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