Video Transcript
In this video, we will learn how to
describe and give examples of thermal decomposition reactions. We’ll explain why thermal
decomposition is a chemical change, use chemical equations to represent these
reactions, and show the safety precautions related to performing thermal
decomposition in the lab.
A decomposition reaction is a type
of chemical reaction in which a compound breaks down into simpler substances. The reactant of a decomposition
reaction is a compound, and the products of a decomposition reaction may be
elements, compounds, or both. The most common type of
decomposition reaction is called thermal decomposition.
Thermal decomposition is the
breaking down of a substance when heated to form two or more other substances. The factor which causes the
chemical reaction to occur is heat. In a chemical equation, the 𝛥
symbol, which looks like a triangle, represents the addition of heat. We know that heat can be added to a
substance to change its state, such as to melt ice, and that this would be
considered a physical change.
In contrast, in a thermal
decomposition reaction, heat causes a chemical change to occur. New substances are formed, and we
may observe signs that a chemical change has occurred, such as the formation of a
gas, a color change, or an odor. Many different types of compounds
can undergo thermal decomposition reactions. In fact, many metal carbonates can
be decomposed when heated to produce a metal oxide and carbon dioxide.
Metal carbonates are chemical
compounds composed of positively charged metal cations and negatively charged
carbonate anions. The carbonate anion has a chemical
formula of CO32−. Some examples of metal carbonates
that undergo thermal decomposition reactions are copper(II) carbonate, lead(II)
carbonate, and zinc carbonate. Potassium carbonate is another
example of a metal carbonate, but it requires much more heat to decompose compared
to the other carbonates listed. The chemical formulas of metal
carbonates are always written with the metal cations first and the carbonate anion
second.
In copper(II) carbonate, the copper
ion has a two plus charge and the carbonate ion has a two minus charge. Chemical compounds are neutral. So the total amount of charge must
be zero, which is true in this case. Therefore, the chemical formula of
copper(II) carbonate is CuCO3. For both lead(II) carbonate and
zinc carbonate, the sum of the ionic charges are also zero. Therefore, the chemical formulas of
these two compounds are PbCO3 and ZnCO3.
When a metal carbonate is heated
and it decomposes, there are two products: a metal oxide and carbon dioxide. Let’s say that solid lead(II)
carbonate in a test tube is heated with a Bunsen burner, causing the lead(II)
carbonate to decompose. Lead(II) carbonate, which is a
white solid, is the reactant of the thermal decomposition reaction. After the reaction, a solid remains
in the test tube, but the solid is yellow. This color change is a sign that a
chemical reaction has taken place. The yellow solid is known as
lead(II) oxide and has a chemical formula of PbO. But lead(II) oxide is only one of
the products of the reaction. The other product is carbon
dioxide.
If we compare the mass of the solid
in the test tube before the reaction to the solid in the test tube after the
reaction, we would see that the mass decreased. We would expect the mass to be the
same before and after the reaction because the law of conservation of mass states
that the mass of the products should equal the mass of the reactants in a chemical
reaction. The problem is that the carbon
dioxide gas formed in the reaction escaped from the test tube before we could find
the mass.
Now that we know that metal
carbonates can be decomposed by heating to produce a metal oxide and carbon dioxide
gas, let’s learn about how these reactions are represented using chemical
equations. Zinc carbonate is a metal carbonate
that decomposes when heated. Let’s write a word equation to
represent this reaction. Zinc carbonate is the reactant of
the reaction. So we will write it before the
reaction arrow. Above the reaction arrow, we need
to use the 𝛥 symbol to represent heating. There are two products of the
reaction. One of the products is zinc oxide,
which is a metal oxide, and the other product is carbon dioxide. Carbon dioxide is always one of the
products in the thermal decomposition of a metal carbonate.
Instead of representing the
reaction using the names of the substances involved, we could also write a chemical
equation using chemical formulas. The chemical formula of zinc
carbonate is ZnCO3. Zinc carbonate is a solid, so we
can also include the state symbol “s.” The product zinc oxide is also a
solid and has the chemical formula ZnO. And the chemical formula of carbon
dioxide, which we are quite familiar with, is CO2. Carbon dioxide is a gas, so we will
use the “g” state symbol here.
