To dicover which compound is being oxidized and which one is being reduced, we need to know the oxidation number of each specie.
The oxidation number (NOX) of an element is the electrical charge it acquires when it forms an ionic bond or the partial character (δ) it acquires when it forms a predominantly covalent bond.
There are some rules for NOX, like:
The NOX of simple substances is always zero.
The NOX of ions is equal to their charge.
The sum of the NOX elements of a compound always equals zero.
The sum of the NOXs of the elements in a composite ion is always equal to the charge on the ion.
The NOX of Oxigen in an compound is -2 almost in all cases.
With this in mind, let's calculate the NOX of each specie of the following equation:
2 Mg + O2 --> MgO2
Reactants
Mg: 0
O2: 0
Products
O: -2
Mg: x + -2= 0; x = +2
Mg: +2
When the nox increases, it means that the substance has oxidized, that is, it has lost electrons.
When the nox decreases, it means that the substance has reduced, that is, it has gained electrons.
In this case, the nox of Mg increased and of O decreased. It means that Mg is being oxidized and O is being reduced.
The half-reactions are:
Mg → Mg2+ + 2e-(oxidation)
O + 2 e- → O-2 (reduction)
Answer:
What is being oxidized?
Mg
What is being reduced?
O
Write the half-reaction:
Mg → Mg2+ + 2e-(oxidation)
O + 2 e- → O-2 (reduction)