Oxidation Numbers and Examples of Redox Reactions

The rules (chill…we will go through them one by one, also, you know where to find this) Pure elements, atoms and compounds have an oxidation number of 0 Oxygen has an oxidation number of -2 Hydrogen has an oxidation number of +1. When combined with metals it will have an oxidation number of -1 (hydride ions) Alkali metals (column 1) will have an oxidation number of +1 Alkaline earths ( column 2) will have an oxidation number of +2 Halogens, when in binary compounds, will have an oxidation number of -1 Silver will have an oxidation number of 1+, Zinc will have an oxidation number of +2, aluminum will have an oxidation number of +3 Other elements in compounds will have an oxidation number such that the total for the compound will be zero, or in the case of ions, the charge given.

“Pure elements, atoms and compounds have an oxidation number of 0” Zn C Cl 2 CO 2 H 2 SO 4  Zn 0  C 0  (Cl 2 ) 0  (CO 2 ) 0  (H 2 SO 4 ) 0

Oxygen has an oxidation number of -2 Oxide (O 2- ) MgO  Mg x+ O 2- (we will get to the + ion later) CO 2  C x+ O 2- 2 note: total of 4 -’s (2 x 2-) Fe 2 O 3  Fe 2 x+ O 2- 3 note: total of 6 -’s (3 x 2-) Exception (!): Peroxide (O 2 2- ) has an oxidation number of 1-. It is rare and unstable.

Hydrogen has an oxidation number of +1. When combined with metals it will have an oxidation number of -1 (hydride ions) H 1+ Cl H 1+ 2 O H 1+ 2 SO 4 C 6 H O 2 NaH 1- CaH 1- 2 These are hydrides! }

Alkali metals (column 1) will have an oxidation number of +1 Alkaline earths ( column 2) will have an oxidation number of +2 Mg 2+ O Na 1+ I Ca 2+ Cl 2 Cs 2 1+ S (each cesium is 1+, there are two, so the total is 2+)

Halogens, when in binary compounds, will have an oxidation number of -1 Halogens are column 17 elements: F, Cl, Br and I “Binary” means just two elements in the compound: – NaCl, KI, MgBr 2, FeF 3 (they all end in “ide”) Each has a 1- charge – NaCl 1-, KI 1-, MgBr 1- 2, FeF 1- 3 This does NOT apply to more complex compounds: – KI 5+ O 3, HCl 1+ O, Fe(BrO 7+ 4 ) 3 (they all end in “ite or “ate”, and the third element is often oxygen) Important!!!! – Column 16, when binary often has a 2- – Column 15, when binary often has a 3- (not always though)

Three you can count on: Silver has an oxidation number of 1+ Zinc has an oxidation number of +2 Aluminum has an oxidation number of +3

The last rules looks complicated, but work it through: Other elements in compounds will have an oxidation number such that the total for the compound will be zero, or in the case of ions, the charge given. HClO – H = 1+, O = 2-, so Cl must be 1+ KMnO 4 – K = 1+ O= 2- (times 4 so, 8-), so Mn must be 7+

Types of reactions (try to give examples of each) All Single Replacement reactions are redox All Combustion reactions are redox Some Synthesis and Decomposition reaction are redox. No Double Replacement or Water Forming reactions are redox (they are precipitation reactions.)