How do you balance redox reactions!!!

help, i dont know how to balance redox reactions. basically because i dont know how to find the oxidation number, i know there are rules but i just dont get it >.< help please, this is one of the equations


PH(3)+ I(2) ------> H(3)PO(2)^(-) + I^(-)

help, i dont know how to balance redox reactions. basically because i dont know how to find the oxidation number, i know there are rules but i just dont get it >.< help please, this is one of the equations

Look at the periodic table.
H = +1, when bonded to a nonmetal, -1 when H is the negative ion of metallic hydride.

O = -2, unless the compound is peroxide, O = -1

Column 1 metals = +1
Column 2 metals = +2
Al = +3
Column 6 = -2 when alone
Column 7 = -1 when alone

The sum of the oxidation number = 0 for compounds or the charge for ions
The oxidation number for atoms or molecules of 1 element = 0



In PH3, H = +1, 3 H’s = +3, so P = -3

I2 is one element = 0


In H3PO2^-, 3 H’s = +3, O = -2, 2O’s = -4
+3 + P + -4 = -1
P = 0 This is wrong!

see web site below
http://en.wikipedia.org/wiki/Hypophospho…
H3PO2 is Hypophosphorous acid, not a -1 charged ion

In H3PO2, H = 3 H’s = +3, 2O’s = -4
+3 + P + -4 = 0
P = +1

I -1 is iodide ion

P changes from -3 to +1
I changes from 0 to -1

General Steps for balancing redox reactions.
Step 1: Determine which atoms have had their oxidation number changed as they changed from a reactant to a product.
Step 2: Write the skeletons of the oxidation and reduction half-reactions. (The skeleton reactions contain the formulas of the compounds oxidized and reduced, but the atoms and electrons have not yet been balanced.) See Example.
PH3 → H3PO2
I2 → I-
Step 3: Balance all elements other than H and O.
I2 → 2 I-

Step 4: Balance the oxygen atoms by adding 1 H2O molecule for each O, where needed.