Oxidation-Reduction Reactions supplement

Kinds of Reactions – Alternative Viewpoint Reactions in which no electrons are transferred – e.g., Reactions in which electrons are transferred, e.g.,

Oxidation - Reduction In the second chemical equation, electrons are transferred. The zinc is oxidized and the copper is reduced. These are oxidation reduction reactions.

Terminology Oxidation process describes losing electrons and formal charge increases. The reducing agent is oxidized. Reduction process describes gaining electrons. The oxidizing agent is reduced.

Oxidation Numbers Used to keep track of electron shifts in a reaction. The oxidation number is defined as the charge which an atom appears to have when electrons are counted according to the following rules. (Note the emphasis on appears.) Oxidation numbers are somewhat artificial as a bookkeeping device.

Rules 1.In the free elements, each atom has an oxidation number of zero. (e.g., H 2, Fe, Na, Cl 2, etc.) 2.In simple ions (monatomic ions), the oxidation number is equal to the charge on the ions. (e.g., For Na +, oxidation number is +1; for O 2-, oxidation number is -2)

Rules 3.In most compounds containing oxygen, the oxidation number of each oxygen atom is In most compounds containing hydrogen, the oxidation number of each hydrogen atom is All oxidation numbers must be consistent with the conservation of charge. For neutral molecules, the oxidation numbers of all atoms must add up to zero. For polyatomic ions, the oxidation numbers of all atoms must add up to the charge on the ion.

Oxidation Numbers and Oxidation and Reduction Processes Oxidation refers to any chemical change in which there is an increase in oxidation number. Reduction refers to any chemical change in which there is a decrease in oxidation number. Oxidation and reduction must always occur together and must compensate each other. Doing the accounting of oxidation numbers is an essential part of balancing oxidation-reduction equations.

Balancing Oxidation-Reduction Equations -- Approach 1.Assign oxidation numbers to the atoms that change 2.Decide on the number of electrons changed per atom 3.Decide on the number of electrons changed per formula unit. 4.Compensate electron gain or loss by writing appropriate coefficients by the oxidizing agent and the reducing agent. 5.Insert other coefficients to obey conservation of matter.

Example Balance the equation

Example Balance the following