OXIDATION NUMBERS. I need help assigning oxadation numbers. A) HI B) PBr3 C) GeS2 D) KH E) AS2O5 F) H3PO4.  I know that A is H1 I-1. And I know that for PBr3, it is X+3(-1)=0. What I don't...

OXIDATION NUMBERS.

I need help assigning oxadation numbers.

A) HI

B) PBr3

C) GeS2

D) KH

E) AS2O5

F) H3PO4.

 

I know that A is H1 I-1.

And I know that for PBr3, it is X+3(-1)=0. What I don't understand is how you know that Br is a -1 to begin with.

Also, I'm having trouble knowing how to start the process if there are no rules for the elelments in the compound I'm tryig to find oxidation numbers for.

Expert Answers
ndnordic eNotes educator| Certified Educator

Oxidation numbers are used to follow where the electrons are going in oxidation-reduction reactions.

There are some basic rules you  can use to assign oxidation numbers:

1.  The oxidation number of every element is zero (0)

2.  The oxidation number for monoatomic ions = the charge of the ion.

3.  Group 1 metal ions are always +1

4.  Group 2 metal ions are always +2

5. H is +1 when bonded to a nonmetal; -1 when bonded to a metal.

6.  F is always  -1

7. Oxygen is almost always -2

8. The sum of all oxidation numbers in a compound is always zero

9. The sum of the oxidation numbers in a polyatomic ions is equal to the charge of the ion.

Now to your examples:

HI:  H = +1 (rule 5) so I has to be -1 (rule 8)

PBr3:  The ionic charge of halogens is -1 (rule 2) and you have 3 Br so the total negative charge is -3; therefore by rule 8 P has to be +3

GeS2: Sulfur in group 16 has a charge of -2 (rule 2) so 2 sulfur = a total of -4. Therefore Ge is +4 (rule 8)

KH: H is -1 when bonded to a metal (rule 5) and K is +1 (rules 2 &3)

As2O5:  O is usually -2 (rule 7) so a total negative charge of -10; therefore 2 As = +10 and each As is +5.

H3PO4:  H is +1 (rule 5); O is -2 (rule 7). So you have +3 - 8 = -5 when you add the H & the O charges. Since total charge is zero (rule 8) the P has to be +5.

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