Chapter 4 - Reactions in Aqueous Solution

Oxidation –Reduction ReactionsLoss of electrons by a substance is called oxidation.The gain of electrons by a substance is reduction.Oxidation of one substance is always accompanied by the reduction of another as electrons aretransferred between them.LEO-GERLoss of Electrons-OxidationGain of Electrons-ReductionOIL RIGOxidation Is Loss of electronsReduction Is Gain of electronsOxidation NumbersThe oxidation number of an atom in a substance is the actual charge of the atom if it is amonatomic ion.• For an atom in its elemental form, the oxidation number is always zero.• For any monatomic ion, the oxidation number equals the charge on the ion.• Nonmetals usually have negative oxidation numbers, although they can sometimes bepositive.• The oxidation number of oxygen is usually –2 in both ionic and molecular compounds. Themajor exception is in compounds called peroxides, which contain O 2- 2 ion, giving eachoxygen an oxidation number of –1.• The Oxidation number of hydrogen is +1 when bonded to nonmetals and –1 when bonded tometals.Ch 4 Reactions in Aqueous Solution - 9 -