Chemistry Revision Notes of Oxidation, Reduction and Electrochemistry for NDA Part-2
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Redox Reactions

The reactions involving oxidation as well as reduction as its two half reactions are called redox reactions. In these reactions, one substance is oxidised and other substance is reduced.

Disproportion reactions are the reaction in which same element is oxidised as well as reduced.

e.g. `overset(0)Cl_2+NaOH → Na overset(-1)Cl +Naoverset(+5)ClO_3+H_2O`

`overset(0)Cl_2 undersettext(Reduction) → Na overset(-1)Cl`

`overset(0)Cl_2 oversettext(Oxidaction) → Na overset(+5)(Cl) O_3`

`=>` Oxidation and reduction take place together in equivalent amounts.
`=>` All metal displacement reactions are redox reactions.
`=>` The formation of ionic solid from its elements is always a redox reaction.

`overset(0)Zn +overset(-2) CuSO_4 → Zn overset(-2)SO_4 +overset(0)Cu`

`overset(0)Zn oversettext(Oxidation)→ Znoverset(-2)SO_4`

`overset(-2)CuSO_4 undersettext(Reduction)→overset(0)Cu`

Note : Compounds like `H_2O, O_3, HNO_3, H_2SO_3, NaNO_2, SO_2, NO_2S_2O_3` act as oxidising as well as reducing agents.
Reducing agents are oxidised and oxidising agents are reduced in a chemical reaction.

Oxidation Number

Oxidation Number shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.

By knowing how the oxidation number of an element changes during a reaction, we can tell whether it is being oxidised or reduced.

Increase in oxidation number is oxidation.
Decrease in oxidation number is reduction.

General Rules for Calculation of Oxidation Number

`=>` The oxidation number for an atom of any free (uncombined) element is ZERO. e.g `Na, Ca`

`=>` The oxidation number of an element in self-combination is always ZERO. e.g, `H_2 , O_2`

`=>` In most hydrogen containing compounds, oxidation number of hydrogen is +1. (Exception is when `H` combines with alkali metals or alkaline earth metals to form hydrides of metals such as : `NaH, LiH, CaH_2`. Then, the oxidation number of `H` is -1).

`=>` In compounds involving the alkali metals, the elements are assigned oxidation number of +1. In compounds involving the alkaline earth metals, the elements are assigned oxidation number of +2.

`=>` Oxygen is usually assigned an oxidation number of -2 for oxides. It has an oxidation number of -1 in peroxide (`H_2O_2`).

`=>` Oxidation number of oxygen in peroxides like `H_2O_2 , Na_2O_2` is `-1` in superoxides like `KO_2 , RbO_2` it is `-1/2` and and compounds in which O-atom bonded to fluorine like `OF_2 , O` is in +2 while in `O_2F_2` is in +1 state.

`=>` Fluorine always has oxidation number of -1 in compounds. The other elements in that group are usually -1 in compound with elements of low electronegativity.

`=>` The sum of oxidation numbers of all the atoms in the formula for a neutral compound is ZERO.

`=>` The sum of oxidation numbers of an ion or complex ion is the same as the charge on that ion.

`=>` Negative oxidation number in compounds of two unlike atoms is assigned to the more electronegative atom.

`=>` In combinations of non-metals not involving hydrogen and oxygen, the nonmetal that is more electronegative is considered negative.

`=>` The common oxidation state of alkali metal (e.g. Li, Na, K etc.) is +1.

`=>` The oxidation number of alkaline earth metals (e.g. Be, Mg, Ca, Ba, etc.) is +2.

`=>` Oxidation number of metal in metal carbonyl is zero. e.g. `Fe` is in 0 oxidation state in `Fe(CO)_5`.

`=>` In `C_3O_2` [carbon suboxide] oxidation number of carbon is `4/3, Br_3O_8`, [tribomooctaoxide ], `Br` is in `16/3` oxidation state and `Na_2S_4O_6` [sodium tetrathionate], `S` is in 2.5 oxidation state.

 
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