Types of Chemical Reactions

Writing Chemical Reactions

Types of Reactions

Many chemical reactions have defining characteristics which allow them to be classified as to type.

Types of Chemical Reactions

The five types of chemical reactions in this unit are:

Synthesis

Decomposition

Single Replacement

Double Replacement

Combustion

Synthesis Reactions

Two or more substances combine to form one substance.

The general form is A + X AX

Example:

Magnesium + oxygen magnesium oxide

2Mg + O2 2MgO

Synthesis Reactions

Synthesis reactions may also be called composition or combination reactions.

Some types of synthesis reactions:

Combination of elements

One product will be formed from multiple reactants

Synthesis Reactions

K + Cl2

Write the ions: K+ Cl-

Balance the charges: KCl

Balance the equation: 2K + Cl2 2KCl

Synthesis Reactions

Some types of synthesis reactions:

Oxides + water

Nonmetal oxide + water acid (don’t

need to memorize this)

SO2 + H2O H2SO3

Metal oxide + water metal

hydroxide (base)

BaO + H2O Ba(OH)2

Synthesis Reactions

Some types of combination reactions:

Metal oxides + nonmetal oxides

Na2O + CO2 Na2CO3

CaO + SO2 CaSO3Notice how the oxygens simply add together

Decomposition Reactions

One substance reacts to form two or more substances.

The general form is AX A + X

Example:

Water can be decomposed by electrolysis. (-lysis means “to split”)

2H2O 2H2 + O2

Decomposition Reactions

Types of Decomposition Reactions:

Decomposition of carbonates

When heated, some carbonatesbreak down to form an oxide and carbon dioxide.

CaCO3 CaO + CO2

H2CO3 H2O + CO2

Decomposition Reactions

Types of decomposition reactions:

Some metal hydroxides decompose into oxides and water when heated.

Ca(OH)2 CaO + H2O

Note that this is the reverse of a similar combination reaction.

Decomposition Reactions

Types of decomposition reactions: Metal chlorates decompose into

chlorides and oxygen when heated.

2KClO3 2KCl + 3O2

Zn(ClO3)2 ZnCl2 + 3O2

Some of these reactions are used in explosives.

Decomposition Reactions

Some substances can easily decompose: Ammonium hydroxide is actually

ammonia gas dissolved in water.

NH4OH NH3 + H2O

Some acids decompose into water and an oxide. (don’t need to memorize this)

H2SO3 H2O + SO2

Combustion Reaction

When a substance combines with oxygen, a combustion reaction results.

The combustion reaction may also be an example of an earlier type such as 2Mg + O2 2MgO (synthesis)

Combustion Reactions

Combustion reactions involve light and heat energy released.

Natural gas, propane, gasoline, etc. are burned to produce heat energy.

Most of these reactions with hydrocarbons produce water and carbon dioxide.

Combustion Reaction

Methane, CH4, is natural gas.

When hydrocarbon compounds are burned in oxygen, the products are water and carbon dioxide.

CH4 + O2 CO2 + H2O

CH4 + 2O2 CO2 + 2H2O

Single Replacement Reactions

A metal will replace a metal ion in a compound. The general form is A + BX AX + B

A nonmetal will replace a nonmetal ion in a compound. The general form is Y + BX BY + X

Look for an element + a compound as reactants

Single Replacement Reactions

Examples:

Ni + AgNO3

Nickel replaces the metallic ion Ag+.

The silver becomes free silver and the nickel becomes the nickel(II) ion.

Ni + AgNO3 Ag + Ni(NO3)2

Balance the equation:

Ni + 2AgNO3 2Ag + Ni(NO3)

Single Replacement Reactions

Not all single replacement reactions that can be written actually happen.

The free metal must be more active than the metal ion it is replacing.

You can use the activity series to predict if a single replacement reaction will occur.

Elements at the top will replace elements below.

Single Replacement Reactions

An active nonmetal can replace a less active nonmetal.

The halogen (F2, Cl2, Br2, I2) reactions are good examples.

F2 is the most active and I2 is the least.

Cl2 +2 NaI 2 NaCl + I2

Predicting Single Replacements

Use the activity series to predict the outcome:

Ag + CuCl2

K + NaCl

Cl2 + NaI

I2 + KBr

Predicting Single Replacements

Use the activity series to predict the outcome:

Ag + CuCl2 No reaction

K + NaCl KCl + Na

Cl2 + NaI I2 + NaCl

I2 + KBr No reaction

Double Replacement Reactions

Ions of two compounds exchange places with each other. The general form is AX + BY AY + BX

Look for two ionic compounds (this includes acids + bases)

Often the driving force is the formation of a solid (precipitate), which we can predict with solubility rules

Double Replacement

CuSO4(aq)+ Na2CO3(aq)

Cu2+ combines with CO32- to form

CuCO3, which is a solid

Na+ combines with SO42- to form

Na2SO4, which remains dissolved.CuSO4(aq)+ Na2CO3(aq) CuCO3(s)+ Na2SO4 (aq)

Double Replacement

A special type of double replacement involves acids reacting with bases. It is called a neutralization.

The general format:Acid (H in front) + base (OH in back) salt + H2O

Double Replacement -Neutralizations

Example: Hydrochloric acid + sodium hydroxide

HCl + NaOH

HCl + NaOH NaCl + H2O

The Na+ and Cl- ions combine to make NaCl

The H+ from the acid combines with OH- from the base to make water (H2O or HOH)

Practice

Classify each of the following as to type:

H2 + Cl2 2HCl

Combination

Ca + 2H2O Ca(OH)2 + H2

Single replacement

Practice

2CO + O2 2CO2

Combination and combustion

2KClO3 2KCl + 3O2

Decomposition

Practice

FeS + 2HCl FeCl2 + H2S

Double replacement

Zn + HCl ?

Single replacement

Zn + 2HCl ZnCl2 + H2