electrochemical cells (1 Viewer)

[mazza_728]

im really confused about the anode and cathode and whether they are positive or negative in a voltaic cell and an electrolyitc cell (one where electricity needs to be put in for electrolysis)
can anyone explain this.. and why it is such?


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[abdooooo!!!]

A voltaic or galvanic cell is a device in which a chemical reaction generates electricity (converts chemical energy into electricity energy)
A cell in which electrolysis occurs is called an electrolytic cell; electrolysis is a process in which electrical energy is used to bring about chemical change (converts electrical energy into chemical energy)

Anode is where an oxidation occurs
Cathode is where a reduction occurs

The oxidation state of a species increases with its loss of electrons, thus the species is oxidised
The oxidation state decreases when electrons are gained; the species is reduced

Electrons travel from -ve to a +ve potentials; electrons are -vely charged and thus we can think of them as migrating from high concentration (-ve end) to an area of low concentration (+ve)

by using these facts we can say that voltaic cell will have its anode as -ve as electron is transferred from anode (an oxidation) to cathode (an reduction) as it would under normal chemical conditions without any external input energy.

but in electrolytic cell things are reversed. an external voltage (potential difference in charges) is applied to the electrodes, and hence the reaction is reversed ie the normal oxidation electrode now becomes the reduction electrode. thus anode (an oxidation) now is connected to the +ve end of the circuit.

so anode is -ve if its voltaic cell, while anode is +ve when its an electrolytic cell.

me can't think of specific examples... i can't remember any redox pairs, but if you just use the fact that anode is where oxidation occurs you will see why different cells have differing charges on its electrodes.

read the books if you still don't understand... if you like me diagrams are much easier to interpret than using words.

hope this helps... i can never explain properly... stupid hsc

 

[mazza_728]

In a voltaic cell the anode is -ve and the cathode is +ve
and in an electrolytic cell the anode is +ve and the cathode -ve
Right?

Does anyone else find this confusing?

 

[mazza_728]

I just read thru ur post again .. i understand a lil better -- i just have to remember anode = oxidation , cathode = reduction
Right?
Thankyou xoxo

 

[abdooooo!!!]

yep. thats all it is. if you can get a diagram of a cell it would be much easier to understand.

 

[Misturi]

I have a simple way that my teacher taught me

OIL RIG

Oxidation Is Loss, Reduction Is Gain.
You just simply have to remember that cat is +ve and an is -ve.

If a cat is your favourite animal...it might help

 

[Misturi]

Try these sites l found while l was looking around.

http://www.carleton.ca/~rburk/chem1000/on_line_tuts/redox/redox11.jpg

http://www.chem.uidaho.edu/~honors/redox.html

http://library.tedankara.k12.tr/chemistry/vol1/redox/trans85.htm

Hope they help

NB: I am not sure what the ON increases and the ON decreases are in the first picture.

 

[abdooooo!!!]

i am pretty certain they stand for Oxidation Number.

but im usually wrong with my guesses.

 

[Misturi]

Originally posted by abdooooo!!!
i am pretty certain they stand for Oxidation Number.

but im usually wrong with my guesses.

l've never heard of that before.

 

[abdooooo!!!]

my god... even rote learners know that. there is a syllabus dot point on it which you need to rote learn for the hsc.

do you use the syllabus for your study? i doubt that you do... i recommend you use it because its not whether you understand the theory or not... its whether you remember it and how well you can bullshit about it in the exams that gets you the top marks. if you're any good in english hsc science shouldn't be hard at all if you just try to assimilate a bunch of facts.

 

[Misturi]

Originally posted by abdooooo!!!
my god... even rote learners know that. there is a syllabus dot point on it which you need to rote learn for the hsc.

do you use the syllabus for your study? i doubt that you do... i recommend you use it because its not whether you understand the theory or not... its whether you remember it and how well you can bullshit about it in the exams that gets you the top marks. if you're any good in english hsc science shouldn't be hard at all if you just try to assimilate a bunch of facts.

l've finished chemistry..l got 51

 

[abdooooo!!!]

i know that... i was referring to your biology

why do i always seem to have some sort of communication brake down with you? its like we're comming from totally different worlds.

meh... good luck with your other subjects... hopefully it won't be as low as your chemistry mark.

 

[Misturi]

Originally posted by abdooooo!!!
i know that... i was referring to your biology

why do i always seem to have some sort of communication brake down with you? its like we're comming from totally different worlds.

meh... good luck with your other subjects... hopefully it won't be as low as your chemistry mark.

I have absolutly no idea..maybe its because we don't really know each other?

I use the syllabus for biology, but l've only finished the first module in prelim.

 

[mazza_728]

cat is +ve and an is -ve.

but doesnt this change in relation to the cells.
in a voltaic cell this is true but in a electrolytic cell cat is -ve and an is +ve??

