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.
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.
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:
- Thermolysis:
- Heat-induced decomposition.
- Electrolysis:
- Electricity-induced decomposition.
- 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:
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.
Explanations 
Textbooks 
Exams 
Magazine 
