Label each element with an oxidation number?

May 16, 2017

See below.

Explanation:

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

${\stackrel{\textcolor{b l u e}{0}}{\text{Mg") + 2 stackrelcolor(blue)("+1")("H") stackrelcolor(blue)("-1")("Cl") → stackrelcolor(blue)(0)("H")_2 + stackrelcolor(blue)("+2")("Mg") stackrelcolor(blue)("-1")("Cl}}}_{2}$

Listing the atoms in order, the oxidation numbers are:

$\text{Mg} = 0 \textcolor{w h i t e}{l l}$ (Rule 1)
$\text{H = +1} \textcolor{w h i t e}{l l}$ (Rule 5)
$\text{Cl = -1} \textcolor{w h i t e}{m}$ (Rules 2 and 6)
$\text{H} = 0 \textcolor{w h i t e}{m l}$ (Rule 1)
$\text{Mg = +2}$ (Rule 3)
$\text{Cl = -1} \textcolor{w h i t e}{m}$ (Rules 2 and 6)

Equation 2

$4 {\stackrel{\textcolor{b l u e}{0}}{\text{Fe") + 3stackrelcolor(blue)(0)("O")_2 → 2stackrelcolor(blue)("+3")("Fe")_2stackrelcolor(blue)("-2")("O}}}_{3}$

The reactants, $\text{Fe}$ and ${\text{O}}_{2}$, are both elements, so their oxidation numbers are zero.

For ${\text{Fe"_2"O}}_{3}$, we argue as follows:

The oxidation number of $\text{O}$ is -2 (Rule 4) — "Fe"_2stackrelcolor(blue)("-2")("O")_3

The sum of the oxidation numbers of the three $\text{O}$ atoms is -6.

The sum of the oxidation numbers of the two $\text{Fe}$ atoms is +6 (Rule 2).

The oxidation number of each $\text{Fe}$ atom is +3 — $\textcolor{w h i t e}{m l l l} \stackrel{\textcolor{b l u e}{\text{+3")("Fe")_2stackrelcolor(blue)("-2")("O}}}{_} 3$
$\textcolor{w h i t e}{{\text{The oxidation number of each Fe atom is +3" — )stackrelcolor(blue)("+6")color(white)("Fe"_2)stackrelcolor(blue)("-6")color(white)("O}}_{3}}$