Some rules to help you find oxidation numbers are:
- The oxidation number of a free element is zero.
- The sum of all oxidation numbers in a compound is zero.
- The oxidation number of a Group 2 metal in a compound is +2.
- The oxidation number of oxygen in a compound is usually –2.
- The oxidation number of hydrogen in a compound is usually +1.
- The oxidation number of chlorine is usually –1.
Equation 1
# stackrelcolor(blue)(0)("Mg") + 2 stackrelcolor(blue)("+1")("H") stackrelcolor(blue)("-1")("Cl") → stackrelcolor(blue)(0)("H")_2 + stackrelcolor(blue)("+2")("Mg") stackrelcolor(blue)("-1")("Cl")_2#
Listing the atoms in order, the oxidation numbers are:
#"Mg" = 0color(white)(ll)# (Rule 1)
#"H = +1"color(white)(ll)# (Rule 5)
#"Cl = -1"color(white)(m)# (Rules 2 and 6)
#"H" = 0color(white)(ml)# (Rule 1)
#"Mg = +2"# (Rule 3)
#"Cl = -1"color(white)(m)# (Rules 2 and 6)
Equation 2
#4stackrelcolor(blue)(0)("Fe") + 3stackrelcolor(blue)(0)("O")_2 → 2stackrelcolor(blue)("+3")("Fe")_2stackrelcolor(blue)("-2")("O")_3#
The reactants, #"Fe"# and #"O"_2#, are both elements, so their oxidation numbers are zero.
For #"Fe"_2"O"_3#, we argue as follows:
The oxidation number of #"O"# is -2 (Rule 4) — #"Fe"_2stackrelcolor(blue)("-2")("O")_3#
The sum of the oxidation numbers of the three #"O"# atoms is -6.
The sum of the oxidation numbers of the two #"Fe"# atoms is +6 (Rule 2).
The oxidation number of each #"Fe"# atom is +3 — #color(white)(mlll)stackrelcolor(blue)("+3")("Fe")_2stackrelcolor(blue)("-2")("O")_3#
#color(white)("The oxidation number of each Fe atom is +3" — )stackrelcolor(blue)("+6")color(white)("Fe"_2)stackrelcolor(blue)("-6")color(white)("O"_3)#
