What is the charge of Carbon on a CN- group?

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pfaction

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I'm not going to post TPR's answers until I see yours. I still cannot get what I am doing wrong. I'll post the passage in reference and hopefully one of you can explain it to me.
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pfaction said:
I'm not going to post TPR's answers until I see yours. I still cannot get what I am doing wrong. I'll post the passage in reference and hopefully one of you can explain it to me.
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negative on the carbon...?
 
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Okay, here's the relevant information from the passage.

VESWA.jpg


You'll kindly notice that -1 is not an answer here.

One more shot at it before I post their answer.
 
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N is -3

So I would immediately assume that C is +3 ....but I feel like the negative sign on CN- has something to do with this. ...

Sent from my Nexus S 4G
 
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sudo said:
N is -3

So I would immediately assume that C is +3 ....but I feel like the negative sign on CN- has something to do with this. ...

Sent from my Nexus S 4G
Click to expand...

Yes, the sum of all oxidation states should be the charge of the ion. If N is -3, C should be two.

pfaction, your initial question was about the charge of C. There is a difference between charge and oxidation state for atoms in covalent bonds.
 
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They are asking for oxidation states. Not charge. Not the same thing. I think N might be 3- here so C would be +2
 
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milski said:
Yes, the sum of all oxidation states should be the charge of the ion. If N is -3, C should be two.

pfaction, your initial question was about the charge of C. There is a difference between charge and oxidation state for atoms in covalent bonds.
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Lol u beat me to it.
 
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Pfaction- oxidation states are not the same thing as chrages. CN- actually has an N that has no charge and a C with a 1- charge. Oxidation state of N however is 3-. And the oxidation states of both elements must add up to the overall charge of the ion which is -1. So if the oxidation state of C here is 2+ and N is 3-, they add up to the ion's charge of -1.
 
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I don't understand that though.

H2CO3

O3 = -6
H2 + +2
Carbon is thus +4

I understand this.

I don't understand nitrogen. How did you come up with those numbers? The # of bonds?
 
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pfaction said:
I don't understand that though.

H2CO3

O3 = -6
H2 + +2
Carbon is thus +4

I understand this.

I don't understand nitrogen. How did you come up with those numbers? The # of bonds?
Click to expand...

Nitrogen exists as -3, +3, or +5. Just like oxygen is almost always -2. It's just stuff you need to know.

The oxidation state is typically the same as the charge on the monatomic ion. As you know (maybe), monatomic nitrogen is azide, which is N3-.
 
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God damn it.

I looked in TPR and I saw:

HNO3
I got it right, obviously it's +5, but that's with other rules, like O = -2, H = +1.
Is there a section in TBR with these? It's seriously confusing the **** out of me.
 
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pfaction said:
I don't understand that though.

H2CO3

O3 = -6
H2 + +2
Carbon is thus +4

I understand this.

I don't understand nitrogen. How did you come up with those numbers? The # of bonds?
Click to expand...
There are certain rules for oxidation numbers.
Rule 1: The oxidation number of an element by itself in an equation is always 0. Some examples are solid metals (Ca, Fe, Zn, etc) and diatomic molecules (O 2 , N 2 , Cl 2 , H 2 , etc).
Rule 2: The oxidation number of a monatomic ion is the same as its charge.
Rule 3: a) Oxygen has an oxidation number of -2, except in the polyatomic ion of peroxide in which it is -1.
b) Hydrogen has an oxidation number of +1 when it is bonded to a nonmetal and -1 when it is bonded to a metal.
c) Fluorine is always -1. Other halogens are usually -1 except when bonded to oxygen.
Rule 4: The sum of the oxidation numbers of each atom must equal 0 in a neutral compound. For polyatomic ions, the sum of the oxidation numbers will be equal to the charge of the polyatomic ion.

Other elements such as C and N have multiple oxidation numbers, so it really depends on what compound they are in.
 
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pfaction said:
God damn it.

I looked in TPR and I saw:

HNO3
I got it right, obviously it's +5, but that's with other rules, like O = -2, H = +1.
Is there a section in TBR with these? It's seriously confusing the **** out of me.
Click to expand...

Halides are always -1. Oxygen almost always -2, Hydrogen almost always +1, Nitrogen on the MCAT will probably always be -3.

TBR has a good section on it.
 
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I would actually think N has a good chance of not being -3 on an MCAT passage. I think the key here though is that N as the more electronegative atom is going to "call the shots" if you will. I've never really seen rules on oxidation states go into when exceptions occur for group 5A elements, but the impression I've always gotten is that the most electronegative atom is going to be reduced to whatever will fill its valence shell. Hence H is -1 with really metallic elements, O will assume a different charge than -2 with F in a molecule, N will assume a different charge than -3 with O in a molecule, etc.
 
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Yeah, I found it. The last chapter of Gen Chem, I'm only on Ch4! Haha. Thanks Chiddler, I drew out what they were talking about and kind of got it.

Also LOL @ the fact I posted in that topic, wow.
 
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