MCQ 19

[D27RII]

The reductant is Fe(s) not Fe+ ions

the anode is Fe(s)

no logical VCAA question would have an answer in which electrons flow from the reductant (iron) to the anode (also iron).

that's just ridiculous, even if mattys explanation makes any sense, that is not sound chemistry, this question is absolutely pathetic.

the only logical answer is D
the least illogical answer is D

HERE IS HOW U RECHARGE A FUEL CELL

first of all, it is possible to recharge fuel cells, even though it is unprofitable, and illogical to do so, yet it can still be done.

there is always some reactants in a fuel cell and some products. periodically reactants are fed in and products are taken out, but at any given time there is always hydrogen gas, oxygen gas and water in the fuel cell (since both hydrogen and oxygen are in the fuel cell, this could mean that all the reactants, as in all types, are in the fuel cell hence rendering option A correct - albeit through ridiculous wording).

Hence to recharge the fuel cell the terminals are simply switched, and water is fed in and the hydrogen gas and oxygen gas are fed out. (the opposite of what normally happens). water is constantly in contact with the electrodes SINCE THE ELECTROLYTE IS AQUEOS!!!, even if the electrolyte isn't aqueous common sense can be used to reason that with water continually leaving the cell and being continually produced, come water at all time will be in contact with the electrodes.

VCAA ASSESSOR REPORTS

now, we have heard a lot of ppl citing assessors' reports.

the assessor reports DO NOT state the fuel cells are unrechargeable.
they simple state that "fuell cells being non rechargeble is an ACCEPTABLE answer"
the key word there is acceptable, and the answer is acceptable because in common practice there is NO NEED to recharge a fuell cell, however in theory, practice and for the sake of learning first principles, fuels cells CAN be recharged.

[D27RII]

Which of the following is true for both fuel cells and rechargeable cells?

Okay first lets examine all the responses.

OPTION A

All reactants are stored within the cell.

this response has some truth to it. all reactants can be referring to the type of reactant, in which case this option is correct as all the reactants are present within all cells at any given time. even in a fuel cell, at any given time, there is always oxygen and hydrogen/whatever other fuel, within the cell. periodically oxygen and the fuel is replaced, but nevertheless at any particular time, all the reactants (types) are within the cell.

that being said, this is HORRIBLE wording, and on a normal day i would deem this response wrong.

OPTION B

Reaction products are continuously removed from the cell.

this is not applicable for rechargeable cells, hence wrong.

OPTION C

Electrons pass from the reductant to the anode as electricity is produced.

The most popular response, and the once chosen by Mao ( hence naturally it holds a lot of weight   ). The reason i believe this is wrong because one of the FUNDAMENTAL principles preached by VCAA is that electrons never EVER flow to the anode.

U can look into and analyze this option in as much depth as u want to, but it only warrants a shallow inspection to realize that it is incorrect. don't draw too much out of this option, this sort of question has been the bread and butter of galvanic/electrolysis questions for YEARS. the traditional response has always been electrons flow to the cathode and NEVER to the anode.

furthermore, the question states AS ELECTRICITY IS BEING PRODUCED, no electricity is produced when electrons flow from reductant to anode.... Electricity is only produced when electrons flow from anode to cathode across TWO DIFFERENT species which involve a reductant and an oxidant. hence this question is wrong, Even if mao can argue that electrons flow from reductant to anode, this would not effect any electricity to be produced. Electricity is produced when electrons flow from reductant/anode to the oxidant/cathode.

OPTION D

the one that seems least stupid, and still a lot stupid. before i explain why believe this is correct, i must admit, this is an absolutely horrific response in its own credit.


HERE IS HOW U RECHARGE A FUEL CELL

first of all, it is possible to recharge fuel cells, even though it is unprofitable, and illogical to do so, yet it can still be done. (but why would u need to?, i don't know)

there is always some reactants in a fuel cell and some products. periodically reactants are fed in and products are taken out, but at any given time there is always hydrogen gas, oxygen gas and water in the fuel cell (since both hydrogen and oxygen are in the fuel cell, this could mean that all the reactants, as in all types, are in the fuel cell hence rendering option A correct - albeit through ridiculous wording).

Hence to recharge the fuel cell the terminals are simply switched, and water is fed in and the hydrogen gas and oxygen gas are fed out. (the opposite of what normally happens). water is constantly in contact with the electrodes SINCE THE ELECTROLYTE IS AQUEOS!!!, even if the electrolyte isn't aqueous common sense can be used to reason that with water continually leaving the cell and being continually produced, some water at all time will be in contact with the electrodes.


this type of reaction is not at all far-fetched, instead it is a very common reaction that VCAA expects us to have an intimate knowledge about - electrolysis of water. this is basically what happens when fuel cells are recharged ---> very possible.


VCAA ASSESSOR REPORTS

now, we have heard a lot of ppl citing assessors' reports.

the assessor reports DO NOT state the fuel cells are unrechargeable.
they simple state that "fuell cells being non rechargeble is an ACCEPTABLE answer"
the key word there is acceptable, and the answer is acceptable because in common practice there is NO NEED to recharge a fuell cell, however in theory, practice and for the sake of learning first principles, fuels cells CAN be recharged.




considering all the options, A surprisingly seems the only one that has potential of being a straight out correct answer, but even then the wording is so pathetic that everyone should get the mark. B is just wrong and C is polar to the fundamental rule concerning electron flow. No one expects u to look at reductants and anodes in such great detail, normally, questions simply expect students to understand the relationship between cathode/oxidants and anode/reductants etc.

