Decomposition Reaction

When it's fall, the trees turn into a multicolor display of reds, yellows, and oranges. As the leaves fall to the ground, they will decompose and release Carbon back into the atmosphere.

Decomposition Reaction Decomposition Reaction

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Table of contents

    When we talk about decomposition in chemistry, we are often referring to the decomposition reaction. In this article, we will learn what this reaction is, what its types are, and look at some examples.

    Tree, Wikimedia Commons

    Fig. 1: When it's fall, the trees turn into a multicolor display of reds, yellows, and oranges.

    • The article is about decomposition reactions.
    • First, we will define what a decomposition reaction is and then look at the basic formula.
    • Then, we will learn about and look at some examples of the different types of decomposition reaction: heat-induced, electricity-induced, and light-induced.
    • Next, we will compare decomposition reactions to a similar type of reaction called a radioactive decay reaction.
    • Lastly, we will summarize the characteristics of a decomposition reaction.

    Decomposition reaction definition

    A decomposition reaction is a reaction where a compound breaks down into two or more simpler substances.

    These “simpler substances” can be either compounds or elements. They are considered simpler since they have fewer atoms within them (ex: compound with 4 elements decomposes into two compounds with 2 elements).

    Decomposition reactions usually require an input of energy, such as heat, to proceed.

    Decomposition reaction formula

    The basic formula for a decomposition reaction is:

    $$AB \rightarrow A + B$$

    While the reaction looks like the compound is splitting into its elements, it's really just splitting into simpler things,

    For example, let's look at the decomposition of calcium carbonate:

    $$CaCO_{3\,(s)} \rightarrow CaO_{(s)} + CO_{2\,(g)}$$

    Here we see that the products aren't just Ca and CO3, which is what you might assume from the general formula.

    The main thing to remember from this formula is that one species into breaking into two or simpler (and stable) things.

    Decomposition reaction types

    As I mentioned earlier, these reactions typically require some addition of energy to proceed. Because of this, we can divide decomposition reactions into three types:

    1. Thermolysis:
      • Heat-induced decomposition.
    2. Electrolysis:
      • Electricity-induced decomposition.
    3. Photolysis:
      • Light-induced decomposition.

    Essentially, we can sort these reactions based on what type of energy causes the compound to split/decompose.

    Decomposition reaction examples

    Now that we know the different types, let's look at some examples:

    First up, we have the decomposition of potassium chlorate, which is a thermolytic reaction:

    $$2KClO_{3\,(g)} \xrightarrow {heat} 2KClO_{(s)} + 2O_{2\,(g)}$$

    As will also be the case with the next examples, this energy is added, so the reaction can exceed the Activation Energy.

    Activation Energy is the energy required for a reaction to proceed. All reactions have that requirement.

    Adding heat gives the reaction the “push” it needs to get past that energy barrier and proceed.Also, the state of the decomposing compound isn't always the same as the products made from it. As you can see in the above reaction, even though potassium chlorate is a gas, it produced a solid potassium hypochlorite (KClO). When a compound decomposes, it will always produce the most stable form of its components.

    Next we have electrolytic decomposition reaction, with the decomposition of water:

    $$2H_2O_{(l)} \xrightarrow {elec} 2H_{2\,(g)} + O_{2\,(g)}$$

    The way this Electrolysis works is that a current is passed through the water, which helps break down the water.

    Below is what this process might look like:

    Decomposition reactions Water electrolysis

    Fig.2: Water electrolysis

    The battery is connected to the cable, which sends an electric current through the water. The upside-down tubes are there to collect the hydrogen and oxygen gas formed as the water decomposes.

    The last type of decomposition reaction is a photolysis reaction, shown below by the decomposition of silver chloride (AgCl):

    $$2AgCl_{(s)} \xrightarrow {light} 2Ag_{(s)} + Cl_{2\,(g)}$$

    This reaction can happen when silver chloride is exposed to sunlight. The sunlight gives AgCl the energy is needs to decompose.

    Light is sometimes written as hv in a reaction. Hv represents the energy of a photon (light particle), where h is Plank's constant and v is the frequency of the light.

    When this reaction occurs, there is a color change. AgCl is white, but silver (Ag) is grey, therefore the color change is a good indicator of whether the reaction occurred.

    The photolysis of water in plants

    Photolysis also plays a key part in photosynthesis. Light is absorbed by chlorophyll in a plant's cells. This light is used to split water into hydrogen and oxygen.

    The oxygen is released into the air as a byproduct. We breathe in this oxygen, which is why plants are so important!

    The hydrogen is used for further steps in the photosynthesis process, so the plant can produce food for itself.

    Decomposition versus decay

    It's important to not confuse decomposition with a similar process called nuclear or radioactive decay.

    Radioactive decay is a process where an unstable atomic nucleus emits radiation and is transformed into the nucleus of one or more other elements.

    Essentially, radioactive atoms are unstable for one reason or another (such as too many neutrons for the amount of protons), so they emit a type of radiation (such as a neutron) to stabilize themselves. Decay reactions are similar to decomposition reactions, since they involve one species breaking down into two or more simpler ones (for decay, simpler means smaller atomic number). However, there are key differences, such as that decay releases energy instead of requiring it, and it involves the breaking down of an element instead of a compound.

    Characteristics of decomposition reaction

    To summarize, let's break down the different characteristics of decomposition reactions:

    • One compound breaks down into two or more simpler species
    • These reactions usually require energy, such as:
      • heat
      • electricity
      • light
    • The products are in their most stable form, meaning:
      • They may be a different state of matter
      • They are not always just the component elements or ions

    Decomposition Reactions - Key takeaways

    • A decomposition reaction is a reaction where a compound breaks down into two or more simpler substances.
    • The basic formula for a decomposition reaction is:

      $$AB \rightarrow A + B$$

    • There are three types of decomposition reaction:

      1. Thermolysis
        • Heat-induced decomposition
      2. Electrolysis
        • Electricity-induced decomposition
      3. Photolysis
        • Light-induced decomposition
    • Decomposition is separate from radioactive decay
      • Radioactive decay is a process where an unstable atomic nucleus emits radiation and is transformed into the nucleus of one or more other elements.
    Frequently Asked Questions about Decomposition Reaction

    What is decomposition reaction?

    A decomposition reaction is a reaction where a compound breaks down into two or more simpler substances.

    What are the three types of decomposition reactions?

    The three types of decomposition reaction are:

    1. Thermolysis
      • Heat-induced decomposition
    2. Electrolysis
      • Electricity-induced decomposition
    3. Photolysis 
      • Light-induced decomposition


    What is the decomposition of CaCO3?

    The decomposition reaction for calcium carbonate (CaCO3) is:


    CaCO3 (s) --> CaO(s) + CO2 (g)

    What are three examples of decomposition reaction?

    Here are some examples of a decomposition reaction:


    CaCO3 (s) --> CaO(s) + CO2 (g)


    2H2O(l) --> 2H2 (g) + O2 (g)


    2KClO(g) --> 2KClO(s) + 2O2 (g)


    What happens when ferrous sulphate decomposes?

    When heat is added to ferrous sulphate (FeSO4) it decomposes as in the reaction below:


    2FeSO4(s) --> Fe2SO4 (s) + SO2 (g) + SO3 (g)

    Test your knowledge with multiple choice flashcards

    What is the basic formula for a decomposition reaction?

    True or False: In a decomposition reaction, either an element or compound can decompose

    True or False: Decomposition reactions release heat

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