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blogfast25
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Quote: Originally posted by DraconicAcid  |
No, because metal ions in solution are hydrated. A generic metal(II) ion in water is better represented as [M(H2O)6]2+.
[M(H2O)6]2+ + 2 NH3 -> M(OH)2 + 2 NH4(+) + 4 H2O
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The solvated metal cation as a weak acid reacting with the weak base NH3? Interesting. I've always seen it more as a case of ligand
exchange (H2O < == > OH<sup>-</sup> . Evidence for your pathway?
[Edited on 6-5-2015 by blogfast25]
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gatosgr
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So if that's the case the reduction potential for Cu(NH3)4 for 1 molar solution and 0.1 M for ammonia
Cu2+ +4NH3-> Cu(NH3)4(2+)
stability constant of formation for metal complex Cu(NH3)4(2+)
β=[Cu(NH3)4]/[Cu2+][NH3]^4
http://www.vaxasoftware.com/doc_eduen/qui/kfcomplejos.pdf
β=1.1x10^13
Reduction of Cu
Cu2+ +2e -> Cu E0=0.34V
Nernst:
E=E0-RT/2Flog(αCu/(αCu2+)(ae2-)^2)<=>E=E0-RT/2Flog(1/(αCu2+))
[Cu2+]=β[NH3]^4/[Cu(NH3)4]
E=0.39+0.0592/2log([Cu(NH3)4]/(β[NH3]^4))=0.11V
Is that right?
[Edited on 6-5-2015 by gatosgr]
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DraconicAcid
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by DraconicAcid |
No, because metal ions in solution are hydrated. A generic metal(II) ion in water is better represented as [M(H2O)6]2+.
[M(H2O)6]2+ + 2 NH3 -> M(OH)2 + 2 NH4(+) + 4 H2O
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The solvated metal cation as a weak acid reacting with the weak base NH3? Interesting. I've always seen it more as a case of ligand
exchange (H2O < == > OH<sup>-</sup>. Evidence for your pathway?
[Edited on 6-5-2015 by blogfast25] |
Not off of the top of my head, but it is well-known that hydrated metal cations are acidic because the metal cation withdraws electron density from
the water ligands.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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gatosgr
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Some metal salts are acidic some are basic according to lewis theory... ammonia doesnt need water according to bronsted theory.. I was taught these
first year along with arhenius theory.. they're ok for beginners I guess.. according to lewis theory the amount of protonation depends on
electronegativity of the metal.
That chinese paper is weird it doesnt cancel the ratios... http://chem.xmu.edu.cn/teach/fxhx/fxhxwenxian/Biochemists%20...
https://ch301.cm.utexas.edu/help/ch302/ab/fracspecies.pdf
The fraction of ascorbic acid and dehydroascorbic isnt that the same as α=γ(c/c0)??
-0.05916/2log(fH2A/D) why is this here it's still dependent on H+ ... how is this solved ?
nobody knows?
I found the graph for ascorbic acid:
if you read in the literature ascorbic acid is used for iron and cobalt compounds as well..
So this is why you needed to boil the water for the AA to reduce Cu2+ ΔG=ΔH-ΤΔS.. now you can try with adjusting the ph, ALTHOUGH some metal
complexes have ph dependent E0 as well.
[Edited on 6-5-2015 by gatosgr]
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blogfast25
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| Quote: Originally posted by DraconicAcid | | Not off of the top of my head, but it is well-known that hydrated metal cations are acidic because the metal cation withdraws electron density from
the water ligands. |
I was also taught that some solvated cations behave like weak acids because of the repulsion exerted by the central electrical force on the protons:
[M(H<sub>2</sub>O)<sub>6</sub>]<sup>z+</sup> + H<sub>2</sub>O < === >
[M(H<sub>2</sub>O)<sub>5</sub>(OH)]<sup>(z-1)+</sup> + H<sub>3</sub>O<sup>+</sup>
Especially for z = 2, 3, ... and small ionic radius.
Does it have to be solved? You have an expression for E<sub>red</sub> = f(pH) anyway.
Can it be expressed only in concentrations and equilibrium constants? I think so but it’s not worth the effort, I think. It's likely to lead to a
polynomial [H<sup>+</sup>]<sup>3</sup> + etc. Not neat. That's why they keep it implicit, IMO.
[Edited on 6-5-2015 by blogfast25]
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