galvanic reaction :S (1 Viewer)

[shinigami66]

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]

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

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

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]

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]

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

 

[annabackwards]

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

No worries, glad i helped

rust refers to ferrous oxide, or iron oxide

Yupo, specifically Fe2O3 ^^