6.5 Types of Chemical Reactions: Synthesis and Decomposition

Types of Chemical Reactions: Synthesis
and Decomposition
There are currently about 10 million known compounds. Each compound
can react in many different ways. It is impossible for anyone to memorize all the
reactions. To make it easier to predict what reactions will take place, chemists
have grouped similar reactions into categories.
One way of grouping chemical reactions is based on recognizing patterns
in the chemical formulas. For example, Figure 1 shows three chemical
reactions that at first seem unrelated. But when you compare the chemical
equations for these reactions, a pattern emerges. Can you tell what it is?
LeaRning Tip
State Symbols
It is good practice to write state
symbols after chemical formulas to
clarify the states of the chemicals
during the reaction. For example,
HCl(g) tells us that the hydrogen
chloride exists as a gas.
Zn(s) + S(s) → ZnS(s)
2 Na(s) + Cl2(g) → 2 NaCl(s)
HCl(g) + NH3(g) → NH4Cl(s)
Figure 1 (a) Powdered zinc metal reacts with sulfur to produce zinc sulfide powder. (b) A small
piece of sodium metal ignites as it is placed in a flask of chlorine gas (yellowish gas). (c) Hydrogen
chloride gas and ammonia gas both diffuse out of their aqueous solutions. When the two gases
come in contact, they react to produce a cloudy white powder of ammonium chloride.
synthesis Reactions
The three reactions shown in Figure 1 are examples of synthesis reactions. In
a synthesis reaction, two simple reactants combine to make a larger or more
complex product (Figure 2). The chemical equations for synthesis reactions
follow the general pattern:
A + B → AB
synthesis reaction a reaction in which
two reactants combine to make a larger
or more complex product; general pattern:
A + B → AB
Figure 2 In some cases, the reactants are atoms (elements), while in others, they are molecules
(elements or compounds).
6.5 Types of Chemical Reactions: Synthesis and Decomposition
Table 1 shows how the three synthesis reactions in Figure 1, on the
previous page, follow the general pattern.
Table 1 Examples of Synthesis Reactions
Synthesis reaction
zinc sulfide
(Figure 1(a))
zinc + sulfur → zinc sulfide
Zn(s) + S(s) → ZnS(s)
sodium chloride
(Figure 1(b))
sodium + chlorine → sodium chloride
2 NaCl(s)
2 Na(s) + Cl2(g) →
ammonium chloride
(Figure 1(c))
ammonia + hydrogen chloride → ammonium chloride
NH3(g) +
General pattern
A + B → AB
Decomposition Reactions
decomposition reaction a reaction in
which a large or more complex molecule
breaks down to form two (or more) simpler
products; general pattern: AB → A + B
We can think of decomposition reactions as being the opposite of synthesis
reactions. During a decomposition reaction, large compounds are broken
down into smaller compounds or elements (Figure 3). The general pattern
for decomposition reactions is
AB → A + B
Figure 3 In a decomposition reaction, a complex molecule breaks down, or decomposes, into
simpler products. The products can be elements or compounds.
LeaRning Tip
Just the Opposite
Decomposition reactions (AB → A + B)
are the reverse of synthesis reactions
(A + B → AB).
Decomposition reactions usually absorb energy (such as thermal or
electrical energy) from an external source. This energy is then used to
convert reactants into products. For example, water can be decomposed into
its elements using electricity as
its energy source.
Table 2 shows how two decomposition reactions follow the general pattern.
Table 2 Examples of Decomposition Reactions
Decomposition reaction
energy + water → hydrogen + oxygen
energy + 2 H2O(l) → 2 H2(g) + O2(g)
sodium azide
energy + sodium azide → sodium + nitrogen
2 NaN3(s)
→ 2 Na(s) + 3 N2(g)
Ontario Science 10 SB
General pattern
AB → A + B
nitrogen compounds undergo important decomposition reactions.
Deborah Crowle the use of airbags in cars is a lifesaving application of a
Figure 4 Airbags are designed to slow
decomposition reaction (Figure 4). Airbags contain a nitrogen compound
your forward motion
during a collision. 2nd pass
An airbag inflatesApproved
in about 1/20th of a
second and staysNot
fully Approved
inflated for only
about 1/10th of a second.
called sodium azide, NaN3(s).Duringacollision,asuddenflowofelectricity
is automatically sent to the airbag. This electrical energy triggers the rapid
decomposition of sodium azide to produce nitrogen gas and sodium metal.
Chapter 6 • Chemicals and Their Reactions
SKILLS: Researching, Analyzing the Issue, Defending a Decision, Communicating
4.A., 4.C.
Ammonium nitrate is one of the cheapest and most widely used
fertilizers. However, it can also be used to make explosives (Figure 5):
ammonium nitrate → water + nitrogen + oxygen + energy
2 NH4NO3(s)
→ 4 H2O(g) + 2 N2(g) + O2(g) + energy
Because of this, some politicians have proposed that the sale of
ammonium nitrate should be restricted or banned.
1. Research the facts regarding the use of ammonium nitrate as
a fertilizer and in the manufacture of explosives.
2. Research the arguments for and against the proposed ban.
3. Organize your findings in a “pros” and “cons” chart.
A. Do you think the proposed ban is fair? Defend your decision
in a letter to a local politician. C
Figure 5 A few well-placed explosive charges can demolish old,
vacant buildings quickly and safely. Many explosives release
their destructive energy through decomposition reactions.
Within each category, the reactions follow the
same pattern.
combine to make a larger or more complex
product and follow the general pattern
A + B → AB.
breaks down to make two or more simpler
product and follow the general pattern
AB → A + B.
YOUR LeaRning
1. Classify each of the following as either a synthesis or a
decomposition reaction: K/U
4. Write a balanced chemical equation for each of the
following reactions. Include state symbols. Classify each
one as either synthesis or decomposition. K/U T/I
(a) zinc chloride → zinc + chlorine
(a) Hydrogen gas reacts explosively with chlorine gas to
form hydrogen chloride gas.
(b) potassium + iodine → potassium iodide
(c) potassium oxide + water → potassium hydroxide
(d) calcium carbonate → calcium oxide + carbon dioxide
2. Write a balanced chemical equation for each of the word
equations given in question 1. K/U T/I
3. Copper metal was first made over 3 000 years ago by
heating a mineral containing copper(II) oxide. The other
product of this reaction is oxygen. K/U T /I
(b) A solution of hydrogen peroxide, H2O2, breaks down
into water and oxygen.
(c) Solid potassium chlorate breaks down into solid
potassium chloride and oxygen when heated.
(d) Ammonia gas, NH3, can be made by combining
hydrogen gas and nitrogen gas.
(e) Aluminum metal reacts with oxygen from the air to
form a hard coating of aluminum oxide. This coating
prevents aluminum objects from corroding.
(a) Write a word equation for this reaction.
(b) Is this a synthesis reaction or a decomposition
reaction? Explain.
(c) Write a balanced chemical equation for the reaction.
6.5 Types of Chemical Reactions: Synthesis and Decomposition