Electrolytic cells (1 Viewer)

[Suvat]

Using the table of reduction potentials, how do you determine which substance is reduced at the cathode and oxidised at the anode? For example in most cases the dissolved cations and anions in the water are respectively reduced and oxidised but sometimes it is the water molecules which undergo reduction and/or oxidation (eg if the dissolve anion is sulfate). How do you determine which substance is electrolysed?
Also, what are the equations for the oxidation and reduction of the water molecule?

Thanks

 

[jims]

the most positive (electrowhatever) species will be preferenciated over the less positive species.

pretend u have a dilute solution of CuCl2

Anode:
the 2 species that can be oxidised are Cl(-) and H2O
from ur table:
2Cl(-) --> Cl2 (the gas) + 2e(-) -1.36V
2H2O --> O2 + 4H(+) + 4e(-) -1.23V
so the H2O will be oxidised in preference to the Cl(-)

Cathode:
for reduction you have Na(+) and H2O:
Cu(+2) + 2e(-) --> Cu +0.34V
2H2O + 2e(-) --> H2 +OH(-) -0.83V
so the Cu(+2) will be reduced

however, the concentration of your solution and your voltage may affect what is reduced/oxidised. so if you increased the concentration of this solution or the voltage, the Cl(-) may start to be oxidised along with the water.

all the equations you need are provided for you.

 

[Constip8edSkunk]

in an aqueous system

inert electrodes

at the anode, the oxidation of the salt ion must have E⁰ more positive than -1.23V for that reaction to take place, otherwise water will b oxidised. exceptions being SO4 and NO3 which are never oxidised doe to their high oxidation states, also if the oxidation potential is less than that of water, the ion may still oxidise at higher concentrations or pH (i think) levels. eg. Br will oxidise in preference 2 water, but Cl will only oxidise at medium - high concentrations

at the cathode, ions with reduction potentials >0.83V are never oxidised, those with positive ie. >0 potentials will be reduced in preference to water, those in between, eg iron, etc will reduce at a certain concentration. if not, then water is reduced.

reactive electrodes

at the anode, there is a possibility that the electrode will oxidise. to see if this will b the case check the potential 4 each reaction and the most positive will take preference. eg electrolysis of a solution containing Br ions with copper electrode will cause the copper 2 oxidise.

Molten systems

in molten systems, as tehre are no other substances to influence the reactions, the redox reaction at each electrode is much simpler, and the respective ions will be oxidised/reduced

jims has the eqns 4 water above

btw, r u tony?

 

[Suvat]

Thanks guys, still confused about some aspects but developed a simplistic method: whichever substance is closer to the centre of the standard reduction potential at either end is the substance that's electrolysed except sulfates and nitrates.

Yeah, it's Tony here, who may you be?

 

[iambored]

crap, i was just thinking, on the BOS table, the ones further up are oxidised and go on the anode right???? i hope so or i just stuffed a number of qs

 

[Constip8edSkunk]

john

...so much for my monopoly of plagarism off here, now theres like 40 ppl from our school here

 

[iambored]

Originally posted by Suvat
Thanks guys, still confused about some aspects but developed a simplistic method: whichever substance is closer to the centre of the standard reduction potential at either end is the substance that's electrolysed except sulfates and nitrates.

simplistic??

 

[Suvat]

Kinda simplistic...

Which John are you?

 

[iambored]

i'm john brown