Oxidation-reduction reactions · The oxidation number
Calculating Oxidation Numbers in Compounds
At the pharmacy they sell a bottle of violet crystals, potassium permanganate, KMnO₄. Its formula doesn't say how many electrons the manganese has lost or gained. How can you find out? With a small equation with a single unknown. At the end of the page you'll come back to this bottle.
The idea: a sum that must equal the total charge
You already know the basic rules: an element on its own is 0, oxygen is almost always $-2$, hydrogen is almost always $+1$, and a monatomic ion equals its charge. Now we use them to find a number we don't know.
The key is this: the oxidation number is an "accounting" of electrons. Electrons are neither created nor lost, so when you add up all the oxidation numbers of the atoms in the formula, you must get the total charge of the species.
How to set it up and solve it
Always follow the same steps: write the formula, put in the known numbers, call the unknown $x$, multiply by the subscripts, set the sum equal to the total charge, and solve.
Practice★★☆Not started
The mineral hematite, from which iron is extracted, is iron(III) oxide, Fe₂O₃. Calculate the oxidation number of iron by setting up the condition that the sum of the…
The mineral hematite, from which iron is extracted, is iron(III) oxide, Fe₂O₃. Calculate the oxidation number of iron by setting up the condition that the sum of the oxidation numbers in the whole compound must be 0 (oxygen is $-2$).
In an ion, the sum is not $0$ but the charge of the ion. It is the same equation, with a different result on the right side of the equals sign.
Practice★★☆Not started
The nitrate ion, NO₃⁻, is present in fertilizers. Since it is an ion with charge $-1$, the sum of the oxidation numbers must be $-1$. Calculate the oxidation number of…
The nitrate ion, NO₃⁻, is present in fertilizers. Since it is an ion with charge $-1$, the sum of the oxidation numbers must be $-1$. Calculate the oxidation number of nitrogen, showing the equation you set up.
The same element can have a different oxidation number depending on the compound. Calculating it lets you tell in which one it is more oxidized: the greater the number, the more electrons it has lost (or the less it shares them).
To oxidize: to lose electrons; the oxidation number increases.
To be reduced: to gain electrons; the number decreases.
Practice★★★Not started
Potassium permanganate (KMnO₄) is a violet disinfectant sold in pharmacies. Calculate the oxidation number of manganese knowing that potassium is $+1$ and oxygen is…
Potassium permanganate (KMnO₄) is a violet disinfectant sold in pharmacies. Calculate the oxidation number of manganese knowing that potassium is $+1$ and oxygen is $-2$, and that the compound is neutral. Compare your result with the oxidation number of manganese in MnO₂ ($+4$) and state in which one it is more oxidized.