Decomposition
Related Examples and Practice Problems
Additional Worked Out Examples/ Practice
Identifying classification types: Differentiation between elements, compounds or mixtures and homogeneous and heterogenous mixtures
Separation techniques: Selected and explaining limitation of appropriate separation
Relating Properties to Composition: Predicting classification based on descriptive properties
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\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3→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\text{2H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_22H2O→2H2+O2 (electrolysis of water produces hydrogen and oxygen gases).
Photodecomposition (Photolysis):
The compound decomposes when exposed to light.
Example: 2AgCl→2Ag+Cl2\text{2AgCl} \rightarrow 2\text{Ag} + \text{Cl}_22AgCl→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\text{2H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_22H2O2→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\text{HgO} \rightarrow \text{Hg} + \text{O}_2HgO→Hg+O2
Metal Carbonates:
Metal carbonates decompose upon heating to produce a metal oxide and carbon dioxide.
Example: CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3→CaO+CO2
Metal Hydroxides:
Metal hydroxides decompose upon heating to produce a metal oxide and water.
Example: Ca(OH)2→CaO+H2O\text{Ca(OH)}_2 \rightarrow \text{CaO} + \text{H}_2\text{O}Ca(OH)2→CaO+H2O
Metal Chlorates:
Metal chlorates decompose upon heating to produce a metal chloride and oxygen gas.
Example: 2KClO3→2KCl+3O2\text{2KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_22KClO3→2KCl+3O2
Oxyacids:
Oxyacids (acids containing oxygen) decompose to form nonmetal oxides and water.
Example: H2CO3→CO2+H2O\text{H}_2\text{CO}_3 \rightarrow \text{CO}_2 + \text{H}_2\text{O}H2CO3→CO2+H2O