Can someone pleeeeaasseee explain galvanic/electrolytic stuff to me?

[MDtoBe777]

This seems to be one of the hardest concepts for me to grasp. Do any of you wonderful people have a good way to memorize/understand this stuff?

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[sdnstud]

i just kinda look at what is oxidized and what is reduced....then add the voltages...if the voltage comes out positive...it's galvanic (spontaneous). it the voltage comes out negative...it's electrolytic (non spont). the important thing is to figure out what is being oxidized/reduced

 

[Clemson Doc]

Here are some things about cells off the top of my head...

Galvanic cell
-also called a voltaic cell
-positive cell potential
-spontaneous (delta G < 0)
*I like to remember that "gals (females) are spontaneous"
-cathode is positive
-anode is negative

Electrolytic cell
-negative cell potential
-requires energy input to induce a reaction
-not spontaneous (delta G > 0)
-cathode is negative
-anode is positive

*In both cells, reduction takes place at the cathode, and oxidation takes place at the anode (mnemonic: RED CAT, AN OX). Remembering this is the key to answering most questions on electrochemistry
*In both cells, electrons flow from the anode to the cathode. This should make sense, because the anode releases electrons (oxidation) and the cathode accepts electrons (reduction)
*Remember OIL RIG: Oxidation is loss (of electrons); reduction is gain (of electrons). You can also remember that an oxidized species shows the charge increase (e.g., from +2 to +4) and a reduces species shows the charge decrease (e.g., from -1 to 0).

EMF = E (oxidation) + E (reduction)

*Galvanic cells have a positive EMF, and electrolytic cells have a negative EMF.
*When you are given a table of reduction potentials, the higher (more positive) values are more suitably reduced. The more negative potentials would rather be oxidized. So when a passage gives you two half-reactions and gives the reduction potentials of both, the higher potential will be reduced, and the lower potential will be oxidized.
*If a half-reaction has a reduction potential of -1.46, the reverse half-reaction has an oxidation potential of +1.46.
*If a table on the MCAT gives you potentials of half-reactions, check to see if each reaction is an oxidation (electrons on the products side) or a reduction (electrons on the reactants side). I have seen passages where you have to flip one of the reactions in order to get the correct potential you're after (i.e., so that you can look only at reduction potentials, for instance)

The cell diagram is written as follows:
anode electrode | oxidized species || reduced species | cathode electrode

*the "|" designates a phase change (usually solid to liquid), and the "||" means you're changing from one side of the cell to the other
*If a substance is dripped into one side or the other, it should be included in the above cell diagram as well on the appropriate side (this is not likely, but it could happen)

 

[Csv321]

Clemson Doc....AWESOME! I just got stuck/confused on a question in 4R about that

 

[MDtoBe777]

Here are some things about cells off the top of my head...

Galvanic cell
-also called a voltaic cell
-positive cell potential
-spontaneous (delta G < 0)
*I like to remember that "gals (females) are spontaneous"
-cathode is positive
-anode is negative

Electrolytic cell
-negative cell potential
-requires energy input to induce a reaction
-not spontaneous (delta G > 0)
-cathode is negative
-anode is positive

*In both cells, reduction takes place at the cathode, and oxidation takes place at the anode (mnemonic: RED CAT, AN OX). Remembering this is the key to answering most questions on electrochemistry
*In both cells, electrons flow from the anode to the cathode. This should make sense, because the anode releases electrons (oxidation) and the cathode accepts electrons (reduction)
*Remember OIL RIG: Oxidation is loss (of electrons); reduction is gain (of electrons). You can also remember that an oxidized species shows the charge increase (e.g., from +2 to +4) and a reduces species shows the charge decrease (e.g., from -1 to 0).

EMF = E (oxidation) + E (reduction)

*Galvanic cells have a positive EMF, and electrolytic cells have a negative EMF.
*When you are given a table of reduction potentials, the higher (more positive) values are more suitably reduced. The more negative potentials would rather be oxidized. So when a passage gives you two half-reactions and gives the reduction potentials of both, the higher potential will be reduced, and the lower potential will be oxidized.
*If a half-reaction has a reduction potential of -1.46, the reverse half-reaction has an oxidation potential of +1.46.
*If a table on the MCAT gives you potentials of half-reactions, check to see if each reaction is an oxidation (electrons on the products side) or a reduction (electrons on the reactants side). I have seen passages where you have to flip one of the reactions in order to get the correct potential you're after (i.e., so that you can look only at reduction potentials, for instance)

The cell diagram is written as follows:
anode electrode | oxidized species || reduced species | cathode electrode

*the "|" designates a phase change (usually solid to liquid), and the "||" means you're changing from one side of the cell to the other
*If a substance is dripped into one side or the other, it should be included in the above cell diagram as well on the appropriate side (this is not likely, but it could happen)

Just one question:
When you said that electrons flow from the anode to the cathode, the electrons in the salt bridge flow in the opposite direction, right? Anions flow to the anode, and cations flow to the cathode? Is this true in both electrolytic and galvanic cells?

Also...you do not multiply the standard reduction potentials depending on the number of electrons that were either reduced or oxidized. Right? So if the value they gave you is for lets say fe2+ + 2e- -->fe. And you have 2 moles of Fe, you just leave the value alone. Haha..okay that makes 2 questions! Thanks for your help!

 

[imsotired]

Just one question:
When you said that electrons flow from the anode to the cathode, the electrons in the salt bridge flow in the opposite direction, right? Anions flow to the anode, and cations flow to the cathode? Is this true in both electrolytic and galvanic cells?

Also...you do not multiply the standard reduction potentials depending on the number of electrons that were either reduced or oxidized. Right? So if the value they gave you is for lets say fe2+ + 2e- -->fe. And you have 2 moles of Fe, you just leave the value alone. Haha..okay that makes 2 questions! Thanks for your help!

electrons flow from the anode to the cathode so there will be an accumulation of positive charge in the half cell with the anode and an accumulation of negative charge in the half cell with the cathode so therefore, a salt bridge is needed to neutralize those charges. Anions flow to the anode and cations flow to the cathode. I don't think it's electrons because salt bridges are usually composed of KCl, I think... not too sure on that.

Standard reduction potentials are intensive properties that do not depend on the amount). You don't multiply anything to the reduction potentials when finding the emf of the cell. You only consider the number of moles of electrons when using the Nernst equation.

 

[Nakhrewali]

I couldn't have said it better myself!

 

[sdnstud]

one more question:

For both galvanic and electrolytic cells, electrons migrate from the anode to the cathode. The ions in the salt bridge try to neutralize that by having anion migrating to anode and cation migrating to cathode. So why are the anode and cathode negative and positive respectively in galvanic....and opposite in electroytic cell? in other word, if the anode is negatively charged in a galvanic cell, shouldn't it attract cation instead of anion?

 

[ya herd]

one more question:

For both galvanic and electrolytic cells, electrons migrate from the anode to the cathode. The ions in the salt bridge try to neutralize that by having anion migrating to anode and cation migrating to cathode. So why are the anode and cathode negative and positive respectively in galvanic....and opposite in electroytic cell? in other word, if the anode is negatively charged in a galvanic cell, shouldn't it attract cation instead of anion?

The way I try to think of it is like this...The anode is negative because oxidation is occurring there, and its building up a negative charge and shipping those electrons over to the cathode. Since its sending those electrons, the anode needs something negative to fill its place, hence, the anions from the salt bridge. The cathode is receiving this influx of negative charge, so its needs something to balance it out...hence, the salt bridge gives it some cations.

I know I know...maybe not the *best* way to think about it, but it works pour moi.