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Classifying Chemical Reactions
Chapter 8
Types of Chemical Rxns
Classifying reactions There are 5 general types of reactions
Combination (aka synthesis or composition) Decomposition Single-replacement Double-replacement (acid-base & precipitation) Combustion
Not all chemical reactions fit neatly into only one of these classes - sometimes, one reaction may fit 2 or 3 different types (or none of the types)
Combination reactions (synthesis)
Two or more substances combine to form ONE product. The product will always be a compound
Metal + nonmetal ionic compound EX: 2Na + Cl2 2NaCl
Nonmetal + nonmetal molecular compound Sometimes, 2 nonmetals may produce more than one
product S + O2 SO2
2S + 3O2 2 SO3
Decomposition reactions
ONE single compound is broken down into two or more products
The products can be any combination of elements or compounds
It is usually very difficult to predict the products of decomposition reactions
Most decomposition reactions usually require energy in the form of heat, light, or electricity
EX: 2KClO3 + heat 2KCl + 3O2
CaCO3 + heat CaO + CO2
Single-replacement reactions A free element replaces an element within an ionic
compound (or an acid) Also known as single-displacement reactions Whether one metal will replace another can be
determined by the chemical reactivities of the two metals
The activity series lists metals in order of decreasing reactivity
A metal can replace any metal listed below it on the series EX: Zn + CuSO4 Cu + ZnSO4
Zn + Na2SO4 No reaction
Single-replacement reactions (cont.)
A halogen can also replace another halogen in a compound
This replacement is usually limited to halogens The activity of halogens decreases as you move
down the group Fluorine can replace any of the halogens,
chlorine can replace any halogen beneath it, etc.
EX: 2NaBr + Cl2 2NaCl + Br2
NaBr + I2 No reaction
Double-replacement reactions
Precipitation reactions involve an exchange of positive ions between two ionic compounds (or an ionic compound and an acid)
Generally take in aqueous solution Characterized by the formation of a
precipitate (a solid formed from mixing two aqueous compounds)
EX: BaCl2(aq) + Na2SO4(aq) BaSO4(s) + NaCl(aq)
Double-replacement reactions Acid-Base reactions involve an exchange of
positive ions between an acid and a metal hydroxide
Generally take in aqueous solution Characterized by the formation of water and
a salt (an ionic compound that is not an oxide or hydroxide)
EX: Ba(OH)2(aq) + H2SO4(aq) BaSO4(s) + H2O(l)
Combustion reactions
An element or compound reacts with oxygen, often producing energy in form of heat and light
These reactions commonly involve hydrocarbons, compounds of H and C
The complete combustion of a hydrocarbon produces CO2 and H2O
If the supply of O2 is insufficient during the reaction, combustion will be incomplete
A large amount of heat is usually released
EX: C3H8 + 5O2 3CO2 + 4H2O + heat