Redox Equilibrium Homework Solutions

[Big-Daddy]

Homework Statement

I.) Give an estimate for the stability constant of the complex [FeCl]²⁺, given that the standard redox potential for the Fe³⁺/Fe²⁺ system in 1 moldm^-3 HCl is 0.710 V.

The "standard electrode potential of the reaction Fe³⁺ + e^- -> Fe²⁺" (quote from the question) had previously been calculated as 0.772 V but I myself cannot see how this should be different just because 1 moldm^-3 HCl is present.

IV.) H3AsO4 and K4Fe(CN)6 are dissolved in water in a stoichiometric ratio. What will the ratio of [H3AsO4]/[H3AsO3] be at equilibrium if pH = 2.00 is maintained?

V.) Are the following equilibrium concentrations possible in an aqueous solution? If yes, calculate the pH of the solution. [H3AsO4]=[H3AsO3]=[I₃^-]=[I-]=0.100 M (where on Earth does the I₃^- come from?!)

Available data: [Fe(CN)6]3-/[Fe(CN)6]4- has E° = +0.356 V. H3AsO4/H3AsO3 has E° = +0.560 V. I2/2 I- has E° = +0.540 V.

Homework Equations

ΔG°(rxn) = -n(rxn) * F * E°(rxn) = - R * T * ln(K(rxn))

The Attempt at a Solution

My thinking was that we need the value of K for reaction H3ASO4 + 2H+ + 2e- -> H3AsO3 + H2O. This can easily be found from the value of the standard electrode potential for this reaction itself. Then take the inverse, before dividing by [H+]²=10^-4. I got 1.17*10^-15, but the correct answer is 0.107. For the next question, I soon as I saw I₃^- instead of I2 I was defeated.

 

[Adrmay]

Any help greatly appreciated!I.) The stability constant of the complex [FeCl]2+ can be estimated using the standard reduction potential of the Fe3+/Fe2+ reaction. The standard reduction potential for this reaction in 1 moldm-3 HCl is 0.710 V. The standard reduction potential for the reaction Fe3+ + e- -> Fe2+ is 0.772 V. Using this value, we can estimate the stability constant of the complex [FeCl]2+ as follows:K = exp(-ΔG°/RT) = exp(-nFΔE°/RT) = exp(-1 * 96485 * (0.772 - 0.710)/(8.314 * 298)) = 3.37 x 10^4