electrolysis of water

[megan09]

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hi
i'm having a little trouble understanding the half equations for the electrolysis of water:
from the table of standard potenstials the two equations involving water are:

02 + 4H+ +4e- --> 2H2O (E0 = 1.23V)

2H2O + 2e- --> H2 + 2OH- (E0 = -0.83V)

for the half equations i was always told that the most positive (ie 1.23V) equation stayed the same and the other one reversed and changed signs....so this should then become:

02 + 4H+ +4e- --> 2H2O (E0 = 1.23V)

H2 + 2OH- --> 2H2O + 2e- (E0 = 0.83)

but every text book i own and all the websites i've searched have it the other way around....why?

any help would be appreciated

[brenton1987]

Originally Posted by megan09

hi
i'm having a little trouble understanding the half equations for the electrolysis of water:
from the table of standard potenstials the two equations involving water are:

02 + 4H+ +4e- --> 2H2O (E0 = 1.23V)
2H2O + 2e- --> H2 + 2OH- (E0 = -0.83V)

any help would be appreciated

This is an electrolytic reaction so energy needs to be input into the system. The reaction is not spontaneous so you need the most negative potential.

O₂ + 4 H⁺ + 4 e^- --> 2 H₂O (+1.23 V)
4 H₂O + 4 e^- --> 2 H₂ + 4 OH^- (-0.83 V)

2 H₂O --> O₂ + 4 H⁺ + 4 e^- (-1.23 V)
4 H₂O + 4 e^- --> 2 H₂ + 4 OH^- (-0.83 V)
---
2 H₂O --> O₂ + 2 H₂ (-0.40 V)

Originally Posted by megan09

for the half equations i was always told that the most positive (ie 1.23V) equation stayed the same and the other one reversed and changed signs....so this should then become:

but every text book i own and all the websites i've searched have it the other way around....why?

That would be for a Galvanic cell. They are spontaneous electrochemical reactions so youre looking for the most positive potential.

[megan09]

thanks, that actually makes sense now!

[mrs grasshopper]

does electrolysis always occur in an electrolytic cell?

[AndreRieu]

Originally Posted by mrs grasshopper

does electrolysis always occur in an electrolytic cell?

...pretty much the definition of an electroytic cell
otherwise its a galvanic cell with a spontaneous reaction

off topic but does anyone have english papers especially comphrension questions

[megan09]

Originally Posted by AndreRieu

off topic but does anyone have english papers especially comphrension questions

for belonging?
sorry no, i just hope they are good questions and texts, if not i know that most people doing the exam will feel the same way about it since everyone wouldn't have much practice on belonging section 1

[Divinity_]

Originally Posted by AndreRieu

...pretty much the definition of an electroytic cell
otherwise its a galvanic cell with a spontaneous reaction

off topic but does anyone have english papers especially comphrension questions

Try looking in the AOS forum.