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Offline dudebuddyguy

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Someone help me understand this
« on: November 25, 2013, 02:47:18 PM »
Oxidation half-reactions and Reduction half-reactions.

I've read over the explanation given:

Quote
  • Assign oxidation numbers to each symbol.
  • Determine which element has lost electrons from reactant to product. (The reactant charge will be larger, for example, –3 to 0).
  • Determine which element has gained electrons from reactant to product. (The product charge will be larger. Ex +2 to –1).
  • Write out each half reaction.
  • Balance the half reactions so that electrons lost in oxidation = electrons gained in reduction.

And I'm still confused. Hopefully someone can "dumb it down a shade" (so to speak) and help me understand this.

Offline Borek

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Re: Someone help me understand this
« Reply #1 on: November 25, 2013, 04:04:53 PM »
Have you tried to apply it to any reaction? Say,

Cl2 + H2SO3 + H2O :rarrow: HCl + H2SO4
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Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #2 on: November 25, 2013, 04:09:47 PM »
Have you tried to apply it to any reaction? Say,

Cl2 + H2SO3 + H2O :rarrow: HCl + H2SO4

Well the one question on my coursework is

CH4 + O2 -> CO2 + H2O

But I'm totally confused when I reach the 4th step (writing half reactions). I'm a little confused on the 2nd and 3rd step

Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #3 on: November 25, 2013, 04:42:18 PM »
CH4 + O2 -> CO2 + H2O

So I've been watching a few videos on this, and I think I'm starting to get it, and it seems to be a bit easier than I first thought.

So, adding in the oxidation state numbers of everything, I get:

C(-4) H(+1) + O2(0)  :rarrow: C(+4) O(-2) + H(+1) O(-2)

Carbon starts out with a -4 charge, and on the right, ends up with a +4 charge, so it ended up losing 4 electrons, correct?

Hydrogen starts out with a +1 charge, and ends up with a +1 charge, so I disregard that altogether for the oxidation/reduction equations?

O2 starts out with 0 charge, and ends up turning into two Oxygen molecules, each with a -2 charge. (-2) + (-2) = 0, so I disregard this as well?

So the oxidation half-reaction would be C(-4)  :rarrow: C(+4) .

uh..+4e- ?

EDIT: Wait a mo, the Oxygen would be the reduction half-reaction. So (-2) + (-2) = (-4), so it gained +4e-?

EDIT 2: No wait, +2e-, because you don't add the two -2's together, they count as one -2.

Offline Borek

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Re: Someone help me understand this
« Reply #4 on: November 26, 2013, 03:19:41 AM »
Carbon starts out with a -4 charge, and on the right, ends up with a +4 charge, so it ended up losing 4 electrons, correct?

No. After it lost 4 electrons its ON (oxidation number) was 0, and it changed further.

Quote
O2 starts out with 0 charge, and ends up turning into two Oxygen molecules, each with a -2 charge. (-2) + (-2) = 0, so I disregard this as well?

(-2) + (-2) is not zero.

Quote
So the oxidation half-reaction would be C(-4)  :rarrow: C(+4)

Yes.

Quote
uh..+4e- ?

No. See above. Imagine you owe me $4 (so you are at -$4). How many bucks do you need to have $4 after you pay your debt?

Quote
EDIT: Wait a mo, the Oxygen would be the reduction half-reaction. So (-2) + (-2) = (-4), so it gained +4e-?

Yes.

Quote
EDIT 2: No wait, +2e-, because you don't add the two -2's together, they count as one -2.

No. There are two atoms per molecule, each gaining two electrons, so the reaction goes like

O2 + 4e- :rarrow: 2O(-2)
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Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #5 on: November 26, 2013, 02:16:47 PM »
I know I've got it correct now, let me explain the +4e-. After watching YouTube videos, that seems to be the common way to write down if electrons have been lost or gained.

I didn't see anyone using -4e-, so I naturally assumed that all electron statements, be it gain or loss, use "+"

EDIT: Wait...I'd need $8. What does 8 have to do with it?
« Last Edit: November 26, 2013, 02:27:01 PM by dudebuddyguy »

Offline Borek

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Re: Someone help me understand this
« Reply #6 on: November 26, 2013, 02:31:58 PM »
Initially ON was -4, at the end it is +4. It is exactly the same situation. You need to remove 4 electrons to get to ON zero, then you need to remove another 4 electrons to get to ON +4. 8 in total.
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Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #7 on: November 26, 2013, 02:45:08 PM »
Initially ON was -4, at the end it is +4. It is exactly the same situation. You need to remove 4 electrons to get to ON zero, then you need to remove another 4 electrons to get to ON +4. 8 in total.

