Oxidation numbers

[orangepopsicle]

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Hello,

I am having problems with this question. It is asking me to assign oxidation states to all the elements in this equations:

C5H12 + 8O2 --> 5CO2 + 6H2O

i have got the oxidation states for all the elements except C5H12 figured out. I know 8O2 has an oxidation state of 0 since it is a natural element. Then the 5CO2, the C has +4 oxidation state and O2 has -2 oxidation state, and for 6H20, the oxidation state for H is +1 and for the O is -2 but I can't seem to figure out the oxidation states for C5H12. Shouldnt it be +4 for C and -1 for H? but that gives me a total of +20 charge for C and -12 charge for the H so the overall charge of C5H12 would end up becoming +8 when it is a neutral atom. someone help?

 

[FeralisExtremum]

H's oxidation state is only -1 when it's attached to metals. It's default oxidation state is +1. Try assigning oxidation states now with that information.

 

[orangepopsicle]

Wouldn't that still give C5H12 a positive overall charge? If the H is +1 then the total charge for H in the molecule would be +12, then C which has a +4 oxidation state would be +20?

 

[FeralisExtremum]

I suggest reviewing the rules on assigning oxidation states. C does not have a set oxidation state, it varies depending on what it's attached to:

Oxidation States
1. Elements in their elemental form are in the 0 oxidation state.
2. Group 1 metals are +1 and Group 2 metals are +2.
3. Hydrogen is +1 except when bonded to metals (when it’s -1)
4. Transition elements must be determined by context (except Al = +3, Zn = +2, Cd = +2, Ag = +1)
5. The most electronegative elements get their typical oxidation state
6. The last element not assigned balances the charge of the compound/ion

Work your way down the list from 1 to 6 in order of priority.

 

[orangepopsicle]

thanks!

 

[FeralisExtremum]

When there are only two elements and you've assigned the oxidation state for one, then you skip 5 and go to 6 because the compound must be balanced for overall charge. Rule 5 is more so something you use when there are three atoms in a compound and you've assigned the oxidation state for one already and still have two left.

 

[DAT Destroyer]

Hello,

I am having problems with this question. It is asking me to assign oxidation states to all the elements in this equations:

C5H12 + 8O2 --> 5CO2 + 6H2O

i have got the oxidation states for all the elements except C5H12 figured out. I know 8O2 has an oxidation state of 0 since it is a natural element. Then the 5CO2, the C has +4 oxidation state and O2 has -2 oxidation state, and for 6H20, the oxidation state for H is +1 and for the O is -2 but I can't seem to figure out the oxidation states for C5H12. Shouldnt it be +4 for C and -1 for H? but that gives me a total of +20 charge for C and -12 charge for the H so the overall charge of C5H12 would end up becoming +8 when it is a neutral atom. someone help?

The rules presented here are nice.....BUT......It is a bit tricky for the hydrocarbon, hence must be handled a bit differently. OK.....Draw out 5 carbons.....place the hydrogens in...thus we get CH3CH2CH2CH2CH3. All carbons are NOT at the same oxidation level !!!!! For the CH3 carbons.....each hydrogen is +1......thus we have a total of +3.....therefore carbon is at -3. Now lets do the CH2 groups.....only look at hydrogens ATTACHED,,,,,,not the other carbons.....we see 2 Hydrogens, hence a +2,,,,,,,,therefore Carbon is at the -2 oxidation level.
I hope this helps,,,,,,,the other atoms should be straightforward.

Dr. Romano