Core Concepts
In this tutorial, you will learn what a decomposition reaction is and its types. You will also get to explore some of its fascinating examples!
Topics Covered in Other Articles
- What is a Chemical Reaction?
- Types of Chemical Reactions
- Balancing Chemical Equations
- Endothermic vs Exothermic Reactions
- Physical and Chemical Properties
- Reaction Order
What is a Decomposition Reaction?
Decomposition reactions are chemical reactions in which a starting compound is broken down into two or more simpler constituents. These simpler constituents can be either elements or compounds, or both. Here is the general formula of a decomposition reaction:
AB → A + B
(AB is the reactant. A and B are the products)
Decomposition reactions are generally endothermic because the breaking of chemical bonds requires an input of energy. Yes, the formation of products does release energy when they form new bonds, but such energy isn’t usually enough to compensate for the initial energy used to break the bonds of the reactant.
However, in rare cases where the products’ bond-forming energy is greater than the reactant’s bond-breaking energy (likely because the reactant is highly reactive), the decomposition reaction can be exothermic. The decomposition of hydrogen peroxide, which we will explore later, is one such case.
Types of Decomposition Reactions
Remember when I said that in order to start a decomposition reaction, you need an input of energy? Well, that input of energy can be heat, light, or electricity.
Thermal decomposition (Thermolysis)
Thermolysis happens when heat chemically breaks down a compound. That is, heat is a reactant; we supply heat to get the reaction going.
Let’s take a look at an example: the decomposition of calcium carbonate (CaCO3).
CaCO3 (s) + Δ → CaO (s) + CO2 (g) (Δ stands for heat)
(I put Δ as a reactant because it’s easier to do so on a computer. You are perfectly fine putting the Δ on the reaction arrow. The same is true for the hν below.)
If you were to perform this reaction in real life, you’d notice that your initial solid sample of CaCO3 would become quite crumbly after you heated it, and some gas would rise during the process. These are signs that some chemical changes have occurred. Indeed, your CaCO3 has decomposed into the crumbly CaO and CO2 gas.
Photodecomposition (Photolysis)
Photolysis happens when light (or more specifically, photons) chemically breaks down a compound. An example of photolysis can be found among the light-dependent reactions of a plant’s photosynthesis process:
H2O + hν → 2 H+ + 2 e– + 1/2 O2 (hν stands for light)
Here, water absorbs light (specifically, 2 photons) and decomposes into protons (H+), electrons (e–), and molecular oxygen.
Source: Wikimedia Commons.
Electrolysis
We have an entire article dedicated to electrolysis: Electrolytic Cells and Electrolysis! Check it out if you want to know more about this electrifying technique (pun definitely intended)!
Anyway, electrolysis happens when electricity chemically breaks down a compound. An electrolysis reaction is the combination of 2 simultaneous half-reactions (a reduction reaction and an oxidation reaction).
Let’s take a look at an example: the electrolysis of sodium chloride (NaCl).
2 NaCl (l) → 2 Na (l) + Cl2 (g)
In this reaction, NaCl decomposes into liquid sodium metal and chlorine gas. The 2 half-reactions involved are the reduction of Na+ ions and the oxidation of Cl– ions.
Source: Wikimedia Commons.
Examples of Decomposition Reactions
As promised earlier, let’s explore the decomposition of hydrogen peroxide (H2O2).
2 H2O2 (aq) → 2 H2O (l) + O2 (g)
Since the oxygen-oxygen bond in hydrogen peroxide is highly unstable, hydrogen peroxide stored in bottles decomposes into water and oxygen gas all time. However, if you want something more fun (and dangerous), you can try speeding up its decomposition by using a catalyst like potassium permanganate (KMnO4), as shown in the video below. Such a reaction is highly exothermic—it can reach very high temperatures and release a lot of gas (most of which is water vapor). Thus, please don’t carry out the reaction at home, unless you’re qualified to do so.
By the way, the decomposition of hydrogen peroxide also plays a crucial part in the making of the Elephant Toothpaste!
Another popular decomposition reactions are the decomposition of metal hydroxides. Metal hydroxides decompose into metal oxides and water upon heating. Here are some examples:
- Decomposition of sodium hydroxide: 2 NaOH (s) + Δ → Na2O (s) + H2O (g)
- Calcium hydroxide: Ca(OH)2 + Δ → CaO + H2O
- Iron (III) hydroxide: 2 Fe(OH)3 + Δ → Fe2O3 + 3 H2O