Video Transcript
What is the oxidation number of oxygen in hydrogen peroxide?
The oxidation number of an element such as oxygen describes its degree of
oxidation. We can think of the oxidation number as the number of electrons removed from that
element. Oxidation numbers, sometimes called oxidation states, are often either negative or
positive. When an oxidation state is negative, electrons have been gained. But when an oxidation state is positive, electrons have been removed.
There is a set of oxidation state rules we can use to predict oxidation numbers of
different elements. When an element occurs in a reaction in its elemental form, such as oxygen existing
as molecular oxygen, its oxidation state is zero. The oxidation state of an ion is its charge, so a sodium one plus ion would have an
oxidation state of plus one. When elements form a neutral compound, the sum of the oxidation numbers should be
zero. The oxidation state of alkali metals is plus one when in a compound. The oxidation state of alkaline Earth metals is plus two when in a compound.
When halogens appear in compounds, their oxidation states are most often minus one,
with some exceptions. When hydrogen appears in a compound, its oxidation state is often plus one. However, in metal hydrides, such as magnesium hydride, hydrogen takes on an oxidation
state of minus one. Oxygen most often has an oxidation state of minus two, for example, in the compound
water. In H2O, there are two hydrogens present with oxidation states of plus one, and oxygen
has an oxidation state of minus two. Overall, the sum of the oxidation states will be zero for this neutral compound.
However, we find that oxygen does sometimes take on other oxidation states in
peroxides, superoxides, and dioxygen difluoride. In the peroxide ion of O2 2-, oxygen has an oxidation state of minus one. In superoxides, such as sodium superoxide, oxygen has an oxidation state of minus
one-half. In dioxygen difluoride, oxygen has an unusual oxidation state of plus one.
Using these rules, let’s determine the oxidation number of oxygen in hydrogen
peroxide. Hydrogen peroxide has the chemical formula H2O2. Hydrogen will have the oxidation state of plus one. As the name implies, this compound is a peroxide, and thus oxygen will have the
oxidation state of minus one.
We can confirm this by finding the sum of the oxidation states, starting with the two
hydrogens present. The total contributed by the hydrogens is plus two. The two oxygens in the peroxide have oxidation numbers of minus one. This gives a total contributed by the oxygens of minus two. The sum of these oxidation numbers should yield zero for this neutral compound. We find that the sum of plus two and minus two is zero. Therefore, the oxidation number of oxygen in hydrogen peroxide is minus one.
