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Monday, 9 July 2018

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Chemistry: Oxidation numbers, IUPAC names, rules and examples

oxidation number rules


Oxidation number is the number assigned to an element in chemical combination which represents the number of electrons, lost or gained by the atom of that element in the compound.

 

History of atoms

Rules of assigning oxidation number.

1) The oxidation number of an uncombined atom or any atom in a molecule of an element has an oxidation number = 0. For example, hydrogen in h2, Sulphur in S8,phosphorus in p4, all have an oxidation number = 0.

2) In most compounds containing oxygen. The oxidation number of each oxygen atom is - 2, but in compound like H2O2, the oxidation number is - 1.

3) In simple ions (ions containing one atom), the oxidation number is equal to the charge on the ion. For example, Al+3 has an oxidation number of +3, fe+2 has an OXN of +2 and so on.

4) The algebraic sum of all the oxidation numbers of all the atom of an ion is equal to the overall charge on that ion.

5) The sum of all the oxidation numbers of all the atoms in a neutral compound is zero.

  With these fee basic and rather arbitrary rules, let us consider some compounds and how their oxidation numbers are determined, we can conveniently name of an organic compound by determining the oxidation number of the suffice.

Examples

1) calculate the oxidation number of Mn in KMnO4 and give the IUPAC name of the compound.
Solution.
KMnO4,
 Let 'y' represent the OXN of Mn
+1 + y + (4 x -2) =0
+1 + y - 8 =0
Y = +7

IUPAC Name(KMnO4) = Potassium tetraoxmanganate(vii)

2) Calculate the oxidation number of Chromium in K2CR2O7, and give the name of the compound.
Solution.
K2Cr2O7,
Let 'y' represent the OXN of Cr2
+2 + (2y) + (7 x (-2)) =0
+2 + 2y - 14 = 0

y = +6
IUPAC Name(K2Cr2O7) = Potassium heptaOxodichromate(vi)


3) Calculate the oxidation number of chlorine ClO-.
Solution,
Assume the OXN of CL is 'f',
 ClO-,
f + (3 x (-2)) =  - 1

f + (-6) =  - 1

f = +5
IUPAC Name(ClO-) = TrioxoChlorate(v)ion

4) Calculate the oxidation number of Nitrogen in HNO3.
Solution,
Let's assume that the OXN of Nitrogen is 'm'
HNO3,
+1 + m + (3 x (-2)) = 0
m = +5

IUPAC Name(HNO3): Hydrogen trioxonitrate(v)

Practice:
What are the IUPAC name of the following,
* No
* N2O
* H2SO4

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