4.5 - 4.6 : Oxidation - Reduction Reactions

• Oxidation-reduction reaction is a reaction in which electrons are transfered between species or in which atoms change oxidation numbers
• Oxidation number (oxidation state) is the actual charge of the atom if it exists as a monatomic ion, or a hypothetical charge assigned to the atom on the substance by simple rules
• In elements the oxidation number is 0
• In monoatomic ions the oxidation number of an atom equals the charge on the ion
• The oxidation number for oxygen is -2 in most compounds. Execet in hydrogen peroxide, H2O2, where the oxidation number is -1
• The oxidation number of hydrogen is +1 in most of its compounds. Except the number of hydrogen is -1 in binary compounds with a metal.
• The oxidation number is -1 for the halogen fluorine in al of its compounds.
• The other halogens (Cl,Br,I) has an oxidation number of -1 in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen.
• The sum of the oxidation numbers of the atoms in a compound is zero.
• The sum of the oxidation numbers in a polyatomic ion equals the charge on the ion.
• A half-reaction is one of the two parts of an oxidation-reduction reaction, one part of which involves a loss of electrons and the other a gain of electrons
• Loss of electrons = increase of oxidation number
• Gain of electrons = decrease of oxidation number
• Oxidation is the half-reaction in which there is a loss of electrons by a species
• Reduction is the half-reaction in which there is a gain of electrons by a species
• Species that are oxidized loses electrons
• Species that is reduced gains electrons
• An oxidizing agent is a species that oxidizes another species, it is itself reduced (gains electrons).
• A reducing agent is a species that reduces another species, it is itself oxidized (loses electrons).
• A combination reaction is a reaction in which two substances combine to form a thrid substance
• Common types of oxidation-reduction reactions are combination, decomposition, displacement and combustion.
• A decomposition reaction is when a single compound reacts to give two or more substances.
• 2Na(s) + Cl2(g) --> 2NaCl(s)


  Combination reaction
• 2HgO(s) --> 2Hg(l) + O2(g)

  Decomposition
• A displacement reaction (single-replacement reaction) is a reaction in which an element reacts with a compound, displacing another element from it.
• Cu(s) + 2AgNO3 (aq) --> Cu(NO3)2 (aq) + 2Ag(s)

  Displacement reaction
• 4 Fe(s) + 3O2(g) --> 2Fe2O3 (s)


  Combustion
• A combustion reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame