Decomposition Reaction

Decomposition reaction definition

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.

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:

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.

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.

Decomposition versus decay

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

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