Anode and Cathode

[scotties123]

every source i look at says something different. hopefully i wont get the same here lol.
this is for a galvanic cell
correct me if im wrong: Anode is the site of oxidation and electrons flow through the wire from anode to cathode, but ions flow through solution from cathode to anode

question: is the anode positive or negative?
examkrackers says the anode is -, but ive always learned its +
ive checked like 6 sources and they all contradict one another.
thanks.

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[emquiksilver]

The anode is always the site of oxidation and the cathode is always the site of reduction, regardless of the type of electrochemical cell. In a galvanic cell, the anode is - and the cathode is +. The charges are switched for an electrolytic cell.

 

[scotties123]

The anode is always the site of oxidation and the cathode is always the site of reduction, regardless of the type of electrochemical cell. In a galvanic cell, the anode is - and the cathode is +. The charges are switched for an electrolytic cell.

ok, but if a cathode attracts cations, wouldnt it have to be negative? or is this how it is only in electrolytic cells?

 

[dipak_ramkumar]

Okay,

This is always a source of confusion because of the two conventions. How you define the anode/cathode really depends on if you are a physicist or a biochemist.

Regardless of which field you belong to, remember that these statements are always true:

1. Reduction always occurs at the cathode
2. Oxidation always occurs at the anode
3. Cations always flow towards the cathode
4. Anions always flow towards the anode
5. Cells are cyclic therefore ions must flow to balance the charge difference caused by electron flow.

Physicists generally determine charge of the anode/cathode by considering the flow of charge through a wire. Thus, the anode and cathode charges are assigned according to the type of low that poles of the battery induce. Since electron flow always goes anode to cathode, anode of abttery caries a negative charge (repelling electrons) and cathode carries a positive charge (attracting electrons)

Biochemists on the other hand focus on the flow of ions through electrical fields. COnsequenctly, the charges of anodes and cathodes are assigned according to the type of ion that they attract. Anions migrate to the anode and hence anode is considered to be positive. Cations migrate to the cathode so cathode is considered to be negative.

A more uniform way to approach this would be to say that cathodes have a positive "core" and accumulate negative charges on the surface (hence accounting for both the biochemist/physicist accounts) and anodes have a negative "core" and accumulate positive charge on their surfaces.

Good Luck

 

[emquiksilver]

ok, but if a cathode attracts cations, wouldnt it have to be negative? or is this how it is only in electrolytic cells?

I think you're confusing the electrode with its solution. Let's say you have a galvanic cell with a copper cathode. Reduction is occurring at the cathode so that means Cu2+ + 2e- --> Cu. As a result the CuSO4 solution that the cathode is in is getting more positive because Cu2+ is being used up in reduction. That's why cations in the salt bridge is being attracted towards the cathode.

For further clarification, the anode is negative in a galvanic cell because that's where the electrons are coming from. However, in an electrolytic cell, the battery is calling the shots. So it's reversed to do what we want it to do.

I hope that helps (unless I'm wrong)

 

[scotties123]

I think you're confusing the electrode with its solution. Let's say you have a galvanic cell with a copper cathode. Reduction is occurring at the cathode so that means Cu2+ + 2e- --> Cu. As a result the CuSO4 solution that the cathode is in is getting more positive because Cu2+ is being used up in reduction. That's why cations in the salt bridge is being attracted towards the cathode.

For further clarification, the anode is negative in a galvanic cell because that's where the electrons are coming from. However, in an electrolytic cell, the battery is calling the shots. So it's reversed to do what we want it to do.

I hope that helps (unless I'm wrong)

thanks for trying to help, i still dont get how a cathode has cations attracted to it when its +.

 

[emquiksilver]

See if this diagram helps. From wiki:

 

[scotties123]

See if this diagram helps. From wiki:

so, the CuSO4 is dissociated in aqueous solution, and because reduction is occurring at the cathode, the + charged Cu2+ ions are "attracted" to the cathode in order to form Cu(s) and complete the reaction. and in order to balance out the transfer of electrons and maintain electrostatic equilibrium, the negative SO4 ions from the CuSO4 are going towards the anode's half cell to cancel out the Zn+ ions that are forming as a result of its oxidation.

wow, what a mouthful. i think thats right...? thanks again for all ur help. i never thought such a basic concept would confuse me when i can memorize equations left and right with no problem lol.

 

[corbis11]

yeah, i think your reasoning is correct. A lot of times you will see a salt bridge of KCl connecting two seperate containers and the K+ goes into the solution where the cathode is and the Cl- would go into the solution where the anode is to maintain electrical neutrality

 

[jade1013]

an ox and red cat help me remember it

 

[QofQuimica]

http://forums.studentdoctor.net/showpost.php?p=3777346&postcount=12