Video: Selecting the Classification for the Breakdown of a Metal Carbonate

Which classifications best describes the reaction represented by the equation below? MgCO₃(s) ⟶ MgO(s) + CO₂(g) [A] Oxidation [B] Decomposition [C] Precipitation [D] Acid-base [E] Reduction


Video Transcript

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 component.

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.

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