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Offline Big-Daddy

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Latimer calculation
« on: February 05, 2014, 04:12:01 PM »
How far will VO2+ be reduced by Zn  :rarrow: Zn2+ + 2e- reaction (E°=-0.76 V for the reduction, E°=0.76 V for the oxidation), given that I have calculated E° for these reactions to be:

E°(VO2+ + 2H+ + e-  :rarrow: VO2+ + H2O) = 1.00 V
E°(VO2+ + 4H+ + 2e-  :rarrow: V3+ + 2H2O) = 0.68 V
E°(VO2+ + 4H+ + 3e-  :rarrow: V2+ + 2H2O) = 0.3683333 V
E°(VO2+ + 4H+ + 5e-  :rarrow: V + 2H2O) = -0.231 V

It is tempting to hypothesize that the last reaction with positive E° will be the last that occurs, i.e. it will be reduced to +2 oxidation state but no further - but this appears to have no dependence on E°(oxidation) of the reducing agent, which can't be right. So how do we work out?

Offline Big-Daddy

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Re: Latimer calculation
« Reply #1 on: February 06, 2014, 12:18:28 PM »
The original source of this problem is International Chemistry Olympiad 2008 Preparatory Problem 13, http://www.icho.hu/files/prep_problems_icho40.pdf , if someone wants to check that I've done E° calculations right.

I considered calculating ΔG° for the overall redox reaction, but I don't think it is fair to say that the most negative ΔG° will show how far VO2+ is reduced, because then we could just multiply one of the (less) negative (but still negative) reactions by some large number through all its stoichiometric coefficients and say "now this reaction has most negative ΔG°".

Offline Rutherford

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Re: Latimer calculation
« Reply #2 on: February 06, 2014, 12:27:14 PM »
Convert the voltages to Gibbs' energy and calculate the Gibbs' energy of the overall reactions for each case and see whether the reaction is spontaneous.

Offline Big-Daddy

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Re: Latimer calculation
« Reply #3 on: February 06, 2014, 12:37:44 PM »
Convert the voltages to Gibbs' energy and calculate the Gibbs' energy of the overall reactions for each case and see whether the reaction is spontaneous.

Well we can't use ΔG since we don't have enough information about concentrations, but we can use ΔG°. My results are


ΔG°(2VO2+ + 4H+ + Zn :rarrow: 2VO2+ + 2H2O + Zn2+) = -339.6 kJ/mol
ΔG°(VO2+ + 4H+ + Zn :rarrow: V3+ + 2H2O + Zn2+) = -277.9 kJ/mol
ΔG°(2VO2+ + 8H+ + 3Zn :rarrow: 2V2+ + 4H2O + 3Zn2+) = -653.2 kJ/mol
ΔG°(2VO2+ + 8H+ + 5Zn :rarrow: 2V + 4H2O + 5Zn2+) = -510.4 kJ/mol

So you'd think then that VO2+ is reduced all the way to V, since every reaction on the line has (strongly) negative ΔG°. But this isn't the correct answer... instead it stops at V2+. Most negative ΔG° argument, refer to my post above?

Offline Rutherford

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Re: Latimer calculation
« Reply #4 on: February 06, 2014, 12:48:02 PM »
No, calculate the Gibbs' energy for zinc oxidation and then calculate the Gibbs energy for the overall reaction in each of the four cases (just as using Hess law).

Offline Big-Daddy

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Re: Latimer calculation
« Reply #5 on: February 06, 2014, 01:04:53 PM »
No, calculate the Gibbs' energy for zinc oxidation and then calculate the Gibbs energy for the overall reaction in each of the four cases (just as using Hess law).

Didn't I just calculate the (standard) Gibbs' energy for the overall reaction in each of the four cases?

Offline Rutherford

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Re: Latimer calculation
« Reply #6 on: February 06, 2014, 01:56:27 PM »
Oops. Sorry, didn't see. Most negative ΔG° argument is correct, you can prove it. Calculate the Gibbs energy for V2+ to V reduction. It should be positive, so as the V2+ production has the most negative Gibbs energy, and V2+ isn't reduced further to V, it can be concluded that VO2+ will be mostly reduced to V2+.

Offline Big-Daddy

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Re: Latimer calculation
« Reply #7 on: February 06, 2014, 02:24:06 PM »
Oops. Sorry, didn't see. Most negative ΔG° argument is correct, you can prove it. Calculate the Gibbs energy for V2+ to V reduction. It should be positive, so as the V2+ production has the most negative Gibbs energy, and V2+ isn't reduced further to V, it can be concluded that VO2+ will be mostly reduced to V2+.

I see the logic. One point - if we consider VO2+ to V3+ reduction, do we have to use ΔG°(2VO2+ + 8H+ + 2Zn :rarrow: 2V3+ + 4H2O + 2Zn2+) = -277.9*2 = -555.8 kJ/mol to consider it fairly alongside the others? So if, e.g. the ΔG° for the reduction by Zn to V2+ was -520 kJ/mol rather than -653 kJ/mol, then we'd get reduction to V3+ and no further?

Offline Rutherford

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Re: Latimer calculation
« Reply #8 on: February 06, 2014, 02:52:39 PM »
Yes.

Offline Big-Daddy

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Re: Latimer calculation
« Reply #9 on: February 06, 2014, 03:12:22 PM »
Ok. Now let's say we make it more complicated and use Cu metal instead of Zn. Cu can be oxidized to +1 or +2 oxidation states or not at all (http://hyperphysics.phy-astr.gsu.edu/hbase/tables/electpot.html). Would you still know how to solve it? (by solve it, I mean find the main reaction of the system)

I think mastering Latimer diagrams would be good for this year's IChO.

Offline Rutherford

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Re: Latimer calculation
« Reply #10 on: February 07, 2014, 08:14:40 AM »
Then you would have 8 reactions plus 4 more to compare the copper(I) oxidation to copper(II) in each case.

Offline Big-Daddy

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Re: Latimer calculation
« Reply #11 on: February 07, 2014, 08:47:51 AM »
Why plus 4 more? We are starting with Cu, so isn't it true that either Cu remains Cu (no reaction occurs, the VO2+ does not get to form a redox equilibrium at all), or it gets oxidized to Cu+ (by one of the 4 reductions), or it gets oxidized to Cu2+ (by one of the 4 reductions)?

So then, in this case, the most negative ΔG° approach is still going to be for the main reaction (which shows how far Cu gets oxidized and VO2+ gets reduced)?

Note: if Cu remains Cu, we could call this a null reaction with K=1 and ΔG°=0, meaning that if all the possible reactions in our system had ΔG°>0, the null reaction would 'occur' and nothing will happen.

Offline Rutherford

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Re: Latimer calculation
« Reply #12 on: February 07, 2014, 08:51:05 AM »
Maybe Cu+ is even easier oxidized to Cu2+ than Cu. I am not sure exactly if it is, but this should be checked.

Offline Big-Daddy

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Re: Latimer calculation
« Reply #13 on: February 07, 2014, 11:27:43 AM »
Maybe Cu+ is even easier oxidized to Cu2+ than Cu. I am not sure exactly if it is, but this should be checked.

From general knowledge I know that Cu+ tends to disproportionate to Cu2+ and Cu. But I would think that all these possibilities are included when you calculate ΔG° for all 8 reactions you mentioned before? I do not think "the main reaction" in this question could start with Cu+ because there is no Cu+ initially present. And if there was, it would just disproportionate.

So if we have a set of the reactions that can happen starting from the initial reactants present (here, seems to be 8 - starting from Cu and VO2+) are you sure most negative ΔG° is the right way to find the main reaction?

Offline Rutherford

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Re: Latimer calculation
« Reply #14 on: February 07, 2014, 11:41:04 AM »
I don't see why not. I didn't say that the main reaction would start with Cu(I). If Cu(I) disproportionates easily, then in the overall reaction Cu(II) would be produced even if you previously get the most negative Gibbs energy for an overall reaction with Cu(I) being produced (by checking the 8 reactions).


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