Question

How to find oxidation number in a chemical reaction?

Answer 1

Answer:
You assign oxidation numbers to the elements in a compound by using the Rules for Oxidation Numbers.

Explanation:
1.The oxidation number of a free element is always 0.

2.The oxidation number of a monatomic ion equals the charge of the ion.

3.The oxidation number of
H
is +1, but it is -1 in when combined with less electronegative elements.

4.The oxidation number of
O
in compounds is usually -2, but it is -1 in peroxides.

5.The oxidation number of a Group 1 element in a compound is +1.

6.The oxidation number of a Group 2 element in a compound is +2.

7.The oxidation number of a Group 17 element in a binary compound is -1.

8.The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.

9.The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

EXAMPLE:

What is the oxidation number of
Cr
in
CrCl
3
?

Solution:

We use what rules we can to determine the oxidation numbers.

Rule 7 states that the oxidation number of Cl is -1.

We write the oxidation number of the element above its symbol and the total for 3 Cl atoms below the symbol.

This gives
Cr
-1
Cl
3

m
m
m
m
m
m
l
l
-3
m
m
.

Rule 8 states the numbers along the bottom must add up to zero. So the number under
Cr
must be +3.

This gives
Cr
-1
Cl
3

m
m
m
m
m
+3
l
l
-3
m
m
.

There is only one
Cr
atom, so its oxidation number is +3.

This gives
+
3
Cr
-1
Cl
3

m
m
m
m
m
+3
l
l
-3
m
m
.

The oxidation number of
Cr
in
CrCl
3
is +3.