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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: 777888 on October 12, 2004, 03:47:09 PM

Title: Oxidation numbers problems(Grade 12)
Post by: 777888 on October 12, 2004, 03:47:09 PM
I read all the rules of assign oxidation numbers for particles, but I don't really know how to do the following questions! Can someone please help me? (I am most confused about the steps of the followings, so I hope you can tell me or explain the steps, too) Thank you very much!

a) Au(NO3)3

b) NaN3

c) NaBH4

d) Na2HPO4

e) B2H6

f) C2H4O2
Title: Re:Oxidation numbers problems(Grade 12)
Post by: Donaldson Tan on October 12, 2004, 04:51:34 PM
first, consider the structure of the substance [molecular or ionic?], then draw out the structure. the oxidation number of an element in a monoatomic ion is its charge.

eg. O=C=O
the oxidation number of of C is +4. It can be derrived using this method:
carbon form 4 bonds (2 sigma and 2 pi)
in each bond, carbon is bonded to a more electronegative atom (oxygen), thus it "loses" an electron and thus "gains" a single positive charge.
The overall "gain" charge is +4 and thus the oxidation number of C in CO2 is +4

try applying this method. for structures that exihibits reasonance, use one of the cannonical structure. any one of them will do.
Title: Re:Oxidation numbers problems(Grade 12)
Post by: 777888 on October 12, 2004, 06:07:34 PM
First of all, thank you for your time! However, We are not taught about the method with the Lewis structures.

We are taught to determine the oxidation number using rules such as:
-For nonbinary ionic compounds, assign the mteal first then work on the polyatomic ion.
-In compounds that have a nonmetal and a transition metal, assign the nonmetal its "normal" oxidation number then assign the oxidation number to the transition metal to make a sum of zero or the charge on an ion
-The oxidation number of F in any compound is –1
,etc...

That's why I don't get question a and b!
For Au(NO3)3, I don't know how to assign oxidation number for Au first(it's a transition metal)..Someone told me that "becuase NO3 in the formula has a charge of 1-, then (NO3)3 has a charge of 3-, to balance out, Au should have an oxidation number of +3"..WHY? Are oxidation numbers AND electric charges related?

For NaN3, will the oxidation number for N be -1/3 (a fraction)?

Thanks for your *delete me*
Title: Re:Oxidation numbers problems(Grade 12)
Post by: AWK on October 13, 2004, 01:45:13 AM
The rule exists - for simple ions oxidation number is equal to charge of ion
Title: Re:Oxidation numbers problems(Grade 12)
Post by: Demotivator on October 13, 2004, 10:20:55 AM
777888,
Don't confuse oxidation numbers with real charges. Oxidation numbers are an accounting scheme to track electron transfer in redox reactions.

for example, (NO3)-. The electric charge is -1. The oxidation number of N is +5, for O it's -2. Does that mean N has a charge of +5 and 0 -2? No. N has a partial positive charge that is difficult to quantify. This scheme simplifies accounting by assigning all the negative charge to the more electronegative Oxygens. It works because no matter how the charges are really distributed, the sum of them will equal the total electrical charge.
So does Au in Au(NO3)3 really have a charge of +3? No, but its oxidation number is +3.
Title: Re:Oxidation numbers problems(Grade 12)
Post by: 777888 on October 13, 2004, 10:25:29 AM
777888,
Don't confuse oxidation numbers with real charges. Oxidation numbers are an accounting scheme to track electron transfer in redox reactions.

for example, (NO3)-. The electric charge is -1. The oxidation number of N is +5, for O it's -2. Does that mean N has a charge of +5 and 0 -2? No. N has a partial positive charge that is difficult to quantify. This scheme simplifies accounting by assigning all the negative charge to the more electronegative Oxygens. It works because no matter how the charges are really distributed, the sum of them will equal the total electrical charge.
So does Au in Au(NO3)3 really have a charge of +3? No, but its oxidation number is +3.


Thanks! But how do you know that Au in Au(NO3)3 has an oxidation number of +3, not +1?
Title: Re:Oxidation numbers problems(Grade 12)
Post by: Demotivator on October 13, 2004, 10:29:15 AM
because Au(NO3)3 has a total electric charge of 0. each NO3 has a charge of -1, but there are three of them. So Au must be +3 to add up to 0.
(+3) + 3(-1) = 0
Title: Re:Oxidation numbers problems(Grade 12)
Post by: 777888 on October 17, 2004, 12:25:15 AM
Is it possible to determine the oxidation numbers for C and N in CNO-  ?
Title: Re:Oxidation numbers problems(Grade 12)
Post by: Demotivator on October 17, 2004, 04:30:05 PM
Yes.
In CNO C is bonded to N and N is bonded to O.
When C bonds to N as in CN-, C is +2 (because N is -3).
In CNO, N is oxidized, C remains the same (+2) and 0 is -2. That leaves N as -1.
Title: Re:Oxidation numbers problems(Grade 12)
Post by: AWK on October 18, 2004, 01:21:37 AM
CNO(-) has two isomers, namely cyanate and fulminate anions with atom sequences:
O-C=N or O-N=C (charges are omited). The best idea is to calculate a formal oxidation number using -3 for nitrogen and -2 for oxygen. Then in both cases carbon shows oxidation number +4, though, in fact, fulminic acid is a formal derivative of carbon oxide CO.
Title: Re:Oxidation numbers problems(Grade 12)
Post by: 777888 on October 18, 2004, 08:23:32 PM
Au(CN)4 1- (aq)
How can I determine the oxidation number for Au in Au(CN)4 1- (aq) using oxidation number rules instead of lewis structure method! (The answer is +3, but WHY?)
Thanks!
Title: Re:Oxidation numbers problems(Grade 12)
Post by: AWK on October 19, 2004, 01:07:19 AM
By analogy to KCN. Each CN(-) group has 1- charge and net charge of complex anion is 1-.
Subtracting one CN(-) grup and one negative charge from Au(CN)4(-) you wil obtan molecule Au(CN)3 with the same oxidation state as complex anion. Treating Au(CN)3 as ionic compound you can calulate charge of Au in this compound, and finally according to rule - oxidation state  of simple ion is equal to its charge you will obtain oxidation state of Au.
Title: Oxidation numbers problem
Post by: sarvani on July 08, 2011, 10:19:27 AM
Can I Know the oxidation state of NH4NO3
Title: Oxidation numbers problem
Post by: DevaDevil on July 08, 2011, 10:41:59 AM
attempt? with all the help here in the thread you should be able to make a good start
Title: Oxidation numbers problem
Post by: AWK on July 11, 2011, 06:56:02 AM
Can I Know the oxidation state of NH4NO3
NH4NO3 is a compound consisted of two ions. You can easily calculate both oxidation numbers of nitrogen using default oxidation numbers for oxygen oand hydrogen in compounds (-2 and +1 respectively).