Thermal Decomposition Reactions: Definition & Equation

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Thermal energy can be used to physically and chemically change substances. In this lesson, we will go through decomposition reactions that are generated by heating various substances.


Here's a riddle for you. What is used in most recipes, but not eaten? The answer is...heat! Heat can be used to simply warm food, which is a physical change. Enough heat can chemically change food, which is a chemical change. Think of the difference in flavor of a warm piece of bread and a burnt piece of bread. The burnt bread is a result of heat causing a chemical change.

Thermal energy is the the level of movement of the molecules in a material. When molecules move they rub against each other and the friction between them generates heat, which can be transferred from one substance to another. The more extensive the movement of the molecules, the more heat generated.

Decomposition reactions involve a single substance being separated into multiple compounds or elements. Let's look at how heat can decompose substances physically and chemically.

Physical Decomposition

Physical decomposition is when substances are separated that are physically attached. Let's look at an example.

Hydrate Copper(II) Sulfate

Hydrated copper(II) sulfate is a blue crystal. The word 'hydrated' means that water molecules are physically attached to a compound. In the case of hydrated copper(II) sulfate there are five water molecules attached and its formula is CuSO4 • 5H2 O.

Hydrated copper(II) sulfate is a blue crystal with water molecules physically attached to the compound

Let's look at the equation for the physical decomposition of this substance.

CuSO4 • 5H2 O (s) + heat → CuSO4 (s) + 5H2 O (g)

Copper(II) sulfate without the physically attached water molecules is a white solid. It can be re-hydrated by simply adding water, which will make it blue again. When water is added to the copper(II) sulfate, heat is released making this a reversible physical reaction.

CuSO4 (s) + 5H2 O (g) → CuSO4 • 5H2 O (s) + heat

Chemical Decomposition

Thermal energy can also break apart compounds chemically, a process called chemical decomposition.

Copper(II) Carbonate

Copper(II) carbonate is an ionic compound with the chemical equation CuCO3. It is a green compound.

Copper(II) carbonate is a green compound

When it is heated, it decomposes into copper(II) oxide (a black solid) and carbon dioxide gas.

CuCO3 (s) + heat → CuO (s) + CO2 (g)

Copper (II) carbonate can by heated to decompose into copper(II) oxide, a black solid

Potassium Permanganate

Potassium permanganate is an ionic compound with the chemical formula KMnO4. When it is heated it decomposes into potassium manganate, magnesium oxide and oxygen gas.

2KMnO4 (s) + heat → K2 MnO4 (s) + MnO2 (s) + O2 (g)

Notice that both sides of the equation have the MnO4 ion. They are different even though they look the same. The difference is the oxidation state of manganese. Oxidation state is the electric charge of an ion.

The manganese in potassium permanganate has the oxidation state +7, and the manganese in potassium manganate has the oxidation state +6. This is the reason two potassium ions are needed in potassium manganate and only one is required in potassium permanganate.

The subscripts are multiplied by the superscripts to get the net charge. All of the charges have to cancel.

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