Types of Chemical Reactions
Consider for a moment the number of possible chemical reactions. Because there are millions of
chemical compounds, it is logical to expect that there are millions of possible chemical reactions.
It would be very difficult to memorize the equations for all the different chemical reactions that
occur so chemists have grouped them according to the similarities in the way they react. It is not
quite as simple as this though because some chemical reactions can belong to more than one type.
One method groups nearly all the chemical reactions into five main types.
1. synthesis (or combination)
2. decomposition
3. single-replacement (or single displacement)
4. double replacement (or double displacement)
5. combustion
1.
Synthesis reactions
When two reactants combine to form one product. For example:
Word
equation
Potassium
+
Chlorine

Potassium
Chloride
Chemical
equation
K(s)
+
Cl2(g)

2 KCl(s)
2.
Decomposition reactions
When one reactant breaks down into two or more products or two of more of the same molecules.
It is hard to predict the products of decomposition reactions. For example:
1.
2.
Word equation
Calcium
carbonate

Calcium
Oxide
+
Carbon
dioxide
Chemical equation
CaCO3(s)

CaO(s)
+
CO2(g)
Word equation
Mercury oxide

Mercury
+
Oxygen
Chemical equation
2 HgO(s)

2 Hg(l)
+
O2(g)
Write a word and balanced symbol equation for the decomposition reaction you observed.
1
3.
Single Replacement reactions
Occur between a metal element and a compound. The metal element replaces an element in the
compound. Common elements that swap are two metals or a metal and hydrogen. For example:
1.
Word
equation
Magnesium
+
Zinc nitrate

Magnesium
nitrate
+
Zinc
Chemical
equation
Mg(s)
+
Zn(NO3)2(aq)

Mg(NO3)2(aq)
+
Zn(s)
The element magnesium, Mg is replaced by the element zinc, Zn in the compound zinc nitrate.
Zinc becomes the product.
2.
Word
equation
magnesium
+
Hydrochloric
acid

Magnesium
chloride
+
Hydrogen
Chemical
equation
Mg (s)
+
2 HCl (aq)

MgCl2 (aq)
+
H2(g)
In this reaction the element magnesium is replaced by the element hydrogen, H in hydrochloric
acid. Hydrogen gas, H2 becomes the product.
Write a word and balanced symbol equation for the single replacement reaction you observed.
4.
Double Replacement reactions
Occur between two compounds. The two positive metal ions in each reactant swap places with
one another. For example:
1.
Word
equation
Potassium
Carbonate
+
Barium
Chloride

Potassium
Chloride
+
Barium
carbonate
Chemical
equation
K2CO3(aq)
+
BaCl2(aq)

2 KCl (aq)
+
BaCO3(s)
ions
K+, CO32-
Ionic
Equation
CO32-(aq)
Ba2+, Cl+
K+, Cl
Ba2+(aq)
2
BaCO3(s)
Ba2+, CO32-
2.
Word
equation
Sodium
sulfide
+
Magnesium
nitrate

Sodium
nitrate
+
Magnesium
sulfide
Chemical
equation
Na2S(aq)
+
Mg(NO3)2(aq)

NaNO3 (aq)
+
MgS(s)
ions
Na+, S2-
Mg2+, NO3-
Na+, NO3-
Mg2+, S2-
Ionic

S2-(aq)
+
Mg2+(aq)
MgS(s)
Equation
In these reactions the two positive metal ions swap places with one another to form two new
compounds. In most double displacement reactions the reactants are aqueous solutions. The
products formed are a solid and another aqueous solution. The solid product formed when two
aqueous solutions react is called a precipitate. To identify the precipitate solubility rules are used.
The ionic equation show the ions involved in the formation of the precipitate only.
Solubility Rules
1. All nitrates are soluble
2. All chlorides, bromides and iodides are soluble except those of Pb 2+, Ag+ and Hg2+
3. All sulfates are soluble except those of Ba2+, Sr2+, and Pb2+. CaSO4, Ag2SO4, Hg2SO4 are
slightly soluble.
4. All hydroxides are insoluble except those of Group I in the periodic table, NH4+, and Ba2+.
Ca(OH)2 and Sr(OH)2 are slightly soluble.
5. All carbonates and phosphates are insoluble except those in Groups 1 and 2 in the periodic
table and NH4+.
Write a word and balanced symbol equation, plus the overall ionic equation for one of the double
displacement precipitation reactions you observed.
3
Reactions between acids and bases are also double displacement reactions. They are also called
neutralization reactions. For example the reaction between nitric acid and the base sodium
hydroxide.
Word
equation
Nitric acid
+
Sodium
hydroxide

Sodium
nitrate
+
water
Chemical
equation
HNO3 (aq)
+
NaOH (aq)

NaNO3 (aq)
+
H2O(l)
Write a word and balanced symbol equation for the acid-base neutralization reaction you
observed.
5.
Combustion reactions
Reactions that involve the reactant reacting with oxygen are called combustion reactions by
scientists. We commonly called these reactions burning. For example paper burning in oxygen in
the air is a combustion reaction. Combustions reactions are all exothermic because they release
large amounts of heat energy. Energy because it is produced is written on the product side of the
chemical reaction.
For example if an element is burned in oxygen the element and oxygen combine to form on
product like in a synthesis reaction.
Word
equation
Carbon
+
Oxygen

Carbon
dioxide
+
Heat
energy
Chemical
equation
C(s)
+
O2(g)

CO2(g)
+
Heat
energy
If a compound burns in oxygen the products are either carbon dioxide and water or carbon
monoxide and water.
Word
Carbon
Heat

methane
+ Oxygen
+
water
+
equation
dioxide
energy
Chemical
equation
CH4(s)
+
2 O2(g)

4
CO2(g)
+
2 H2O(g)
+
Heat
energy
Write a word and balanced symbol equation for the combustion reaction you observed.
Now that you have an understanding of different reaction types, can write chemical formulae and
can balance chemical equations it is possible to predict the products of a chemical reaction.
Exercises
1.
For the following unbalanced chemical reactions
i)
Identify as a synthesis, combustion, decomposition, single replacement or double
replacement reaction.
iii)
Write a balanced chemical equation for the reaction.
a)
Ag (s)
+
b)
N2O4 (g) 
NO2 (g)
c)
KCl (aq) +
ZnSO4 (aq)
d)
Cu (s)
+
e)
C2H6 (g) +
O2 (g)
f)
H2O2 (aq)

2.

Cl2 (g)

ZnO (s) 

O2 (g)
AgCl3 (s)
K2SO4 (aq)
+
ZnCl2 (aq)
Cu2O3 (s)
+
Zn (s)
CO2 (g)
+
H2O (l)
+
+
Heat
H2O (l)
Prepare your own balanced single replacement and double displacement reaction. Be
sure all compound formula are correct and all elements are in their proper pure form. The
reactions must be different to the ones shown in this packet.
[4 marks each: 2 for formula, 1 for balancing, 1 for proper reaction type]
5