Question

Label each element with an oxidation number?

Answer 1

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)`