Video Transcript
Which of the following chemical
equations shows the thermal decomposition of zinc carbonate? (A) ZnCO3 solid reacts to form ZnO
solid plus CO2 gas. (B) ZnO solid plus CO2 gas react to
form ZnCO3 solid. (C) ZnC2 plus O2 gas react to form
two ZnO solid plus CO2 gas. (D) Two ZnO solid plus C solid
react to form two Zn solid plus CO2 gas. (E) Two Zn solid plus O2 gas react
to form two ZnO solid.
In this question, we are being
asked to identify which of the chemical equations provided in the answer choices
correctly represents the thermal decomposition of zinc carbonate. Thermal decomposition is defined as
the breaking down of a substance when heated to form two or more other
substances. Because the process of thermal
decomposition forms new substances, it is a type of chemical reaction and can be
represented with a chemical equation.
By carefully reading this
definition, we notice that during a thermal decomposition reaction, one reactant
breaks down to form two or more products. Keeping this definition in mind,
let’s take a look at the answer choices provided to us. In all of the answer choices,
except answer choice (B) and (E), there are two or more products formed. In answer choice (B), ZnCO3 solid
is the only product formed. And in answer choice (E), ZnO solid
is the only product formed. Because these two chemical
equations only have one product, they cannot represent thermal decomposition
reactions. So we can eliminate them.
Now, let’s take a closer look at
answer choices (C) and (D). In both of these chemical
equations, we notice that there are two reactants. By definition, a thermal
decomposition reaction happens when one reactant gets broken down by heating. Therefore, these two chemical
equations cannot represent the thermal decomposition of zinc carbonate.
So answer choice (A) must be the
correct answer. But let’s discuss a little bit more
why that is. Zinc carbonate is a type of
compound called a metal carbonate. When metal carbonates undergo a
thermal decomposition reaction, they produce a metal oxide and carbon dioxide
gas. Metal carbonates are composed of
positively charged metal ions and negatively charged carbonate ions. One unit of zinc carbonate is
composed of one zinc two plus ion and the carbonate ion. The sum of the charges of these two
ions is zero. Therefore, the chemical formula of
zinc carbonate can be written as ZnCO3.
When zinc carbonate is heated, it
will decompose to produce zinc oxide, which is the metal oxide, and carbon dioxide
gas. Therefore, the chemical equation
that shows the thermal decomposition of zinc carbonate is answer choice (A). ZnCO3 solid reacts to form ZnO
solid plus CO2 gas.
