Combination, Decomposition and Combustion Reactions

In the end, being able to write chemical equations requires some background information on what to expect the products of a given reaction will be. Even with limited experience, it is possible to predict reaction products for many reactions using just a few guidelines. The table below summarizes the 2 types of reactions that can be identified by examining the number of reactants and products present. All reactions between elements are combination reactions, especially combination with oxygen. In decomposition reactions, one reactant forms two or more products. The reactant is decomposed by energy sources such as heat, light, or electricity. Combustion reactions occur when compounds rapidly combine with oxygen and require some kind of ignition like a flame.


Other Tips: Seven frequently used elements occur naturally as diatomic molecules: H2, O2, N2, Cl2, Br2, I2. Ionic species are usually in aqueous solution, (aq); States of matter,(s),(l),(g) should be included after the chemical symbol.


Combination Reactions:
General Form Important Points
X + Y   Z
  • Reactants can be elements, or compounds
  • Metals combine to make ionic compound
  • Nonmetals combine to make covalent compounds
  • Almost every element combines with oxygen to form an oxide
  • Metals form ionic oxides that react with water to form hydroxides(bases)
  • Nonmetals oxides react with water to form acids.
Examples
  • CaO(s) + H20(l) Ca(OH)2(aq)
  • SO3(g) + H20(l) H2S04(aq)
  • 2SO2(g) + O2(g) 2SO3(g)

  • N2(g) + 3H2(g) 2NH3(g)


Decomposition Reactions:
General Form Important Points
Z   X + Y
  • Usually reactant is compound and products are elements or smaller compounds
  • Occur when bonds within the reactant break and the atoms rearrange into the new products
  • Thermal decomposition requires the input of heat that is shown by a triangle over the reaction arrow ()
  • Many ionic compounds with oxygen containing anions react to form a metal oxide and a nonmetal oxide
  • Many metal oxides, chlorates and perchlorates release oxygen
  • Hydroxides, hydrates and oxygen containing acids release water
  • Electric energy causes a decomposition called electrolysis
Examples
  • CaCO3(s) CaO(s) +CO2
  • FeSO3(s) FeO(s) + SO2(g)
  • 2KClO3(s) 2KCL(s) + 3O2(g)
  • 2H2O(l) 2H2(g) +O2(g)


Combustion Reactions

:
General Form Important Points
Z + O2   CO2 + H20 
  • Combustion occurs when element or compound is exposed to oxygen
  • An ignition source like a flame is required
  • The products of combustion  always include water and carbon dioxide
  • Additional products may be the result of incomplete combustion or may result from complete combustion of reactants that contain atoms other than C and H.
Examples
  • CH4(g) + 2O2 CO2 + 2H2O(l)

Quick Quiz: How would you classify the following reaction: Li2O(s) + CO2(g) Li2CO3(s) ?
Decomposition Reaction
Combustion Reaction
Combination Reaction
None of these

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