Both zinc carbonate and zinc oxide
have a similar appearance; they are both white solids. How can we determine if a chemical
reaction has taken place? We need a way to detect the carbon
dioxide that has been produced. One apparatus used to detect the
formation of carbon dioxide gas uses the following parts: a test tube for heating,
which contains the solid zinc carbonate and is clamped to a stand to hold it in
place. A Bunsen burner with a controlled
blue flame is used to continuously heat the zinc carbonate in the test tube.
As carbon dioxide is being produced
inside the test tube that is being heated, it travels through a delivery tube and
into a test tube filled with limewater. Limewater is an aqueous solution of
calcium hydroxide. As carbon dioxide gas bubbles
through the limewater, the limewater solution begins to turn cloudy. This helpful visual sign tells us
that zinc carbonate has indeed decomposed into zinc oxide and carbon dioxide.
Now, we need to discuss the safety
precautions required when conducting thermal decomposition in the lab. Before we begin our experiment, we
should put on our safety goggles; inspect the glassware, such as test tubes, for
chips or cracks; and clear our work area of any flammable materials. After we see the limewater turn
cloudy, the reaction is complete.
After turning off the Bunsen
burner, it’s extremely important that the delivery tube be disconnected from the
test tube containing limewater. If the delivery tube is not
disconnected from the test tube full of limewater, then as the equipment is cooling
down, limewater could flow back into the test tube that was being heated, which
could cause the glass to crack or even shatter. The effect is known as suck back
and can be prevented by removing the delivery tube as soon as the Bunsen burner has
been turned off. Heating chemicals in glassware in
the lab is dangerous. Be sure to adhere to the safety
precautions and the instructions of your teacher.
There are other types of chemical
compounds that can undergo thermal decomposition reactions. For example, copper(II) hydroxide
decomposes by heating to form copper(II) oxide and water. And copper(II) sulfate decomposes
by heating to produce copper(II) oxide and sulfur trioxide gas. These two reactions are very
similar. They both produce copper(II) oxide,
which is a metal oxide. Heating copper hydroxide produces
water vapor, while heating copper(II) sulfate produces sulfur trioxide gas. If we compared these two reactions
to the thermal decomposition of copper(II) carbonate, we’d see that all three of the
reactions produce copper(II) oxide but that different gases are being produced in
each reaction.
Let’s discuss two more thermal
decomposition reactions. Mercury(II) oxide decomposes by
heating to form elemental mercury and oxygen gas. And sodium nitrate decomposes upon
heating to form sodium nitrite and oxygen gas. We notice that in both of these
decomposition reactions, oxygen gas is one of the products. In the lab, we cannot see the
oxygen gas being produced. However, if we ignite a wooden
splint and then gently blow the flame out so that the splint is glowing, then when
we place the glowing wooden splint into the test tube, the splint will reignite and
burn brightly if oxygen gas is being produced.
Before we summarize what we’ve
learned about thermal decomposition in this video, let’s take a look at a
question.
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.
Let’s summarize what we have
learned about thermal decomposition. First of all, thermal decomposition
is a chemical change. New substances are produced, and we
may see signs of a chemical reaction, such as a gas being produced or a color
change. A thermal decomposition reaction
happens when a substance is heated and breaks down into two or more new
substances. When a metal carbonate thermally
decomposes, the products of the reaction are a metal oxide and carbon dioxide
gas. The presence of carbon dioxide gas
produced can be detected by bubbling it through limewater. The limewater, which is an aqueous
solution of calcium hydroxide, will turn cloudy in the presence of carbon dioxide
gas.
We need to follow safety
precautions when heating chemicals in the lab. We should always wear our goggles
and inspect glassware for chips or cracks before using it in an experiment. And it’s very important to avoid
suck back, which can lead to a situation in which a hot test tube full of chemicals
could shatter. Finally, we also learned that
besides metal carbonates, other substances also undergo thermal decomposition
reactions, such as copper hydroxide, copper sulfate, sodium nitrate, and mercury
oxide.
