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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: BaO on July 25, 2006, 09:38:37 PM

Title: standard reduction potential
Post by: BaO on July 25, 2006, 09:38:37 PM
 Ethanol, C2H5OH, is oxidized by an acidified dichromate solution according to the equation shown. The E° for the reaction above is 2.98 V. The E° for the oxidation of ethanol is 

A  0.52 V
B  2.98 V
C  1.75 V
D  1.23 V
 
this is what i'm not sure about:
 dichromate is Cr+2,so its E0 value is -0.41? but with that value i cant find the right answer, because when i use this fomular: E0cell= E0red - E0ox, the answer is always negative.

thank you
Title: Re: standard reduction potential
Post by: Dan on July 26, 2006, 05:27:00 AM
Quote from: bao on July 25, 2006, 09:38:37 PM
Ethanol, C2H5OH, is oxidized by an acidified dichromate solution according to the equation shown.
what equation? you will need to use it.

Quote
this is what i'm not sure about: dichromate is Cr+2

No, Cr2O72- is dichromate ion.
Title: Re: standard reduction potential
Post by: sdekivit on July 26, 2006, 07:36:59 AM
it's a strange question since the standard electrode potential of ethanol should be negative otherwise you need electrolysis to let the reaction happening.

--> it should be -1,75 V

The equation for the E0 should actually be:

E0cell = E0Ox - E0Red

Title: Re: standard reduction potential
Post by: BaO on July 26, 2006, 04:15:34 PM
Quote
The equation for the E0 should actually be:

E0cell = E0Ox - E0Red


really? but my book shows different
Title: Re: standard reduction potential
Post by: BaO on July 26, 2006, 05:10:49 PM
Quote
--> it should be -1,75 V
you got -1.75. then, i just wonder how much E0(C2H5OH) is . in the chart , i only found this: Cr2O7-2 +14 H+ + 6e- ->2Cr3+ +7 H2O.....Eo=+1.23. is that what you got too ?
Title: Re: standard reduction potential
Post by: sdekivit on July 26, 2006, 05:40:23 PM
yes, but when E0 of ethanol as a reductor (it gets oxidized) is higher (here +/- 0,5 V), the reaction won't take place and a voltage is needed to let take place this reaction (the difference between the two standard electrode potentials).

in a table of standard electrode potentials, for a reaction to occur spontaneously, the standard electrode potential should be higher then that of the reductor. This can be shown with the laws of thermodynamics :)