Types of Chemical Reactions

Unit 4a

There are thousands of known chemical reactions.

We can’t memorize them all.

But, we can try to classify or characterize them to help us understand what is going on.

We will look at five basic types of chemical reactions - but this method of classification is not the only way to look at chemical reactions.

Composition

Decomposition

Single-Replacement

Double-Replacement

Combustion

The Five Types of Reactions

We will look at each of these types of reactions individually in more detail.

composition reactions

A + X → AX

Two or more substances combine to form a different compound.

A typical example is the formation of a metal oxide or sulfide.

2Mg(s) + O2(g) → 2MgO(s)

16Rb(s) + S8(s) → 8Rb2S(s)

A + X → AX

Two or more substances combine to form a different compound.

Non-metals will also react with oxygen or sulfur to form compounds.

S8(s) + O2(g) → 8SO2(g)

C(s) + O2(g) → CO2(g)

2H2(g) + O2(g) → 2H2O(g)

composition reactions

2Na(s) + Cl2(g) → 2NaCl(s)

Sr(s) + Br2(l) → SrBr2(s)

Metals will also react with halogens to form new compounds.

A + X → AX

Two or more substances combine to form a different compound.

composition reactions

Metal oxides can also react with water to produce hydroxides.

CaO(s) + H2O(l) → Ca(OH)2(s)

A + X → AX

Two or more substances combine to form a different compound.

composition reactions

Metal oxides can also react with water to produce hydroxides.

CaO(s) + H2O(l) → Ca(OH)2(s)

A + X → AX

Two or more substances combine to form a different compound.

composition reactions

2H2O(l) → 2H2(g) + O2(g)

The simplest example is the decomposition of a binary compound into its elements.

This is called electrolysis.

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + XThis is the opposite of the synthesis reaction.

The simplest example is the decomposition of a binary compound into its elements.

2HgO(s) → 2Hg(l) + O2(g)

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + XThis is the opposite of the synthesis reaction.

Metal carbonates decompose into metal oxides and carbon dioxide.

CuCO3(s) → CuO(s) + CO2(g)

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + X

Metal hydroxides decompose into metal oxides and water.

Ca(OH)2(s) → CaO(s) + H2O(g)

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + X

Metal chlorates decompose into metal chlorides and oxygen.

2KClO3(s) → 2KCl(s) + 3O2(g)

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + X

Certain acids decompose into non-metal oxides and water.

This is what happens when a soda gives off fizz.

H2CO3(aq) → CO2(g) + H2O(l)

Decomposition ReactionsIn a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances.

AX → A + X

Single-Replacement Reactions

In these reactions, one element replaces a similar element in a compound.

A + BX → AX + Bor

Y + BX → BY + XTypical single-replacement reactions include:Replacement of a metal in a compound

with a more active metal.2 Al(s) + 3Pb(NO3)2(aq) → 3Pb(s) +

2Al(NO3)3(aq)Aluminum is more active than lead.

Replacement of hydrogen in water by a metal.

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Single-Replacement Reactions

In these reactions, one element replaces a similar element in a compound.

A + BX → AX + Bor

Y + BX → BY + XTypical single-replacement reactions include:

Replacement of hydrogen in an acid by a metal.

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

Single-Replacement Reactions

In these reactions, one element replaces a similar element in a compound.

A + BX → AX + Bor

Y + BX → BY + XTypical single-replacement reactions include:

Replacement of one halogen in a compound by another, more active halogen.Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)

Single-Replacement Reactions

In these reactions, one element replaces a similar element in a compound.

A + BX → AX + Bor

Y + BX → BY + XTypical single-replacement reactions include:

Replacement of one halogen in a compound by another, more active halogen.

Chlorine is more active than bromine.

Br2(g) + KCl(aq) → no reaction

Single-Replacement Reactions

In these reactions, one element replaces a similar element in a compound.

A + BX → AX + Bor

Y + BX → BY + XTypical single-replacement reactions include:

double-Replacement Reactions

In these reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY → AY + BXOne of the compounds formed is a precipitate, an insoluble gas, or a molecular compound (such as water).

2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)

Examples of reactions that form precipitates include:

double-Replacement Reactions

In these reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY → AY + BX

yellow solid

Examples of reactions that form precipitates include:

K2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2KCl(aq)

double-Replacement Reactions

In these reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY → AY + BX

An example of a reaction that forms an insoluble gas includes:

FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)

double-Replacement Reactions

In these reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY → AY + BX

Acid/base reactions are examples of reactions that form water.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

double-Replacement Reactions

In these reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY → AY + BX

Combustion Reactions

In these reactions, a substance combines with oxygen, releasing a large amount of energy in the form of light and heat and forming water.The combustion of hydrogen produces water.

2H2(g) + O2(g) → H2O(g)

The combustion of hydrocarbons (compounds made of hydrogen and carbon) produces water and carbon dioxide.

Combustion Reactions

In these reactions, a substance combines with oxygen, releasing a large amount of energy in the form of light and heat and forming water.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)

The combustion of hydrocarbons (compounds made of hydrogen and carbon) produces water and carbon dioxide.

Combustion Reactions

In these reactions, a substance combines with oxygen, releasing a large amount of energy in the form of light and heat and forming water.

CompositionA + X → AX

DecompositionAX → A + X

Single-ReplacementA + BX → AX + BAX + Y → AY + X

Double-ReplacementAX + BY → AY + BX

CombustionA + O2 → CO2 + H2O

Summary