Im so confused

 

[abdooooo!!!]

if you understood that anode is where oxidation occurs in any cell then i think whats confusing you is what a positive and a negative terminal is in voltmeter and a external voltage source.

think of it in this way if you have copper and silver as electrodes, in voltaic cell the anode will be copper while cathode will be silver because... a simple answer would be copper has more tendancy to undergo oxidation than silver. so therefore electron is transferred from copper to silver. since electrons migrate from -ve end to +ve end through the voltmeter, so the anode (copper electrode) will be -ve.

but in an electrolysis cell electron is pumped from a -ve end of a external power source to the copper electrode causing reduction to occur rather than oxidation, hence copper is now the cathode because it is reduced. while at the other end electron is being sucked out of silver by a +ve potential that is applied, so silver is now oxidised rather than reduced making it the anode. this is why the sign changes...

i can't explain very well... too hard for me.

 

[Xayma]

Ok Ill try to explain it then.

The ease at which metals will undergo oxidation in a particular electrolyte is determined by the galvanic series.

Thats why to prevent the material corroding naturally they are held in a solution of that metal's nitrate solution.

However, the metals still do corrode, the dissolving metal is replaced equally with the ions in the nitrate solution.

Lets take silver and copper for example.

Silver is less likely to corrode then copper.

When the copper dissolves instead of copper precipitating the electrons (since when the copper loses 2 electrons when it turns into copper ions to be dissolved) flow across the conductor (for school this would be the wire with the voltemeter) and into the silver, which prevents it from dissolving in the solution by keeping it from forming ions.

Hence silver will precipitate as normal however it wont dissolve as normal since it is prevented from forming ions.

However, the copper isnt precipitating as the ions dont get electrons and are kept in solution. The salt bridge is to allow negative ions (in this case nitrate ions) to flow across to keep both solutions electrically neutral.

Ill do the electrolytic cell soon.

 

[Xayma]

Ok for an electrolytic cell:

You use two inert conductors.

One end becomes a positive end and one becomes a negative end depending on which way electrons are flowing from the "electron pump".

What will happen is, the electrons flow to a terminal, this makes the terminal negatively charged, this makes the positively charged ions attracted to it, this terminal then gives up these extra electrons to the positive ion, creating a reduction reaction (hence the +ve terminal is the ANODE).

Meanwhile electrons are taken from the other terminal to fill the now lack of electrons in the electron pump. Hence this terminal becomes positively charged. This means the negative ions are attracted to it, these donate their extra electrons to this terminal creating neutral ions which aren't dissolved in a polar solution. Hence oxidation is occuring at the negative end and therefore it is the CATHODE.

The circuit remains neutral and the energy is lost through the loss of potential difference of the electrons.

NB: It wont always be the ions that are seperated from each other in this way. In the case of NaCl(aq) the water will end up decomposing (the Na is formed then immediately reacts with the water the product then reacts with chlorine then dissolves in the water again).

 

[braindrainedAsh]

An easy way to remember for galvanic cells

aNode (N for negative)
ca+hode (the t looks like a + sign for positive)

Note these are for GALVANIC cells only! Electrolytic is the opposite way around.

 

[abdooooo!!!]

im pretty sure mazza_728 already know that anode is -ve for galvanic cell. she just wanted to know why its reversed for electrolytic cell.

hey Xayma, i think why that people don't understand is because they don't know the difference between terminals in a voltmeter used in voltaic cell, and a external source ie bactery used in electrolysis cell.

the word anode is derived from an-ions (meaning negatively charged ions) which migrate to that electrode ie anode must be positively charged to attract the negatively charged ions. this is where the confusion lays... its not the actual electrode that changes charges, its which terminals they are connected to that define them being either the positive end or negative end.

edit: when i say its not the electrode that changes charges is ambigous, because if you reversed the voltage electrodes will react differently thus changing its charge. but i what mean is anode its self will always be positively charged and cathode will always be negatively charged no matter what type of the cell it is. its just the terminal end thats connected changes due to the difference in voltmeter and battery, and the fact that electrodes have swapped names ie anode to cathode.

 

[Xayma]

Originally posted by abdooooo!!!
when i say its not the electrode that changes charges is ambigous, because if you reversed the voltage electrodes will react differently thus changing its charge. but i what mean is anode its self will always be positively charged and cathode will always be negatively charged no matter what type of the cell it is. its just the terminal end thats connected changes due to the difference in voltmeter and battery, and the fact that electrodes have swapped names ie anode to cathode.

Anode is defined as the terminal at which oxidation occurs. Hence it doesnt matter what charge it is, as long as oxidation of the ions in the solution occurs at that terminal.

You also wouldn't use the same electrodes for galvanic cells and electrolytic cells, since the electrolytically seperated elements may react with the electrode. (As in the case with creating an electrolytic cell with saltwater and iron electrodes, you will generate rust (from the combination of oxygen with iron) and hydrogen gas.