Video Transcript
A piece of magnesium left in air is
gradually converted to magnesium oxide. In this reaction, is magnesium
oxidized or reduced?
Since this problem asks us to
identify if magnesium is oxidized or reduced, let us first explain a redox
reaction. A redox reaction is a chemical
reaction that involves the transfer of electrons between chemical species. In a redox reaction, reduction
occurs within a chemical reaction when a chemical species gains electrons, whereas
oxidation occurs when a chemical reaction allows a chemical species to lose
electrons.
Since we have identified the
differences between oxidation and reduction, let us write a chemical equation for
the problem. The problem states that magnesium
reacts with the oxygen in the air to form magnesium oxide. The reactants would be Mg for
magnesium and O2 for oxygen. Let us not forget that oxygen is
diatomic, and therefore two oxygen molecules are covalently bonded together to
create the oxygen molecule. The product magnesium oxide can be
written by first identifying the charge of the magnesium ion as Mg2+ and the charge
of the oxide ion as O2−.
To form a neutral compound for
magnesium oxide, there is one magnesium ion per one oxide ion present in the ionic
structure. This would create a formula of MgO
for magnesium oxide. We can also balance the chemical
equation: two Mg solid plus O2 gas reacts to produce two MgO solid.
Now that we have a balanced
chemical equation, let us isolate magnesium and identify if it is being oxidized or
reduced. On the reactant side, magnesium has
a charge of zero since it is in elemental form. On the product side, magnesium is
found in MgO, and therefore the magnesium ion has a charge of 2+. Therefore, two electrons were lost
in order for magnesium to change from a charge of zero to 2+.
Since electrons were lost,
magnesium underwent oxidation. In redox reactions, if one species
undergoes oxidation, then the other species must undergo reduction. Let us now confirm that oxygen
undergoes reduction. Oxygen has a charge of zero since
it is in elemental form. On the product side, oxygen is
found in MgO, and therefore the oxygen ion has a charge of 2−. Therefore, each oxygen atom in O2
gains two electrons to change from a charge of zero to 2−. Since electrons were gained, oxygen
underwent reduction. Completing this extra step to
ensure oxygen is reduced can assist in reducing errors when identifying the chemical
species being oxidized or reduced.
Therefore, magnesium is oxidized
when magnesium is left in air and is gradually converted into magnesium oxide.