Ohh yeahhhh. Damn my math skills. So ON would be +8e- (because + in chemical equations = loss, and - = gain, for some odd reason)

Seriously, why is chemistry so ass-backwards? Oxidation means loss of electrons, and reduction (which you think would be loss) means a gain of electrons. I FEEL LIKE I'M TAKING CRAZY PILLS. (obligatory Zoolander reference)

EDIT: O, not ON

Offline Borek

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Re: Someone help me understand this
« Reply #8 on: November 26, 2013, 03:03:53 PM »
Ohh yeahhhh. Damn my math skills. So ON would be +8e- (because + in chemical equations = loss, and - = gain, for some odd reason)

Seriously, why is chemistry so ass-backwards? Oxidation means loss of electrons, and reduction (which you think would be loss) means a gain of electrons. I FEEL LIKE I'M TAKING CRAZY PILLS. (obligatory Zoolander reference)

EDIT: O, not ON

Nothing is backward here, perhaps you math ;) Trick is, electron charge is -e, not e. Blame physicists for that, it was their idea that the current flows in the opposite direction to the electron flow.
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Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #9 on: November 26, 2013, 03:06:52 PM »
Ohh yeahhhh. Damn my math skills. So ON would be +8e- (because + in chemical equations = loss, and - = gain, for some odd reason)

Seriously, why is chemistry so ass-backwards? Oxidation means loss of electrons, and reduction (which you think would be loss) means a gain of electrons. I FEEL LIKE I'M TAKING CRAZY PILLS. (obligatory Zoolander reference)

EDIT: O, not ON

Nothing is backward here, perhaps you math ;) Trick is, electron charge is -e, not e. Blame physicists for that, it was their idea that the current flows in the opposite direction to the electron flow.

Yes, my math is a tad backwards :P But you and Dan have both been most helpful and patient with me and I appreciate that :) I do admit that my original intention on joining this forum was to get quick and easy answers so I could quickly complete my online course, but those aren't my intentions any more. Onto the topic at hand;

So just "8e-" then. Most "teachers" on YouTube put "+" before the electron count which was a tad confusing. Should a + be placed before the electrons in any specific situations?

Offline Borek

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Re: Someone help me understand this
« Reply #10 on: November 26, 2013, 03:16:00 PM »
Not sure what you refer to. It is nothing unusual to see the half reaction written as

Fe :rarrow: Fe2+ + 2e-

+2e- means just two electrons between the products (well, they almost never get separated and freely floating, but we have to list them somehow in the half reaction equation).
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Offline Corribus

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Re: Someone help me understand this
« Reply #11 on: November 26, 2013, 03:25:42 PM »
Oxidation means loss of electrons, and reduction (which you think would be loss) means a gain of electrons.
Easy way to remember it is that reduction is reducing the charge.  Adding electrons reduces the charge.  Therefore reduction is adding electrons.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #12 on: November 26, 2013, 03:34:23 PM »
Oxidation means loss of electrons, and reduction (which you think would be loss) means a gain of electrons.
Easy way to remember it is that reduction is reducing the charge.  Adding electrons reduces the charge.  Therefore reduction is adding electrons.

OILRIG Oxidation is Losing, Reduction is Gaining. Learned that on YouTube :P

Not sure what you refer to. It is nothing unusual to see the half reaction written as

Fe :rarrow: Fe2+ + 2e-

+2e- means just two electrons between the products (well, they almost never get separated and freely floating, but we have to list them somehow in the half reaction equation).

Ohhh, I see now. So the people on YouTube were just using + in their reaction formulas just like you explained, to show electrons between products.

So with my original equation, the correct answer would be

Original Formula: CH4 + O2 :rarrow: CO2 + H2O

Adding in Oxidation States: C-4 H+1 + O20 :rarrow: C+4 O-2 + H+1 O-2

Oxidation Half-Reaction: C-4 + 8e- :rarrow: C+4

Reduction Half-Reaction: O2 + 4e- :rarrow: 2O-2

Hopefully I've done everything properly :D

EDIT: After looking at my Oxidation and Reduction reactions, I'm starting to think I need to switch them around, so that the oxygen formula is the Oxidation reaction, and the Carbon formula is the reduction.

Offline Borek

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Re: Someone help me understand this
« Reply #13 on: November 26, 2013, 03:41:27 PM »
You are almost there, just check your oxygen half reaction.

For the reaction equation to be balanced you need to have not only atoms balanced, but also a charge. What is total charge on the left? What is total charge on the right? (Please remember there is no such thing as C4- nor C4+, although these are reasonable notational tools at this moment).
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Offline dudebuddyguy

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Re: Someone help me understand this
« Reply #14 on: November 26, 2013, 03:53:22 PM »
You are almost there, just check your oxygen half reaction.

For the reaction equation to be balanced you need to have not only atoms balanced, but also a charge. What is total charge on the left? What is total charge on the right? (Please remember there is no such thing as C4- nor C4+, although these are reasonable notational tools at this moment).

Judging from my knowledge gained from YouTube videos, I need to balance each individual chemical.

Carbon has 1 on each side, so that's balanced.

Hydrogen, which is not part of the half reactions, has 4 atoms on the left, 2 on the right. So I'd have to add 2 Hydrogen atoms on the right (but in which reaction? I have no clue lol)

Oxygen had O2 on the left, and 2O2 on the right. So we need to add another Oxygen atom to the left to balance it.

How to write all this out? I'm not sure. Thing is, my coursework says nothing about balancing the half-reactions. As long as I have the reactions written out, I'm good to go. But because this could appear on my final exam in the future, I should probably learn this.

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