Section 8-2: Types of Chemical Reactions
Objectives:
Define and give general equations for synthesis, decomposition, single-displacement,
and double-displacement reactions.
Classify a reaction as a synthesis, decomposition, single-displacement,
double-displacement, or combustion reaction.
List three kinds of synthesis reactions and six kinds of decomposition reactions.
List four kinds of single-displacement reactions and three kinds of double-displacement
reactions.
Predict the products of simple reactions given the reactants.
A. Introduction
1. several ways to classify chemical reactions with none being entirely
satisfactory
2. introduction to five basic types of reactions: synthesis, decomposition, singledisplacement, double-displacement, or combustion reaction.
B. Synthesis Reactions
synthesis reaction – two or more substances combine to form a new
compound; also known as a composition reaction
general equation:
A + X  AX
element + element  compound
1. Reactions of Elements with Oxygen and Sulfur; Figs. 7,8; p. 277
a. oxide – combination of an element with oxygen; general formulas of
M2O and MO: M represents either Gp. 1 or Gp. 2 metals
b. sulfide – combination of an element with sulfur; general formulas of
M2S and MS: M represents either Gp. 1 or Gp. 2 metals
8.2b
c. nonmetals also combine with oxygen to form oxides
2. Reactions of Metals with Halogens
a. Gp. 1 metals react with halogens to form ionic compounds with the
general formula of MX; M, the metal and X, the halogen
b. Gp. 2 metals react with halogens to form ionic compounds with the
general formula of MX2; M, the metal and X, the halogen
c. fluorine is so reactive that it combines with almost all metals
3. Synthesis Reactions with Oxides; Fig. 9, p. 278, bottom left
a. oxides of active metals react with water to produce metal hydroxides
b. many oxides of nonmetals in the upper right portion of the periodic
table react with water to produce oxyacids
c. certain metal oxides and nonmetal oxides react with each other in
synthesis reactions to form salts
8.2c
C. Decomposition Reactions
decomposition reaction – a reaction in which a single compound undergoes a
reaction that produces two or more simpler substances; often endothermic
general equation; reverse of a synthesis reaction
AX  A + X
compound  element + element
1. Decomposition of Binary Compounds
a. electrolysis – decomposition of a substance by an electric current
b. oxides of less-active metals located in the lower center of the periodic
table, decompose into their elements when heated; Fig. 10, p. 281
2. Decomposition of Metal Carbonates; metal carbonate breaks down to
produce a metal oxide and carbon dioxide
3. Decomposition of Metal Hydroxides; metal hydroxides decompose to yield
metal oxides and water
4. Decomposition of Metal Chlorates; metal chlorates decompose to produce a
metal chloride and oxygen
5. Decomposition of Acids; some acids decompose into nonmetal oxides and
water
8.2d
D. Single-Displacement Reactions
single-displacemnt reaction – a reaction in which one element replaces a similar
element in a compound; many occur in aqueous solutions; Fig. 11, p. 281
general equations:
A + BX  AX + B
or
Y + BX  BY + X
element + compound  compound + element
1. Displacement of a Metal in a Compound by Another Metal; a more active
metal will replace a less active one in a compound
2. Displacement of Hydrogen in Water by a Metal
a. most-active metals react vigorously with water to produce metal
hydroxides and hydrogen gas
b. less-active metals react with steam to form a metal oxide and hydrogen
gas
3. Displacement of Hydrogen in an Acid by a Metal; more-active metals react
with certain metals replacing the hydrogen in the acid; products are a
metal compound (a salt) and hydrogen gas
4. Displacement of Halogens; a more active halogen will replace a less active
halogen in a compound; fluorine is most active halogen. In Gp. 17 each
element can replace an element below it but not one above it.
8.2e
E. Double-Displacement Reactions
double-displacemnt reaction – a reaction in which the ions of two compounds
exchange places in an aqueous solution to form two new compounds
a. one of the compounds formed is usually a precipitate, an insoluble gas
that bubbles out of the solution, or a molecular compound, usually
water
b. other compound is often soluble and remains dissolved in solution
c. general equation:
AX + BY  AY + BX
1. Formation of a Precipitate; occurs when the cations of one reactant combine
with the anions of another reactant to from an insoluble or slightly soluble
compound
2. Formation of a Gas; one of the products is an insoluble gas that bubbles out of
the mixture
3. Formation of Water; one of the products is a very stable molecular compound
such as water; occurs in acid-base neutralization
F. Combustion Reactions
1. combustion reaction – a reaction in which a substance combines with oxygen,
releasing a large amount of energy in the form of light and heat
2. products depend upon the amount of oxygen present
a. adequate oxygen: products are carbon dioxide and water
b. limited supply of oxygen: products are carbon monoxide and water
c. very limited supply of oxygen: products; carbon black or soot and water
8.2e
G. Identifying Reactions and Predicting Products; series of questions to ask
1. Is there only one reactant?
a. No: Proceed to question 2.
b. Yes: Decomposition Reaction
2. Are the reactants two elements or two simple compounds?
a. No: Proceed to question 3.
b. Yes: Probably a Synthesis Reaction
3. Are the reactants oxygen and a hydrocarbon?
a. No: Proceed to question 4.
b. Yes: Combustion Reaction
4. Are the reactants an element and a compound other than a hydrocarbon?
a. No: Proceed to question 5.
b. Yes: Single-Displacement Reaction
5. Are the reactants two compounds composed of ions?
a. No: Go back to question 1 because you might have missed the proper
category.
b. Yes: Double-Displacement Reaction