Hey Guys! Now, I am going to talk about the laws of oxidation numbers because this topic is prerequisite in understanding redox reactions. Now, what are Oxidation Numbers?
-
The value assigned to an atom or ion in order to keep track of electrons gained or loss.
-
It can be positive or negative, but zero for elements in free state.
For some examples:
Ca+2-electron loss
Cl-1-electron gained
RULES IN ASSIGNING OXIDATION NUMBERS
-
Elements in uncombined state have an Oxidation Number of zero.
Examples:
The atoms in He and N2, have oxidation number zero.
-
The oxidation number of a monoatomic ion equal to their ionic charge.
Examples:
The oxidation number of Na+1 is +1; the oxidation number of N3- is -3.
-
The usual oxidation number of hydrogen is +1. Metal hydrides such as LiH and CaH2 are exceptions, the oxidation number is -1.
Examples:
H2O=2-2=0
NaH=1-1=0
-
The oxidation number of oxygen in compounds is usually -2. H2O2 is an exception. The oxidation number of oxygen is -1.
Examples:
H2O2=+1(2)+(-1)2=0 CO2=4+(-2)2=0
=2-2=0 =4-4=0
-
In molecular compounds the more electronegative element is assigned a negative oxidation state equal to its charge as an anion.
Examples:
CO2=+4+(-2)2=0 CCl4=+4+(-1)4=0
=4-4=0 =4-4=0
-
Ionic compound are electrically neutral. Hence, the sum of the individual oxidation numbers is equal to the charge of the compound.
Examples:
MgO=2+(-2)=0 Na2SO4=2+x+(-8)=0
=2-2=0 =2+6-8=0
-
Polyatomic ions are electrically charged. Therefore the sum of the individual oxidation number is equal to the charge on the ion.
Examples:
NH4=3+4=+1 C2H3O2=2x+3+(-4)=-1
=0+3-4=-1
