Anyone have a good list of rules for determining oxidation number?

[USC 22]

I'm not talking about "find the oxidation state of V in VO3-"

I'm talking about the tricky ones, like ClO- or KO2.

Is there a "priority list" that you guys know of? Thanks

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[LoLCareerGoals]

You could try to picture the molecule and use electroneativity/octet fullness as your guide.
Say superoxide KO2. Oxygen is clearly more el-neg and one oxygen is not any different than the other so it should carry equal charge. Now will it be K^+1 and O2^-1 or K^+2 and O2^2- or K^+4 and O2^-4?
Ask yourself could oxygen rip off more el-on from the inner shell of K just so it could fill more? Not filling completely seams more stable to me than the energy it would take to ripp out an inner el-on, let alone several -> K^+1, O2^1-.

ClO-. Cl should win the el-neg battle here. Now will it be Cl- and O or Cl^2- and O+ or worse? Cl- is obv prettier than a new shell forming Cl^2- so I like Cl-, O here.

 

[BerkReviewTeach]

You could try to picture the molecule and use electroneativity/octet fullness as your guide.
Say superoxide KO2. Oxygen is clearly more el-neg and one oxygen is not any different than the other so it should carry equal charge. Now will it be K^+1 and O2^-1 or K^+2 and O2^2- or K^+4 and O2^-4?
Ask yourself could oxygen rip off more el-on from the inner shell of K just so it could fill more? Not filling completely seams more stable to me than the energy it would take to ripp out an inner el-on, let alone several -> K^+1, O2^1-.

ClO-. Cl should win the el-neg battle here. Now will it be Cl- and O or Cl^2- and O+ or worse? Cl- is obv prettier than a new shell forming Cl^2- so I like Cl-, O here.

You should rethink ClO-. O is more eletronegtive than Cl, and you end up with an O that is -2 and a Cl that is +1.

 

[USC 22]

You should rethink ClO-. O is more eletronegtive than Cl, and you end up with an O that is -2 and a Cl that is +1.

The original guy was right, actually (according to TPR, at least)

 

[BerkReviewTeach]

The original guy was right, actually (according to TPR, at least)

You probably want to double check that. According to Ebbing (Example 13.4 page 515), according to Zumdahl (Question 43, chapter 4 page 117), and according to Berkeley Review (Example 10.2 page 255) it's -2 for O and +1 for Cl in ClO-.

I'm sure if you consulted any other general chemsitry textbook you'll see that specific example because it's one of the basic ones used to teach the concept. O carries a -2 oxidation state when it is the most electronegative atom in the molecule and not bonded to itself.

Look specifically for the oxidation state of chlorine in the oxyacids HClO, HClO2, HClO3, and HClO4. That might also help with your original question.

 

[Jepstein30]

You probably want to double check that. According to Ebbing (Example 13.4 page 515), according to Zumdahl (Question 43, chapter 4 page 117), and according to Berkeley Review (Example 10.2 page 255) it's -2 for O and +1 for Cl in ClO-.

I'm sure if you consulted any other general chemsitry textbook in addition to the ones I've listed you'll see that specific example because it's one of the basic ones used to teach the concept. O carries a -2 oxidation state when it is the most electronegative atom in the molecule and not bonded to itself.

Look specifically for the oxidation state of chlorine in the oxyacids HClO, HClO2, HClO3, and HClO4. That might also help with your original question.

I think the confusion lies in the electronegativity trends. It does increase from left to right but it also (and more significantly) decreases from top to bottom.

O is more EN than Cl: http://www.green-planet-solar-energy.com/images/PT-small-electroneg.gif

I think it would be tough for the MCAT to ask you about the EN relationships of atoms not in the same period or group though. O and Cl you should probably know considering only F is more electronegative than O.

 

[BerkReviewTeach]

I think the confusion lies in the electronegativity trends. It does increase from left to right but it also (and more significantly) decreases from top to bottom.

O is more EN than Cl: http://www.green-planet-solar-energy.com/images/PT-small-electroneg.gif

I think it would be tough for the MCAT to ask you about the EN relationships of atoms not in the same period or group though. O and Cl you should probably know considering only F is more electronegative than O.

You're absolutely right that electronegativity trends are what is the issue in most of these questions. Based on organic chemistry, it's reasonable to know that O is more electronegative than Cl. Cl and O are two of the nine elements that frequently show up in organic chemistry, and they are a part of the electronegativity mnemonic.

 

[Jepstein30]

You're absolutely right that electronegativity trends are what is the issue in most of these questions. Based on organic chemistry, it's reasonable to know that O is more electronegative than Cl. Cl and O are two of the nine elements that frequently show up in organic chemistry, and they are a part of the electronegativity mnemonic.

FONClBrISCH?

one of the most ridiculous ones out there haha

question while you're here.. any errata published for the TBR sets? Not saying I've found any mistakes but would like to make sure I'm not fooling myself thinking I got a question right when both I and the answer key could potentially be wrong.

 

[MMa23]

For ClO, just remember that HClO is the acid and H is almost always H+ (except for hydride) and you will know ClO is -1. This also applies to the other acids like H3PO4 so you know PO4 is -3. etc etc.

 

[Lunasly]

You probably want to double check that. According to Ebbing (Example 13.4 page 515), according to Zumdahl (Question 43, chapter 4 page 117), and according to Berkeley Review (Example 10.2 page 255) it's -2 for O and +1 for Cl in ClO-.

I'm sure if you consulted any other general chemsitry textbook you'll see that specific example because it's one of the basic ones used to teach the concept. O carries a -2 oxidation state when it is the most electronegative atom in the molecule and not bonded to itself.

Look specifically for the oxidation state of chlorine in the oxyacids HClO, HClO2, HClO3, and HClO4. That might also help with your original question.

This was my first guess, but TPRH claims that O has a -1 oxidation state and chlorine has a 0 oxidation state. I'm assuming that this is a mistake on their part or does anyone agree with them?

 

[PhysioG318]

This is what I use:
Example 1: SO42-

1)set S to equal:X
2) O is always -2....so since there are 4 O then its -2 * 4 which equals -8
3) Now I write my equation: (x) -8=-2 Note: always set your equation equal to the to sum of the oxidation state which in this case is -2.
4) solve for X and you get +6

Hope that helps that's how my chemistry taught me!!

 

[PhysioG318]

sorry didn't realize the thread was from 2013...lol way off topic but still good info lol