Redox Identifying Ox. States Balancing 20-2

A.) Assigning Oxidation Numbers 1.) An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. 20.2

B.) Free Ions Metal Hydrogen Oxygen (FIMHO) 1.) The ox # of any element in the free or uncombined state is zero. The ox # of elements in compounds add to zero 20.2

2.) The ox # on an ion is the charge. The ox # on atoms in a polyatomic ion add up to the charge of the ion. Examples: Na +, Cl -, NO

3.) Metals: Group 1, and 2 are always +1, and +2 ex.) MgCl 2 and NaF

4.) H is usually +1 except when in a hydride, like KH (-1)

5.) O is usually -2 except when in a peroxide in H 2 O 2, or with F in OF 2. 6.) If compound has 3 or 4 elements, find H and O first, then find rest using CRT.

Practice Finding Ox States K 2 CrO 4 Cr 2 O

C.) Oxidation-Number Changes in Chemical Reactions –1.) An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction. 20.2

D.) Identifying Redox Reactions –1.) If the oxidation numbers change, the reaction must be a redox reaction. (LOOK for elements) 20.3

***Identifying Redox Reactions K + H 2 O  KOH + H 2 2Zn + HCl  ZnCl 2 + H

***Identifying Redox Reactions A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air. 20.3

E.) Balance Redox Equations –1.) Using Oxidation-Number Changes Use half reaction method described below (electrons lost = electrons gained).

20.3