TYPES OF CHEMICAL REACTIONS
There are five basic types of chemical reactions that we will be studying in this
unit. They are called synthesis, decomposition, single replacement, double
replacement, and combustion reactions. After studying this unit you should be
able to identify these different reaction types and predict the products of single and
double replacement reactions. You should also be able to predict if single
replacement reactions will take place based on the reactants given. Below you will
find examples of each type of reaction.
Synthesis
In a combination reaction 2 or more substances combine to form a single new
substance. A general formula for this type of reaction is:
A + B ---------------> AB
Examples
2Mg(s) + O2(g) --------------------> 2MgO(s)
I2(s) + Zn(s) -----------------> ZnI2(g)
The examples above were easy to predict the products since there were only two
elements to begin with. Synthesis reactions can also occur when two compounds
combine to form a new compound. Here are a few examples of those reactions.
Na2O + H2O -----------------------> NaOH
In the example above, a metallic oxide reacts with water to form a base (OH).
Whenever a metallic oxide undergoes a synthesis reaction with water, a base will
form.
CO2 + H2O --------------------> H2CO3
In the example above, a nonmetallic oxide reacts with water to form an acid.
Whenever a nonmetallic oxide undergoes a synthesis reaction with water, an acid
will form.
Na2O + CO2 -----------------> Na2CO3
In the example above, a metallic oxide reacts with a nonmetallic oxide to form a
salt. Whenever a metallic oxide undergoes a synthesis reaction with a nonmetallic
oxide, a salt will form.
Decomposition Reactions
In a decomposition reaction a single substance is broken down into 2 or more new
substances. A general formula for this type of reaction is:
AB ---------------> A + B
Examples
2H2O2(aq) ------------------> 2H20(l) + O2(g)
2NI3(s) -----------------> N2(g) + 3I2(g)
Single Replacement Reactions
In a single replacement reaction a metal replaces another metal in a compound or a
nonmetal can replace another nonmetal in a compound. The first examples shown
will be of the more common type, a metal replacing another metal.
A + BC ---------------> AC + B
Examples
Cu(s) + 2AgNO3(aq) -----------------> Cu(NO3)2(aq) + 2Ag(s)
The only way that this reaction will take place is if the metal displacing the other
metal from the compound is more reactive. In this reaction copper must be more
reactive than the silver that it is replacing or the reaction will not take place. You
can compare the reactivity of each metal by comparing them in a table called the
Activity Series of Metals. This table can be found in your textbook. In this case
copper is more reactive than silver so the reaction will take place.
2Al(s) + 3Ca(NO3)2(aq) --------------------> 2Al(NO3)3(aq) + 3Ca(s)
This reaction will not take place because aluminum is not more reactive than
calcium, the metal that it must replace.
The second type of single replacement reaction is when a nonmetal replaces
another nonmetal in a compound. This reaction only occurs with halogens
(fluorine, chlorine, bromine, iodine). The examples below will show this type of
reaction.
A + BC -------------------> BA + C
Example
F2(g) + 2NaCl(aq) ---------------> 2NaF(aq) + Cl2(g)
In order for this reaction to occur fluorine must be more reactive than chlorine (the
nonmetal that it is replacing). This reaction will take place because fluorine is
more reactive than chlorine. You can tell the reactivity of the halogens by looking
at their order on a periodic table. Fluorine, which is at the top of the column is the
most reactive and can replace any halogen below it. The halogens become less
reactive as you move down the column.
Double Replacement Reactions
In a double replacement reaction the two positive ions in ionic compounds are
exchanged. The positive ions are always the first elements in the compound, so the
first elements in the compound are exchanged. In order for a double replacement
reaction to take place one of the following results must occur.
Precipitate forms
Gas is released
Molecular substance forms (usually water)
Below you will find some examples of double replacement reactions.
AB + CD -------------------> AD + CB
Examples
AgNO3(aq) + NaCl(aq) -------------------> AgCl(s) + NaNO3(aq)
FeS(s) + HCl(aq) ------------------> FeCl2(aq) + H2S(g)
Combustion Reactions
In a combustion reaction an element or a compound react with oxygen to produce
energy. The most common combustion reaction involve a hydrocarbon (compound
containing carbon and hydrogen) reacting with oxygen. This type of reaction will
yield carbon dioxide and water. The examples below will be using a hydrocarbon
reacting with oxygen.
CxHy + O2(g) -----------------> CO2(g) + H2O(g) + energy
Examples
CH4(g) + 2O2(g) -------------------> CO2(g) + 2H2O(g) + energy
2C2H2(g) + 5O2(g) ------------------> 4CO2(g) + 2H2O(g) + energy
If the combustion is incomplete in this type of reaction, the toxic gas carbon
monoxide is formed instead of carbon dioxide. This incomplete combustion can
occur in your furnace at home if the furnace is not operating properly. Below is an
example of an equation with incomplete combustion.
2CH4(g) + 3O2(g) ----------------------> 2CO(g) + 4H2O(g) + energy