D sounds silly as it is impractical to recharge a fuel cell, but hey, it can be done. the catalyst electrodes can work both ways.


overall,

[Souljette_93]

I just want to wait till the assessors report and see what they would do.

[m@tty]

C is logical.

The reductant is the iron atoms participating in the reaction. The anode is comprised of the Fe atoms that are not. During the course of the reaction, electrons flow from the iron atoms participating (the reductant) to the anode. How is this not "sound chemistry" ?

Also, dude you're clutching at straws if you are saying that the chief assessor didn't really mean that fuel cells aren't rechargeable when he said it. xD


Okay then, by definitions:

Reductant = A reducing agent

Anode = The positively charged electrode by which the electrons leave a device.

The reductant is the thing that releases electrons. At an atomic level this is the Fe atoms participating in the reaction (is this what you dispute? Dude, it's simple logic xD ). Then, by definition, the electrons produced must leave via the anode - ie. the electrons must "pass from the reductant to the anode as electrons are produced".

Therefore C.

QED.

Moderator Note: I have copied a section of m@tty's post from here to include in this thread. - Mao

[D27RII]

umm it is not specifed what the reductant and anode are

[D27RII]

besdies, electrons flowing from reductant to anode does not produce electricity, option C states "As electricity is being produced" - hence implies that electrons need to flow from reductant to CATHODE, not from reductant to anode.

[D27RII]

glad to see no one has selected option B

the obvious traditional answer is that as electricity is being produced, electrons flow from REDUCTANT TO CATHODE<

hence electrons flowing from REDUCTANT TO ANODE - sounds like a trick question placed there to misguide some students.

[andy456]

glad to see no one has selected option B

the obvious traditional answer is that as electricity is being produced, electrons flow from REDUCTANT TO CATHODE<

hence electrons flowing from REDUCTANT TO ANODE - sounds like a trick question placed there to misguide some students.

so it was a trick placed by the examiners to teach the true knowledge of the students in order to seperate the brilliant from the average

[m@tty]

With A the key word is ALL, it implies that all the reactants that will participate in the reaction are stored in the cell from the beginning.

B is wrong.

With C, as electricity is produced the reductant is oxidised, chucks its electrons onto the anode which then pass over to the cathode and then eventually to the oxidant. This is true in BOTH fuel cells and rechargeable cells.

D is incorrect. Fuel cells are never recharged. In addition the examiner's report said that they can't be recharged. They will not contradict themselves. You're reading into it too much; they didn't say it was 'acceptable' to say that fuel cells can't be recharged because they were being gracious last year.. they said it because it is true.

Hence, C.

[D27RII]

the problem i see with that reasoning matty is too many words along the lines of "atomic level" "intermediate phase" etc,

it was supposed to be a straightforward redox question, in which instances, students are taught that electrons flow from reductant to CATHOdE as electricity is being produced. hence electrons flowing from reductant to ANODE (the opposite) sounds like a trick response.

im not sure if ppl are supposed to draw sooo much out of that question.

BUT, that being said im still glad to see ppl so adamantly arguing that their option is right and i love seeing ppl dwelve into sub-atomic quantum chemistry to prove their responses. its great. it means bloody VCAA will definitely have to give more than one correct answer for that pathetic question, im sure they will have a lot of arguments on their hands if they don't.

and btw cheers for that assumption question, i thought i got it wrong :S

[kenhung123]

I think people think C is wrong because they are assuming the electrons come from somewhere else (presumably the cathode) TO the anode.

[m@tty]

How does explaining the mechanism correctly alter the correctness of my answer? Just because something is typically omitted doesn't mean it isn't needed to answer questions!

It IS a trick question. It was meant to test the understanding of the mechanism of discharge/galvanic cell.

And how on earth do you think it straight forward to somehow deduce that fuel cells can really be recharged?? Even though in the last years exam paper and in many other places it is clearly stated that they cannot be recharged?

It is taught that the reductant is the chemical species which releases electrons, yes?

You understand that the reactions takes place ON THE SURFACE of the electrodes?

It is also taught that the electrons pass from the anode to the cathode, correct?

Then it is simply logical that the electrons must pass from the reductant (remember the electrons were in its possession) to the anode. This MUST happen just so that it can then flow to the cathode.

The critical point is realising the distinction between "anode" and "reductant". In the specific case of Q 19 one is the iron rod while the other is the reacting iron atoms which are releasing electrons.

And I see no possibility that this question will be altered. It will stand, with C being correct.

[m@tty]

No, they think C is wrong because the electrons *magically* appear at the anode. Certainly didn't come from the reductant, right?

[stonecold]

No, they think C is wrong because the electrons *magically* appear at anode. Certainly didn't come from the reductant, right?

gg!

I am not even going to enter into this bull shit argument anymore.

People who got this wrong, please get over it.  You got tricked or were not able to think outside the square of the rote learned definition.  Big deal.  It happens to the best of us.   I hate nothing more than when people can't accept that they are wrong or made a mistake.

And I just have one more thing to say...

VCAA ASSESSOR REPORTS

now, we have heard a lot of ppl citing assessors' reports.

the assessor reports DO NOT state the fuel cells are unrechargeable.
they simple state that "fuell cells being non rechargeble is an ACCEPTABLE answer"
the key word there is acceptable, and the answer is acceptable because in common practice there is NO NEED to recharge a fuell cell, however in theory, practice and for the sake of learning first principles, fuels cells CAN be recharged.

You would make a good politician.  That is all.

[Greggler]

why is this question even being argued?

Process of elimination got me to C. Didnt even think twice...