is not constant using the Nernst's equation calculate the pH range in which you will have the conditions needed.