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Module 1: Oxidation and Reduction Reactions

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Oxidation Numbers

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Rules to calculate oxidation numberThe oxidation number of any atom in the uncombined state is zero.The oxidation number of the alkali metals is +1 in their compounds.The oxidation number of alkaline earth metals is +2 in their compounds.The oxidation number of fluorine is -1 in all its compounds.The oxidation number of oxygen is -2 except peroxides, oxygen fluorides and superoxides of metals.The oxidation number of peroxides is -1, oxygen fluorides and oxygen difluoride is +1 and +2 and for potassium superoxide is -1/2.The oxidation number of hydrogen is +1 and for metal hydrides like sodium hydride, calcium hydride its -1 per atom.The oxidation number of a charged species is equal to the charge of the species.
Oxidation number for chemical compounds
This video explains how to calculate the oxidation number of an elementin a compound.Oxidation number tells us about the number of electrons that are being transferred in a redox reaction.Before getting into oxidation number calculation, some general rules should be followed:Rule #1: The oxidation number of alkali metals or group I = + 1.Rule #2: The oxidation number of alkaline earth metals is +2.Rule #3: Oxidation number for halogens = -1. But, if the halogen is with an oxygen atom, then the oxidation number is +1.Rule #4: The oxidation number of oxygen and elements in group 6 = -2. If oxygen is in the form of peroxide, then the oxidation number is -1.Rule #5: The oxidation number of H is +1 if it is with a non-metal, and -1 if it is with a metal.The rule of thumb for calculating the oxidation numbers are:Sum of the oxidation number for a neutral compound i.e. a compound with no ionic charge = 0.Sum of oxidation number for a polyatomic ion = ionic charge of the compound.Let us consider an example:What is the oxidation number of Mg in MgCl2?The oxidation number of Cl is -1.Since it is Cl2, we multiply the oxidation number with two times of Cl.Hence the oxidation number of Cl2 is -2.Since the compound is neutral, the sum of oxidation numbers is 0.The oxidation number of Magnesium should be +2 because the sum of oxidation number for neutral compound = 0. Hence the oxidation number of Magnesium = +2 and chlorine = -1.Lets look at another example. What is the oxidation number of Phosporous in phosporic acid? The oxidation number of oxygen is -2 and hydrogen is +1.Since there are 4 atoms of oxygen, the oxidation number is 4 x -2 = -8.Similarly for hydrogen, it is 3 x 1 = 3.Using this sum of oxidation number for neutral compound rule, we have -5 + x = 0, so the oxidation number of Phosporous in phosporic acid is +5.Consider another example with an ionic charge like dihydrogen phosphate.
What is the oxidation number of Phosporous in dihydrogen phosphate?The oxidation number of oxygen is -2.Since there are 4 atoms of oxygen, the oxidation number is -8.The oxidation number of hydrogen is +1 and since there are two atoms, it is +2.Using the oxidation number rule for polyatomic ions, the sum of oxidation numbers is equal to the ionic charge of the compound.30. So in this case, the sum of the oxidation number is equal to -1. 2 + x -8 = -1. The oxidation number of Phosporous is +5.