Decomposition

Core Concept

Decomposition reactions involve a single compound breaking down into two or more simpler substances. These reactions follow the general form: AB→A+B

Where:

AB is a single compound (the reactant).
A and B are the products, which can be elements or simpler compounds.

Decomposition reactions are often driven by energy input, such as heat, light, or electricity.

Practice Tips

Spot the Single Reactant: If you only see one chemical formula on the left side of the arrow, it is a decomposition reaction.
Identify the Energy Source: Look for symbols like $\Delta$ (delta) above the reaction arrow, which indicates that heat was added to cause the breakdown.
Check for Diatomics: Remember that elements like Oxygen ($O_2$) and Hydrogen ($H_2$) must be written with a subscript of 2 when they are produced as pure gases.
Balance the Equation Last: Focus on identifying the correct formulas for the simpler products before trying to balance the overall chemical equation.

Test Yourself

Assorted Multiple Choice

What is typically required for a decomposition reaction to occur, where a single compound breaks down into two or more simpler substances?

Practice Problems & Worked Out Examples 🔒

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Recognizing Decomposition Reactions

To identify a decomposition reaction:

Look for a single reactant that produces multiple products.
Common indicators include heat, light, electricity, or a catalyst, which are often required to drive the reaction.

Types of Decomposition Reactions

Thermal Decomposition: The compound decomposes when heated.

Example: $CaCO_{3} \rightarrow CaO + CO_{2}$

Heating calcium carbonate produces calcium oxide and carbon dioxide.

Electrolytic Decomposition: The compound decomposes when an electric current is passed through it.

Example: $2H_{2}O \rightarrow 2H_{2} + O_{2}$

Electrolysis of water produces hydrogen and oxygen gases.

Photodecomposition (Photolysis)

The compound decomposes when exposed to light.

Example: $2AgCl \rightarrow 2Ag + Cl_{2}$
Sunlight decomposes silver chloride into silver and chlorine gas.

Catalytic Decomposition

A catalyst speeds up the decomposition without being consumed.

Example: $2H_{2}O_{2} \rightarrow 2H_{2}O + O_{2}$

Decomposition of hydrogen peroxide is often catalyzed by manganese dioxide ($MnO_{2}$).

Predicting Products in Decomposition Reactions

Binary Compounds

Compounds made of two elements decompose into their individual elements.

Example: $2HgO \rightarrow 2Hg + O_{2}$

Metal Carbonates

Metal carbonates decompose upon heating to produce a metal oxide and carbon dioxide.

Example: $CaCO_{3} \rightarrow CaO + CO_{2}$

Metal Hydroxides

Metal hydroxides decompose upon heating to produce a metal oxide and water.

Example: $Ca(OH)_{2} \rightarrow CaO + H_{2}O$

Metal Chlorates

Metal chlorates decompose upon heating to produce a metal chloride and oxygen gas.

Example: $2KClO_{3} \rightarrow 2KCl + 3O_{2}$

Oxyacids

Oxyacids (acids containing oxygen) decompose to form nonmetal oxides and water.

Example: $H_{2}CO_{3} \rightarrow CO_{2} + H_{2}O$

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