Finding the Oxidation State

1
Finding the Oxidation State

• Formal charges in Molecules and Ions

2
Objectives

• When you complete this presentation, you will be
  able to
• use rules to determine the formal oxidation state
  of individual atoms in
• ionic compounds
• molecular compounds
• polyatomic ions

3
Introduction

• The oxidation state, OS, of an atom is the
  formal charge of that atom.
• We apply formal rules to determine the oxidation
  state of an atom.
• These formal rules take into account ...
• The overall charge of the compound or ion
• The charges of the other atoms in the compound or
  ion.

4
Rules

• The charge on F is always 1-.
• The charge on O is usually 2-.
• The charges on Cl, Br, and I are usually 1-.
• The charge on H is usually 1.
• The charges on Group 16 non-metals are usually
  2-.
• The charges on Group 15 non-metals are usually 3-.

5
Examples
Find the oxidation state of S in H2SO4, sulfuric
acid
The net oxidation state of the compound, OSnet,
is 0.
The oxidation state of each H, OSH, is 1.
? The oxidation state of 2 Hs is 2.
The oxidation state of each O, OSO, is 2-.
? The oxidation state of 4 Os is 8-.
?
OSnet 2OSH OSS 4OSO
OSS OSnet 2OSH 4OSO
?
OSS 0 - (2) - (8-)
?
OSS 6
6
Examples
Find the oxidation state of N in NH4Cl, ammonium
chloride
The net oxidation state of the compound, OSnet,
is 0.
The oxidation state of each H, OSH, is 1.
? The oxidation state of 4 Hs is 4.
The oxidation state of the Cl, OSCl, is 1-.
?
OSnet OSN 4OSH OSCl
OSN OSnet 4OSH - OSCl
?
OSN 0 - (4) - (1-)
?
OSN 3-
7
Examples
Find the oxidation state of N in HNO3, nitric
acid
The net oxidation state of the compound, OSnet,
is 0.
The oxidation state of the H, OSH, is 1.
The oxidation state of each O, OSO, is 2-.
? The oxidation state of 3 Os is 6-.
?
OSnet OSH OSN 3OSO
OSN OSnet - OSH 3OSO
?
OSN 0 - (1) - (6-)
?
OSN 5
8
Examples
Find the oxidation state of Cr in H2Cr2O7,
dichromic acid
The net oxidation state of the compound, OSnet,
is 0.
The oxidation state of each H, OSH, is 1.
? The oxidation state of 2 Hs is 2.
The oxidation state of each O, OSO, is 2-.
? The oxidation state of 7 Os is 14-.
?
OSnet 2OSH 2OSCr 7OSO
2OSCr OSnet 2OSH 7OSO
?
2OSCr 0 - (2) - (14-)
?
?
2OSCr 12
OSCr 6
9
Examples
Find the oxidation state of O in HFO3, fluoric
acid
The net oxidation state of the compound, OSnet,
is 0.
The oxidation state of the H, OSH, is 1.
The oxidation state of the F, OSF, is 1-.
?
OSnet OSH OSF 3OSO
3OSO OSnet - OSH - OSF
?
3OSO 0 - (1) - (1-)
?
?
3OSO 0
OSO 0
10
Examples
Find the oxidation state of Mn in MnO4-,
permanganate ion
The net oxidation state of the compound, OSnet,
is 1-.
The oxidation state of each O, OSO, is 2-.
? The oxidation state of 4 Os is 8-.
?
OSnet OSMn 4OSO
OSMn OSnet 4OSO
?
OSMn (1-) - (8-)
?
OSMn 7
11
Examples
Find the oxidation state of Cr in CrO42-,
chromate ion
The net oxidation state of the compound, OSnet,
is 2-.
The oxidation state of each O, OSO, is 2-.
? The oxidation state of 4 Os is 8-.
?
OSnet OSCr 4OSO
OSCr OSnet 4OSO
?
OSCr (2-) - (8-)
?
OSCr 6
12
Summary

• The oxidation state, OS, of an atom is the
  formal charge of that atom.
• We apply formal rules to determine the oxidation
  state of an atom.

13
Summary

• The Rules
• The charge on F is always 1-.
• The charge on O is usually 2-.
• The charges on Cl, Br, and I are usually 1-.
• The charge on H is usually 1.
• The charges on Group 16 non-metals are usually
  2-.
• The charges on Group 15 non-metals are usually 3-.