Oxidation Numbers

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Oxidation State (or Numbers)

This is a very easy way of determining the oxidation number. The credit for this work goes to Dr. Amar.

The oxidation number represents the limiting ionic contribution to bonding. For simple ionic crystals like NaCl, the oxidation state of each atom is the charge on the atom. The following RULES allow you to determine oxidation states for any atom in any compound. If two rules are in conflict, the lower-numbered rule takes precedence.

0. The sum of the oxidation numbers is 0 for a neutral compound. The sum of the oxidation numbers is equal to the overall charge of a molecular ion.

1. The oxidation number of any atom in its elemental form is 0. Examples: H2 (g), Fe (s), Ar (g), Hg (l).

2. The alkali metals always have the oxidation state of +1.

3. Fluorine always has an oxidation state of -1.

4. Hydrogen is in the +1 oxidation state when bonded to a non-metal (like F or O). Hydrogen is in the -1 oxidation state when bonded to a metal (like Li).

5. Oxygen almost always has the oxidation number -2. Important exceptions are the peroxides like hydrogen peroxide (H2O2)

Beyond these strict rules, it is useful to know a few facts and trends Carbon can have oxidation numbers ranging from -4 to +4 (CH4 to CO2) Nitrogen can have oxidation numbers from -3 to +5 (NH3 to NO3-). Halogens (except for fluorine) can have oxidation states of -1 to +7 (HCl to ClO4-). Sulfur can have oxidation states between -2 and +6 (SH2 to SO4 2-).

Oxidation - Reduction Reaction AND Electrochemistry

a transfer of electrons from the substance oxidized to the substance reduced

Balancing Redox Equation

(1) The Oxidation state Method: This method breaks the reaction into two half-reactions, the reduction reaction and the oxidation reaction. IT is based on the change in oxidation states of the various ions.

(2) Ion- Electron method: More complex but seems to represent the true mechanism of such reaction more closely.

Voltaic Cells

Electrons can be transferred from one side to the other though an external connecting wire. Electric current moves in a closed loop path, or circuit, so this movement of electrons through the wire is balanced by the movement of ions in the solutions.

Electrolytic Cells

Redox reactions do not occur spontaneously but can be forced to take place by supplying energy with an external current. In the electroplating where electrolysis is used to coate a material with a layer of metal, the object to be plated is made the cathode in the reaction.


There are two important differences between the voltaic cell and the electrolytic cell:

1. the anode and cathode of an electrolytic cell are connected to a battery or other direct-current source, whereas a voltaic cell serves as a source of electrical energy

2. in electrolytic cells, electrical energy from an external source causes nonspontaneous redox reactions to occur. IN voltaic cells, however, sontaneous redox reactions produce electricity. Thus, in an electrolytic cells, electrical energy is converted into chemical energy; in a voltaic cell, chemical energy is converted into electrical energy.