Which classifications best
describes the reaction represented by the equation below? MgCO₃ solid reacts to form MgO
solid plus CO₂ gas. A) Oxidation, B) Decomposition, C)
Precipitation, D) Acid-base, or E) Reduction.
The chemical we’re starting with is
magnesium carbonate. Without any direct chemical
intervention, we’re forming magnesium oxide and carbon dioxide. You might recognize this as a
classic example of how a metal carbonate breaks down. Now, let’s have a look at the five
possible descriptions and see which one best applies.
The term oxidation usually refers
to either the loss of electrons or the gaining of oxygen. For an inorganic reaction like
this, the loss of electrons is usually the key consideration. We can check if this reaction can
be considered an oxidation by looking at the oxidation state of each ion or atom in
the equation. Magnesium in magnesium carbonate
has an oxidation state of plus two. Since it’s an ion with a two plus
charge. In a carbonate, oxygen will have a
negative two oxidation state. And carbon will have a plus four
oxidation state. You can check this makes sense by
adding up the oxidation state of every atom in the entire compound. What you get is a total of zero,
which is what you’d expect.
Now, what about magnesium
oxide? Magnesium isn’t its typical
positive two oxidation state. Well, oxygen isn’t its typical
negative two. And finally, carbon dioxide. Since we have two oxygens in their
typical negative two oxidation state, we have carbon in a positive four oxidation
state. If we compare the elements on both
side of the reaction equation, we can see that there is no change in any oxidation
state for any of the elements. Therefore, we can’t consider this
reaction an oxidation.
Let’s have a quick look at
reduction, which is the opposite of oxidation. Reduction is considered the gain of
electrons or the loss of oxygen. In inorganic chemistry, it’s the
gaining of electrons that’s more typically considered. However, since there is no change
in oxidation state for any component whatsoever, we cannot consider this reaction a
reduction reaction. Even though, visually, it looks
like oxygen has been lost from our main component.
So what about B, decomposition? A decomposition reaction is one
where typically one component decays into two or more other components. Decomposition is a perfect word to
describe this reaction. What we’re seeing here is the
decomposition of a metal carbonate. Such decomposition would normally
require heating in order for it to occur at a decent rate. On this basis, decomposition looks
like a strong candidate to be our answer. So let’s have a look at C and D to
see if we can find any better descriptions.
A precipitation reaction requires
the formation of a solid from a solution. A good example is the reaction of
barium and sulfate ions which produces barium sulfate and this characteristic
aqueous-aqueous-solid pattern. However, in this case, we have the
breakdown of a solid. So even though one of the products
is a solid, we don’t have the solution to start with. So we can move on to option D.
Acid-base reactions have many
types. But the most ordinary involves H⁺
ions reacting with something else. A suitable example is the reaction
of hydrochloric acid, HCl, with sodium hydroxide, NaOH, forming NaCl, which is table
salt, and water. In this case, we have no H⁺ at
all. So we’re not likely to have an
acid-base reaction. There are other types of
acid-base. But, in this case, we only have one
So decomposition looks like our
best answer. Meaning that of the five
classifications given, the one that best describes the reaction represented by the
equation MgCO₃ solid reacts to form MgO solid plus CO₂ gas is decomposition.