In this explainer, we will learn how to describe and give examples of thermal decomposition reactions.
A decomposition reaction involves a compound breaking down into simpler compounds or even elements.
Hydrogen peroxide is a chemical that contains oxygen and hydrogen and is a key ingredient in many types of hair dye. Hydrogen peroxide is not a very stable compound, and it will gradually decompose over time to form water and oxygen.
The following equation shows this decomposition process:
By far, the most common type of decomposition reaction is thermal decomposition. As the name suggests, thermal decomposition describes a substance that decomposes when heated.
Many different types of chemical compounds decompose when heated. A common example is metal carbonates.
Definition: Thermal Decomposition
Thermal decomposition is the breaking down of a substance when heated to form two or more other substances.
Many metal carbonates are decomposed by heat to form a metal oxide and carbon dioxide as is shown in the following general word equation.
Reaction: Metal Carbonate Decomposition
We know that thermal decomposition reactions are chemical reactions as new substances are being formed even though we are not reacting the carbonate with any other chemicals.
Thermal decomposition can occur for many different carbonates, but not for all metal carbonates. For example, copper, lead, and zinc carbonates all break down when heated with a Bunsen burner. However, other carbonates, such as potassium carbonate, will not thermally decompose unless heated to very high temperature.
Let us consider some word and symbol equations for these reactions.
The thermal decomposition of copper carbonate is an exciting reaction to witness. We can observe some interesting color changes as blue-green copper carbonate turns into jet black copper oxide, as follows:
Similarly, lead carbonate decomposes when heated:
Example 1: Finding the Mass of the Products in a Thermal Decomposition Reaction
A student placed a small quantity of zinc carbonate into a test tube. She weighed the test tube and zinc carbonate and found their mass to be 55 g. She then heated the test tube strongly for 5 minutes. After the test tube had cooled down, she reweighed it and the remaining powder inside. What is the new mass of the test tube and zinc compound?
- More than 55 g
- Less than 55 g
- 55 g
According to the law of conservation of mass, the mass of the reactants at the start of the reaction should equal the mass of the products at the end of the reaction.
So at the end of the reaction the zinc carbonate will have reacted to become zinc oxide and carbon dioxide. However, the carbon dioxide is a gas, and so it will have escaped from the test tube. Without carbon dioxide, the new mass will only represent the test tube and remaining zinc oxide, which will be less than 55 g. The correct answer is B.
Zinc carbonate can also thermally decompose, although not as quickly as lead and copper carbonate:
Example 2: Identifying the Correct Chemical Equation for the Decomposition of Zinc Carbonate
Which of the following chemical equations shows the thermal decomposition of zinc carbonate?
A decomposition reaction involves a substance being broken down into at least two other simpler substances. In the case of a carbonate, this involves the formation of an oxide and carbon dioxide.
If we look at the options available to us, we can see the correct formula for zinc carbonate () in answer A and answer B. However, only in answer A do we have a substance being broken down, which is our definition of decomposition, and so the correct answer is A.
Let’s look at the apparatus that we might use to perform a thermal decomposition reaction.
There are a few different ways in which this apparatus can be set up, but most setups consist of
- a test tube for heating,
- a delivery tube,
- a test tube containing some limewater.
The carbonate can be heated and the carbon dioxide produced is tested so we know when the reaction has taken place. A typical setup in shown in the figure below.
In the case of copper carbonate, it is reasonably easy to observe that the reaction has taken place because no blue-green solid remains.
In the case of zinc carbonate, it is more difficult to tell because the carbonate and the solid oxide product are both white in color. To determine if zinc carbonate has decomposed, we could test for the production of carbon dioxide.
Carbon dioxide gas can be tested using an aqueous solution of calcium hydroxide known as limewater. When we bubble carbon dioxide through limewater, a chemical reaction takes place and the limewater turns cloudy.
Example 3: Using Statements to Describe a Decomposition Reaction
Which of the following statements is correct?
In this question, the diagram gives us a series of choices. The first choice relates to whether an oxide breaks down into a carbonate or a carbonate breaks down into an oxide. An oxide is a simpler substance than a carbonate.
Thermal decomposition relies on substances being broken down by heat to form simpler substances, and so the bottom statement “a carbonate breaks down into an oxide” is the correct first step.
The second choice we have relates to the other product of this reaction. A carbonate consists of a metal atom as well as atoms of carbon and oxygen.
