ilwdx - 16-5-2004 at 02:11
for the reaction:
Cr2O7 2-, H+, e ---> Cr3+, H20
Why the [H+] is in the Nernst Equation?
It is neither OX nor RED.
thx~~!
vulture - 16-5-2004 at 07:55
Somethings wrong here. The H+ shouldn't be in the nernst equation.
There's no reductor in your equation, which should normally be added to the nernst equation.
unionised - 16-5-2004 at 12:56
I never did like the Nernst equation (though I still think the guy deserves a capital letter) but I think I know why the concentration of H+ is in
there.
It's there because H+ is involved in the reaction. If there were no H+ in the mixture (OK, I know it's virtually impossible) then the
reaction wouldn't happen and no elctrons could flow. The more H+, the better the reaction goes and that will affect the voltage.
BTW, I think the reductor is Cr(III)
[Edited on 16-5-2004 by unionised]
vulture - 16-5-2004 at 13:03
Cr3+ can't be the reductor, but it can be the reduced species.
I think this is a half reaction.
unionised - 17-5-2004 at 14:56
Doh!
I'm so used to seeing the Nernst equation in terms of half reactions...
t_Pyro - 19-5-2004 at 01:55
This is the reduction half- cell reaction which takes place when a substance is oxidised by acidified dichromate. Strictly speaking, the reduction
half-cell reaction for the Nernst equation should be:
Cr<sup>6+</sup> + 3e<sup>-</sup> -> Cr<sup>3+</sup> E<sup>o</sup>
Therefore, E=2E<sup>o</sup> -
RT/(6F)*ln([Cr<sup>3+</sup>]<sup>2</sup>/[Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup>])
The other half-cell could be something like:
Fe<sup>2+</sup> -> Fe<sup>3+</sup> + e<sup>-</sup>
Sorry~ , I didn't speak clearly
ilwdx - 22-5-2004 at 02:35
What I actually want to ask is that when explain "why the Ox ablity of Cr6+ increases while the concentration of H+ increases in a
reaction?" we use the nerst equation to show that when H+ increases, the E increases tremendously.
But [H+] is neither a Ox nor a Red in the reaction, how can it be included in the nernst equation(isn't that the ln() part of nernst eaquation
only includes the Ox and Red?)?
t_Pyro - 22-5-2004 at 07:18
I think what the question meant was that the lower oxidation state to which the oxidising agent is converted, is different at different Ph values.
I'm not sure about the values for the dichromate, but for permanganate ion, Mn<sup>7+</sup> is converted to
Mn<sup>6+</sup> in alkaline solution, Mn<sup>4+</sup> in neutral solution, and Mn<sup>2+</sup> in acidic solution.
Hence, the oxidising power is greatest in acidic solution, as the change in oxidation state is the greatest in acidic medium.
vulture - 22-5-2004 at 07:40
H+ is necessary to "transport" the charges through the solution, it's the wire so to speak.