[vhpk]

Hi everyone,
I have some questions regarding electrochemistry, hope everyone will help me with this
1/
MnO₄^- + 8H⁺ + 5e --> Mn²⁺ + 4H₂O E = 1.51V
Fe³⁺ + e --> Fe²⁺ E = 0.77V
Suggest why permanganate isn't a suitable oxidizing agent for the quantitative estimation of Fe²⁺ in the presence of HCl but becomes so if sufficient Mn²⁺ and PO₄^3-are added into the solution( Hint: PO₄^3- forms a complex with Fe³⁺, thereby stabilizing it)
I have no idea for this question, since the reduction potential of the estimation reaction for Fe²⁺ is positive, i.e. spontaneous, based on the titration, we can find out the concentration of Fe²⁺.
2/ It's often found that O2 is a slow oxidizing agent. Suggest a mechanistic explanation that takes into account the two standard potentials:
O₂(g) + 4H⁺(aq) + 4e --> 2H₂O(l) E = 1.23V
O₂(g) + 2H⁺(aq) + 2e --> H₂O₂(aq) E = 0.7V
Is the question here asking us to find out the mechanism for two half reactions ? If so, I really don't know how to do
3/ Rate of reaction of reduction [Ru^IVO(bipy)₂(py)]²⁺ to [Ru^III(OH)(bipy)₂(py)]²⁺ is strongly influenced by the change of the solvent from H₂O to D₂O. What does this suggest about the mechanism of the reaction ?
Like question 2, I also have no idea for this question.
Any help would be highly appreciated.
Thank you so much

[Schrödinger]

Hi everyone,
I have some questions regarding electrochemistry, hope everyone will help me with this
1/
MnO₄^- + 8H⁺ + 5e --> Mn²⁺ + 4H₂O E = 1.51V
Fe³⁺ + e --> Fe²⁺ E = 0.77V
Suggest why permanganate isn't a suitable oxidizing agent for the quantitative estimation of Fe²⁺ in the presence of HCl but becomes so if sufficient Mn²⁺ and PO₄^3-are added into the solution( Hint: PO₄^3- forms a complex with Fe³⁺, thereby stabilizing it)
I have no idea for this question, since the reduction potential of the estimation reaction for Fe²⁺ is positive, i.e. spontaneous, based on the titration, we can find out the concentration of Fe²⁺.

Using HCl is not helpful, because Cl^- can be readily oxidized by MnO₄^-. This way, you will be reporting a greater titre value since MnO₄^- will be used up for both the oxidation of Cl^- and Fe²⁺.
But in the presence of PO₄^3-, Fe³⁺ is stabilized. This probably ensures that the oxidation of Fe²⁺ is preferred over the oxidation of Cl^-.

[vhpk]

Thank you for question 1

[Borek]

But in the presence of PO₄^3-, Fe³⁺ is stabilized. This probably ensures that the oxidation of Fe²⁺ is preferred over the oxidation of Cl^-.

I was taught that it is presence of Mn²⁺ that lowers oxidating potential of permanganate to the point where chlorides are not oxidized to chlorine, while PO₄^3- is there to ensure iron (III) is in the form of a colorles phosphate complex, not in the form of yellow chlorides complex. That's to make end point detection easier. But obviusly stronger complexing agent (PO₄^3- vs Cl^-) will make the potential at which oxidation of Fe²⁺ proceeds a little bit lower as well.

http://www.titrations.info/permanganate-titration

Also see http://en.wikipedia.org/wiki/Kinetic_isotope_effect