In a decomposition reaction, the carbon and oxygen atoms are driven off as carbon dioxide gas. Limewater, on the other, hand is a saturated solution of calcium hydroxide used to test for carbon dioxide.
Given the information, it is clear to see that the second product of this decomposition reaction is carbon dioxide, and so statement 4 is the correct answer.
There is an important safety consideration to be aware of when performing a thermal decomposition reaction. When the reaction has finished, it is essential that the test tube containing the limewater is separated from the delivery tube. If this is not done, then as the equipment cools, the gas in the delivery tube and test tube will contract. The contraction will suck the cold limewater up the delivery tube and into the still very hot test tube, which can cause the glass to crack or even shatter. This effect is known as suck-back.
Thermal decomposition reactions have many applications worldwide.
For example, calcium carbonate is thermally decomposed to form calcium oxide (), also known as quicklime. Quicklime has been an important ingredient in the production of concrete.
Thermal decomposition is also useful in the roasting of certain impure ores. Malachite is an ore of copper that contains a large proportion of copper carbonate; the characteristic color can be seen in the photograph below.
Copper carbonate in malachite can be turned into copper oxide through thermal decomposition.
Example 4: Identifying the Starting Material Present in Malachite
Malachite is an impure ore used to form copper(II) oxide. The production of copper(II) oxide involves roasting the malachite in a large furnace at high temperatures. What chemical must be present in malachite to form copper(II) oxide during the reactions that take place inside the furnace?
- Copper(II) carbonate
- Copper chloride
- Copper(II) oxide
- Copper metal
- Copper sulfide
When roasting the malachite in a furnace, there are no other chemicals present that will react in significant quantities with the compounds in the ore.
In thermal decomposition reactions, metal carbonates are broken down by heat to form metal oxides and carbon dioxide.
For copper oxide to be present at the end of the roasting process, a copper compound capable of thermal decomposition must have been present in the malachite.
Looking at the list of different options available to us, the only copper substance capable of thermal decomposition to readily form copper(II) oxide from this list is copper(II) carbonate, and so the correct answer is A.
A final example is the decomposition of a substance known as sodium azide. Sodium azide () is a chemical found in certain types of car airbags.
An electronic controller detonates a mixture of the azide and other chemicals when a large impact is detected. The detonation produces enough heat to decompose the sodium azide and produce large volumes of nitrogen gas, filling the airbag:
Carbonates are not the only types of chemical that can be thermally decomposed:
Copper hydroxide, a light-blue solid, can also be decomposed by heating:
Another copper compound, copper sulfate, can also thermally decompose to produce acidic sulfur trioxide gas and copper oxide:
A decomposition reaction made famous by the work of Joseph Priestley and Antione Lavoisier was the decomposition of mercuric oxide, :
The oxygen produced in this reaction can cause a glowing splint to relight.
The last example of thermal decomposition we will consider is the thermal decomposition of sodium nitrate to form the yellow-white sodium nitrite salt and oxygen gas:
Example 5: Identifying the Common Compound Present in the Combustion of Multiple Copper Compounds
Some of the following copper compounds can be thermally decomposed; others are thermally stable:
Of those that decompose, which product is common in all cases?
There are different copper compounds that are able to undergo thermal decomposition reactions.
Considering the list of substances presented to us, copper hydroxide, copper carbonate, and copper sulfate are all capable of undergoing decomposition when heated.
For each of these substances, the decomposition produces a different gas. In the case of the hydroxide, water in the form of steam is released; with the carbonate, carbon dioxide is released; and finally, with the sulfate, sulfur trioxide is released.
However, despite different gases being produced, these three reactions all produce copper(II) oxide as well. The correct answer is therefore D, .
- Thermal decomposition reactions are decomposition reactions that take place through heating.
- Thermal decomposition is a chemical change with new substances being produced.
- Metal carbonates thermally decompose into metal oxides and carbon dioxide gas.
- The presence of carbon dioxide gas can be tested using limewater.
- When performing a thermal decomposition reaction, it is important to avoid suck-back at the end of the experiment.
- Thermal decomposition reactions are essential in many situations, including the production of concrete, the roasting of impure ores, and the reactions in car airbags.
- Mercury oxide, copper carbonate, copper sulfate, copper hydroxide, sodium nitrate, lead carbonate, and zinc carbonate can all undergo thermal decomposition reactions.