A Level Chemistry convention cell notation question

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itslibster
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#1
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Hi! I was given a question recently regarding an oxygen glucose fuel cell and the question requires writing the half equations for each reaction and then finishing the question with a conventional cell notation.

The half equations for the reactions are:

C6H12O6 + 6H2O > 6CO2 + (24H+) + (24e-)

(4H+) + O2 + (4e-) > 2H2O

The full equation is
C6H12O6 + 6O2 > 6H2O + 6CO2

The positive electrode (anode) is made of platinum. The negative electrode (cathode) is made of carbon/ graphite.

Draw a conventional cell diagram.

I’ve been given an answer of

C(s) | C6H12O6(aq) + H+(aq) | CO2 || O2(g) | H2O(l) | Pt(s)

I don’t understand why they are in this order though? I thought the way to draw a conventional cell diagram was

Anode | oxidised species || reduced species | Cathode

In this case, would it not be:

Pt(s) | C6H12O6(aq) || O2(g) | C(s)

?? I’m pretty confused so if anyone could help me understand this that’d be great!! Thank you
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TSRTD
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#2
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(Original post by itslibster)
Hi! I was given a question recently regarding an oxygen glucose fuel cell and the question requires writing the half equations for each reaction and then finishing the question with a conventional cell notation.

The half equations for the reactions are:

C6H12O6 + 6H2O > 6CO2 + (24H+) + (24e-)

(4H+) + O2 + (4e-) > 2H2O

The full equation is
C6H12O6 + 6O2 > 6H2O + 6CO2

The positive electrode (anode) is made of platinum. The negative electrode (cathode) is made of carbon/ graphite.

Draw a conventional cell diagram.

I’ve been given an answer of

C(s) | C6H12O6(aq) + H+(aq) | CO2 || O2(g) | H2O(l) | Pt(s)

I don’t understand why they are in this order though? I thought the way to draw a conventional cell diagram was

Anode | oxidised species || reduced species | Cathode

In this case, would it not be:

Pt(s) | C6H12O6(aq) || O2(g) | C(s)

?? I’m pretty confused so if anyone could help me understand this that’d be great!! Thank you
Firstly, the negative electrode is the one which undergoes an oxidation reaction. This means the cathode is the glucose half cell so that's the electrode that has the carbon. The positive electrode is where reduction takes place so the oxygen half cell is the anode.

By convention, the more positive electrode tends to go on the right hand side of the conventional cell representation, because when you calculate Ecell you do RHS - LHS, and this will come out as positive if you do it this way round.

You need to also show the CO2 and H+ in the glucose cell as and the H2O in the oxygen cell, as the other species are involved in the oxidation/reduction reactions (you could also include water in the glucose half cell but it isn't necessary). The species with the highest oxidation states go next to the salt bridge. So for the glucose electrode, the oxidation state of carbon in CO2 is higher than in glucose (you can tell this is the case since glucose is oxidised to carbon dioxide), so the CO2 goes next to the salt bridge. In the oxygen half cell, the oxidation state of oxygen is higher in O2(0) than in H2O (-2), so O2 goes next to the salt bridge.
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itslibster
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Firstly, thanks for taking the time to respond! question though, seeing as the negative electrode provides electrons to a species, doesn’t that mean that reduction occurs at the cathode? Sorry if I’m getting confused lol
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TSRTD
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(Original post by itslibster)
Firstly, thanks for taking the time to respond! question though, seeing as the negative electrode provides electrons to a species, doesn’t that mean that reduction occurs at the cathode? Sorry if I’m getting confused lol
The graphite electrode is negative because electrons are released there by the species in solution. Electrons are released when oxidation occurs, as oxidation is the loss of electrons. So here, the graphite electrode is the negative electrode because the electrons released make it negative. The platinum electrode is positive because electrons are accepted and removed by the species present.
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itslibster
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#5
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Woah! This makes everything slot right into place lol. I really struggled with electrolysis at gcse and even moving onto electrochemical cells at A2 A level baffled me but this has cleared up most of my confusion over the whole concept lol. Thank you!!
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rahatmansoor5
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#6
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there was a follow up question for this one saying what must be done to maintain the EMF of this fuel cell when in use? what about that one?
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