[Yurij]

I have two problems:

#1: Calculate the potential of silver electrode submerged in a saturated solution of AgCl where Cl^- activity is exactly 11. E⁰ = 0,799V  K_sp = 1.82 e-10

How do I help myself with this data? I know in most cases activity is very similar to concentration but if I do
<Ag⁺> = K_sp/a_Cl where a_Cl = <Cl^-> and then insert what i get in Nernst equation:

E = E⁰ - 0.059log(1/<Ag⁺>), I get an incorrect answer: It should be 0.222V, but I get 0.162. What am I doing wrong?

#2: Calculate the cathode potential necessary for copper to (insert correct term here) excrete (i know, ridiculous but this is literal translation) from a solution that is 0.01M in Cu⁺² and 0.001M in EDTA (pH = 11)

In this case, I do not know how to start. How does presence of EDTA affect excretion of copper? Please help.

Any response is much appreciated

[Borek]

Are you sure about 11? Not 1.1?

Copper is complexed and its concentration goes down. I guess you are looking for a potential at which copper deposition starts.

[Yurij]

Thanks borek.

I guess there was a typo on the exercise sheet. using 1.1 gives the intended result. As for second problem, I see the light now =).

[Yurij]

Okay, I have another one, which I'm confused about.

Calculate the cathode potential necessary to completely reduce Co(II) from 0.1M solution of EDTA. (PH= 7   α= 5e-4)
K_f = 2,0 e+16. Starting concentration of Co(II) is 0.01M E⁰= -0.277V

I tried doing it like this. First Co²⁺ is complexed to yield 0.01M CoY^2-. This leaves us with 0.09 of Y^4-. From K_fcond = α x K_f we get the equilibrium ocncentration of Co(II). I insert it into Nernst Equation and get the potential which is again in conflict with what the solution should be (I get -0,600ish V, sorry I dont have my notebook with me, the solution should be +0,500ish V)

Where did I steer off the right track?

Actually what are the mechanics of such proceses? I'm guessing the Cobalt is complexed but some of it remains, which is then reduced and to keep equilibrium constant happy, some is uncomplexed and again reduced. My course notes are very unclear on what actually goes on in there =).

[Borek]

Calculate the cathode potential necessary to completely reduce Co(II)

Strange. There is no such thing as "completely reduce". You have to define some limit concentration at which you assume reduction was complete.

Actually what are the mechanics of such proceses? I'm guessing the Cobalt is complexed but some of it remains, which is then reduced and to keep equilibrium constant happy, some is uncomplexed and again reduced.

That's what is happening. What is being reduced is the "free" Co²⁺, which is always in an equilibrium with the complex and free ligands.

[Yurij]

Strange. There is no such thing as "completely reduce". You have to define some limit concentration at which you assume reduction was complete.

I believe professor once said whenever we see the word "completely," it means the concentration of the component in question should be 10^-6M at most

Thanks for helping me understand the process. Some problems are much more clear now =).

[Yurij]

Okay, another problem which I do not understand the mechanics of:

We are determining H2S in water using electrolyticalygenerated Iodine. Calculate C(H₂S) in 1 L of water if, if we added 3 grams of KI to 50ml aliquote of our sample. Current is controlled at 0.0073 Amperes and time of titration was 9.2 minutes.

The reaction that was given as help was
H₂S + I₂ ---------> 2H⁺ + S + 2I^-


First we caclulate the charge spent:

Q = i x T = 9.2min x 60 s/min x 0.0073A = 4.03As

This is the charge of 4.03/96500 = 4.18e-5 moles of e^-

What actually happens after KI is added? How does it affect the whole process? What should I take into account when solving such problems?

Thanks for every answer in advance

[Borek]

Hint: where does the iodine come from?

Please start new threads for new problems.

[Yurij]

Hmm ok I'll start new threads from now on

tell me if I got this right.

Oxidation of I- to I2 results in some charge, which in turn produces current. In order to keep that current constant, we use galvanostat which senses changes in current and subsequently changes its internal resistance (I wasnt really acing physics so tell me If I'm wrong). But anyway, we have our current.

Now the moles of e- from previous post are actually moles that were produced when iodine was oxidated so from that number and from moles of KI (n=m/M) we can calculate the moles of I2 produced right?

then we multiply that by 20 and divide by 1L to get our concentration.

I know my problems are really basic but I really want to be sure how things work

[Borek]

You got it reversed. We force the current through the cell to produce iodine. Doesn't matter what we do to keep the current constant, the only thing that matters is that we are able to calculate the charge, as this is what we need. Think Faraday's law.