Label each element with an oxidation number?

1 Answer
May 16, 2017

Answer:

See below.

Explanation:

Some rules to help you find oxidation numbers are:

  1. The oxidation number of a free element is zero.
  2. The sum of all oxidation numbers in a compound is zero.
  3. The oxidation number of a Group 2 metal in a compound is +2.
  4. The oxidation number of oxygen in a compound is usually –2.
  5. The oxidation number of hydrogen in a compound is usually +1.
  6. 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)#