Decomposition
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Topic Summary & Highlights
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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
Memorize Common Ions: Focus on learning the common polyatomic ions, charges, and patterns.
Roman Numerals for Transition Metals: Practice associating transition metals with their possible charges.
Cross-Method for Formulas: To determine the correct formula, use the “criss-cross” method to balance charges between cations and anions.
Types of Decomposition Reactions
Thermal Decomposition:
The compound decomposes when heated.
Example: CaCO3→CaO+CO2 (heating calcium carbonate produces calcium oxide and carbon dioxide).
Electrolytic Decomposition:
The compound decomposes when an electric current is passed through it.
Example: 2H2O→2H2+O2 (electrolysis of water produces hydrogen and oxygen gases).
Photodecomposition (Photolysis):
The compound decomposes when exposed to light.
Example: 2AgCl→2Ag+Cl2 (sunlight decomposes silver chloride into silver and chlorine gas).
Catalytic Decomposition:
A catalyst speeds up the decomposition without being consumed.
Example: 2H2O2→2H2O+O2 (decomposition of hydrogen peroxide is often catalyzed by manganese dioxide).
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.
Predicting Products in Decomposition Reactions
Binary Compounds
Compounds made of two elements decompose into their individual elements.
Example: HgO → Hg + O2
Metal Carbonates
Metal carbonates decompose upon heating to produce a metal oxide and carbon dioxide.
Example: CaCO3 → CaO + CO2
Metal Hydroxides: Metal hydroxides decompose upon heating to produce a metal oxide and water.
Example: Ca(OH)2 → CaO + H2O
Metal Chlorates: Metal chlorates decompose upon heating to produce a metal chloride and oxygen gas.
Example: 2 KClO3 → 2 KCl + 3 O2
Oxyacids: Oxyacids (acids containing oxygen) decompose to form nonmetal oxides and water.
Example: H2CO3→CO2+H2O