Classical Concept
* In a redox reaction oxidation and reduction occurs simultaneously.
* Oxidation reactions are those reactions which include addition of oxygen or an electronegative element to a substance or removal of hydrogen or an electropositive element from a substance.
* Reduction reactions are those reactions which include removal of oxygen or an electronegative element from a substance or addition of hydrogen or an electropositive element to a substance.
Electronic Concept
* Loss of electrons by any species is known as oxidation.
* Gain of electrons by any species is known as reduction.
* In a redox reaction, the substance that denotes electrons to reduce the other substance is known as a reducing agent.
* In a redox reaction, the substance that accepts electrons to oxidize other substance is known as an oxidizing agent.
* The series obtained when the metals are arranged in the decreasing order of their ability to denote electrons is known as metal activity series or chemical electrochemical series.
* The knowledge of competition between the metals to release the electrons helps us to construct galvanic cells in which the chemical energy is converted into electrical energy.
Oxidation Number
* Unlike ionic bonds, an electron transfer does not occur in covalent bonds. Instead there is a partial transfer or electronic charge. This is known as an electronic shift.
* The oxidation number method always assumes that there is a complete transfer of electrons from a less electronegative atom to a more electronegative atom.
* Oxidation number is defined as the charge or the apparent charge that an atom in a compound or ion would have, if all of the electrons in its bonds belonged entirely to the more electronegative atom.
* Stock notation, indicates the oxidation number by placing a Roman numeral within parentheses after the symbol or name of the metal.
Types of Redox Reactions
* A combination reaction is a chemical reaction in which two or more substances combine to form a single new substance. For a combination reaction to be a redox reaction, either one or both the reactants should be in elemental state.
* A decomposition reaction is a chemical reaction in which a compound breaks up into two or more simple substances. Decomposition reaction is the opposite of combination reaction. Not all decomposition reactions are redox reactions.
* For a decomposition reaction to be a redox reaction, it is essential that at least one of the products of the decomposition reaction is in the elemental state.
* In a displacement reaction, the atom or ion in a compound is replaced by the atom or ion of some other element or compound.
* In a metal displacement reaction, the metal in a compound is displaced by some other metal in the elemental state.
* In a non-metal displacement reaction, the metal or a non-metal displaces another non-metal in a compound.
* A disproportionation reaction is a reaction in which the same element is oxidised as well as reduced. For such reactions to occur, one of the reacting elements should exist in at least three oxidation states.
Balancing Redox Equations
* A redox equation can be balanced using two methods – oxidation number method and halfreaction method.
* The oxidation number method is based on the change in the oxidation number of the reducing and oxidising agent.
* In the half-reaction method the redox reaction is split into two halves – the oxidation half and the reduction half.
* The half-equation method is based on the principle that electrons lost during the oxidation half reaction are equal to the electrons gained during the reduction-half of the reaction.
Redox reactions As a Basis for Titration
* Titrations that involve oxidation-reduction reactions are known as redox titrations.
* Redox titrations can be performed using oxidising agents such as potassium permanganate, potassium dichromate and iodine.
* The titrations in which iodine is directly titrated against a reducing agent are called iodimetric titrations.
* Iodometric titrations involve the addition of an oxidising agent to excess iodide ions to produce iodine which is then titrated with standard thiosulphate solution.
* Iodine dissolves in water in the presence of potassium iodide due to the formation of I three minus ions.
Redox Reactions and Electrode Processes
* Redox reactions in which the oxidation and reduction reactions occur in the same vessel are called direct redox reactions.
* A redox couple is defined as a combination of the oxidized and reduced forms of the same substance taking part in an oxidation or reduction-half reaction.
* The tendency of a substance to lose or gain electrodes in galvanic cell is known as the electrode potential.
* The electrode potential of an electrode depends on the concentration of ions and the temperature of the solution the electrode is in.