[johnnyjohn993123]

Fe³⁺ + Y^-4  FeY^-          K_f=1.3X10²⁵

Fe²⁺ + Y⁺⁴  FeY^2-         K_f=2.1x10¹⁴

Calculate the E° for the process
 FeY^-  + e^-    FeY^2-

 The answer is said to be 0.13 V.
 

My attempt since, the half reaction for the following is
 Fe³⁺ + e^-     Fe²⁺   E°= +0.77 V
Fe²⁺ + e^-     Fe_(s)   E°= - 0.44 V

E₁ = 0.77- (RT/Fn)ln 1/Kf

E₁ = 0.77 - 0.0257/(3)(ln 1/ 1.3X10²⁵)   I put three here cuz there's 3 electrons to form Fe(s)

E₁ = 1.27 V



E₂ = -0.44- (RT/Fn)ln 1/Kf

E₂ =-0.44- 0.0257/(2)(ln 1/2.1x10¹⁴)  put three here cuz there's 2 electrons to form Fe(s)

E₂ = -0.0162 V


If add or substract this answers I will still not get 0.13 V please help me DO I still have to use Nert's Equation ?

[mjc123]

With this mixture of Ks and E°s, I would advise converting everything first into ΔG°, then set up a Hess's law cycle to calculate ΔG° for FeY^- + e^-  FeY^2-.

I put three here cuz there's 3 electrons to form Fe(s)

Where does Fe(s) come into it at all? For equation 1, n = 0 because it is not a redox reaction. This sort of confusion is just why I suggest the procedure above.

[mjc123]

The other way to do it would be to imagine a cell containing 1M FeY^- in equilibrium with free Fe³⁺ and Y^4-, and 1M FeY^2- in equilibrium with free Fe²⁺ and Y^4-, and use the Nernst equation in the form
E = 0.77 + RT/nF ln([Fe³⁺]/[Fe²⁺])
This should (and does) give the same answer as the ΔG route. But caution: use the correct value of n, and be careful calculating the free ion concentrations - remember Y^4- is common to both equilibria.

[johnnyjohn993123]

I cant seem to arrange it in a hess's law .. can you help me out again please

[johnnyjohn993123]

The other way to do it would be to imagine a cell containing 1M FeY^- in equilibrium with free Fe³⁺ and Y^4-, and 1M FeY^2- in equilibrium with free Fe²⁺ and Y^4-, and use the Nernst equation in the form
E = 0.77 + RT/nF ln([Fe³⁺]/[Fe²⁺])
This should (and does) give the same answer as the ΔG route. But caution: use the correct value of n, and be careful calculating the free ion concentrations - remember Y^4- is common to both equilibria.

Why do you use E= E° "+ "RT/nF ln kf2/kf1   shouldnt this be in the subtraction form ?

[johnnyjohn993123]

The other way to do it would be to imagine a cell containing 1M FeY^- in equilibrium with free Fe³⁺ and Y^4-, and 1M FeY^2- in equilibrium with free Fe²⁺ and Y^4-, and use the Nernst equation in the form
E = 0.77 + RT/nF ln([Fe³⁺]/[Fe²⁺])
This should (and does) give the same answer as the ΔG route. But caution: use the correct value of n, and be careful calculating the free ion concentrations - remember Y^4- is common to both equilibria.

Can I also use -0.44V for Fe²⁺?? If so how ?

[mjc123]

I cant seem to arrange it in a hess's law .. can you help me out again please

You want ΔG° for FeY^- + e^-  FeY^2-
You have data (from which a ΔG° can be calculated) for the reactions
Fe³⁺ + Y^4-    FeY^- 
Fe²⁺ + Y^4-    FeY^2-   
Fe³⁺ + e-     Fe²⁺
Can you arrange these so that you get from FeY^- to FeY^2- by a path for which you know ΔG° for all the steps?

Why do you use E= E° "+ "RT/nF ln kf2/kf1   shouldnt this be in the subtraction form ?

I haven't used Kfs, I've used concentrations of Fe ions. Think of it in terms of Le Chatelier's principle - increasing the concentration of the oxidised form makes the couple more oxidising (more positive E).

Can I also use -0.44V for Fe2+??

No. This is E° for Fe²⁺/Fe. Elemental Fe is not involved here.