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.

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ndnordic | High School Teacher | (Level 2) Associate Educator

Posted on

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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