Lesson Video: Thermal Decomposition | Nagwa Lesson Video: Thermal Decomposition | Nagwa

Lesson Video: Thermal Decomposition Science • Third Year of Preparatory School

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In this video, we will learn how to describe and give examples of thermal decomposition reactions.

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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.

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