galvanic reaction :S

[shinigami66]

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Okay the question goes
Phenolphthalein indicator turns pink in the presence of OH- ions. Consider a simple galvanic cell in which the two electrodes are zinc and iron, immersed into a saltwater electrolyte containing phenolphthalein...
a) at which electrode will the electrolyte turn pink? (i said Fe and its correct)
b)write the half-equations occurring at the cathode and anode.
so this is where i don't get, the answers says:
Cathode : 2H2O + 2e -> 2OH- + H2
Anode : Zn -> Zn2+ + 2e

when i check the standard potential table, i realised that can't be true... because when you sum the two half equations together, you would achieve a negative value... and from my understanding, you can't have a spontaneous reaction with a negative value right?
i was thinking that the:
Cathode : O2 + 2H2O + 4e -> 4OH-
Anode being the same as b4
please correct me if im wrong... this topic is really confusing=[

[raniaaa]

sorry buddy can't answer your question, but maybe you can answer mine.. why did you say the Fe electrolyte would be turning pink?

[shinigami66]

because in that case, Zn is more reactive, so it will be oxidised preferentially compared to iron, and what is gaining the electrons from zinc is only water and oxygen, i dun think there are any iron ions in there as the question didnt say, so there are no iron ions to be reduced... so what i think is that the iron electrode is acting as a surface for the reduction to occur...
the reduction cell is O2 + 2H2O + 4e -> 4OH-, so more OH- are generated... then therefore, the colour changes to pink on the iron side~ if that makes sense hah

[raniaaa]

hmm but since zinc is being oxidised then doesn't that mean that the negative ions (i.e. OH-) would migrate to the anode, and the positive ions would migrate the cathode.... so shouldn't the zinc electrolyte be turning pink :S

[shinigami66]

im guessing the salt water must hv balanced out the charges b4 OH- migrates bak there~ but im not too certain...

[raniaaa]

can i ask where this question is from?

[shinigami66]

ma teacher's worksheet, dont noe where he got it from sorry~

[mrhorseham]

i think the question is wrong perhaps...maybe its an electrolytic cell

[annabackwards]

Originally Posted by mrhorseham

i think the question is wrong perhaps...maybe its an electrolytic cell

Yeah, otherwise the Iron would probably just rust in the salt water

EDIT: It's also in the shipwrecks topic, which is concerned with rusting an electrolytic cells.

[shinigami66]

but zinc would rust first right??
mabe the question is wrong... arghh at times like these, my teacher has to give me weird questions to confuse me! =[

[annabackwards]

Originally Posted by shinigami66

but zinc would rust first right??
mabe the question is wrong... arghh at times like these, my teacher has to give me weird questions to confuse me! =[

**Checks standard potentials**

Yupo, except Zn would "corrode" as only iron rusts

[shinigami66]

ahh damn, corrosion is different from rust ay? can you tell me what's the difference? hah ma bad

[annabackwards]

Originally Posted by shinigami66

ahh damn, corrosion is different from rust ay? can you tell me what's the difference? hah ma bad

Oxidation - any element that undergoes oxidation loses electrons (so their valency becomes more positive).

Corrosion - the oxidation of metals.

Rusting - the corrosion of iron.

See the difference?

[shinigami66]

ahh so rusting mainly occurs only for iron?? and hey, nice definition for corrosion, never thought of that =p thanks!

[4lettersdown]

rust refers to ferrous oxide, or iron oxide