Chapter 5: Classifying Chemical Reactions
SYNTHESIS AND DECOMPOSITION REACTIONS
A Quick Look at a Synthesis Reaction…

In 1988, an airplane lost a big portion of its upper fuselage due to metal corrosion.

The chemical name for rust is iron (III) oxide – Fe2O3

Iron (III) oxide is produced (synthesized) from the reaction between Iron and Oxygen.

The balanced chemical equation for the formation of rust is:
4Fe(s) + 3O2 (g) →2Fe2O3 (s)
Synthesis Reactions

A chemical reaction in which two or more reactants combine to produce a new product.
A + B → AB [
+
→
]

Two, or more, reactants (A and B) combine together to form one product (AB).

The reactants may be any combination of elements and compounds, but the product will always be
a compound.

Example “A”) When the space shuttle blasts off, the power of the main thruster is generated
from a synthesis reaction. The reactants are liquid hydrogen and liquid oxygen, which combine to
form water vapour (Remember that hydrogen and oxygen are both diatomic molecules….as are
nitrogen, bromine, iodine, chlorine, and fluorine).

2H2 (l) + O2 (l)→ 2H2O (l)
Example “B”) One important product made by a synthesis reaction is ammonia (NH3 (g)).
Ammonia is the product of the synthesis between nitrogen and hydrogen (called the Haber
Process) and is carried out under very high temperatures and pressure. Ammonia is very
important in farming (fertilizer), making paper, extracting zinc and nickel from ores, in
explosives, and in cleaning products.

N2 (g) + 3H2 (g) → 2NH3 (g)
Example “C”) The atmospheric pollutant, nitrogen dioxide (NO2) is produced through the
synthesis reaction of nitrogen gas in the atmosphere with oxygen. Initially, a colourless molecular
compound, called nitrogen monoxide (NO (g)) is formed: N2 (g) + O2 (g) → 2NO(g). Then, the nitrogen
monoxide undergoes another synthesis reaction that produces a brown reactive gas called
nitrogen dioxide (NO2 (g)): 2NO(g) + O2 (g) → 2NO2 (g)
Synthesizing Binary Ionic Compounds
Problem:
Complete and balance the following synthesis reaction: Na
(s)
+ Cl2
(g)
→
Solution:
Step 1:
This is a synthesis reaction since there are 2 reactants that will combine to form a
single product.
Step 2:
Use the ion charges to determine the most likely product. Use the “criss cross
method” to combine the two elements to create a single ionic compound.
Step 3:
The problem also asks for a balance chemical equation for the reaction. First,
write the skeleton equation:
Na(s) + Cl2 (g) → NaCl(s)
Then, balance the equation using coefficients.
2Na(s) + Cl2 (g) → 2NaCl(s)
Step 4:
Check your solution. First, check to make sure that the product consists of a
combination of all the elements in the reactants. Then, make sure that the total
sum of the positive and negative ion charges gives a net charge of zero for the
compound. Finally, count the number of each type of atom in the final chemical
equation to make sure that the equation is balance.
Practice Problems:
1) Complete and balance the following synthesis reactions (all the products will be solids).
a) Ca(s) + N2 (g) →
b) K(s) + O2 (g) →
c) Cs(s) + P4 (S) →
d) Al(s) + F2(g) →
2) Write a balanced chemical equation for the synthesis reaction between hydrogen gas and chlorine
gas (the product is a gas).
A combustion reaction is when all substances in a compound are combined with oxygen, which then
produces carbon dioxide and water. Combustion is commonly called burning. It is an exothermic reaction,
which means heat is produced and is easily distinguished. Combustion occurs predominantly in
automobiles, homes, and in factories. An example of a combustion reaction is as follows:
CxHy + O2 --> CO2 + H2O (always look for oxygen gas in the reactants, and carbon dioxide + water in the
products – these are your indicators of a combustion reaction)
Decomposition Reactions

A chemical reaction in which a compound breaks down (decomposes) into two, or more, simpler
compounds, or elements.
AB → A + B [

→
+
]
The products may be any combination of elements and compounds, but the reactant will always be
a compound.

A decomposition reaction is the opposite of a synthesis reaction.

Example “A”) An essential component in the development of hydrogen as a fuel source is the
ability to produce sufficient amounts of hydrogen. One way that hydrogen gas is produced is
through the decomposition of water. Electric current is used to break apart the bonds between
the hydrogen and oxygen atoms to form the separate elements. This process is referred to as
the electrolysis of water. Approximately 4% of the hydrogen gas used in the world is produced
by decomposition of water. The reaction is represented by the chemical equation:
2H2O (l) → 2H2 (g) + O2 (g)

Example “B”) The tremendous forces associated with creating explosives is a decomposition
reaction. Trinitrotoluene (TNT), C7H5N3O6(s) is used in explosives that decompose into elements
and compounds: 2C7H5N3O6(s) → 3N2(g) + 5H2O(g) + 7CO(g) + 7C(s). Three products of this
decomposition reaction are nitrogen, water, and carbon monoxide – all gases. The formation, and
rapid expansion of these gases, pushes material away from the blast site, producing an explosion.
Sample Problem: Elements formed from a Decomposition Reaction
Problem:
Complete and balance the following decomposition reaction: AgCl(s) →
Solution:
Step 1:
This is a decomposition reaction. The reactant decomposes into the two elements that it
is composed of – silver and chlorine.
Step 2:
Silver is a metal and is represented as Ag(s). Chlorine is a non-metal and a diatomic
molecule. It is represented as Cl2(g).
Step 3:
The problem also asks for a balance chemical equation for the reaction. First,
write the skeleton equation:
AgCl(s) → Ag(s) + Cl2(g)
Then, balance the equation using coefficients.
2AgCl(s) → 2Ag(s) + Cl2(g)
Practice Problems:
1) Complete and balance the following decomposition reactions (metals are solids, non-metals are gases).
a) AuCl3(s) →
b) MgF2(s) →
c) Li2O(s) →
d) CsCl(s) →
2) Write the chemical equation for the decomposition of chromium (III) oxide into solid chromium
and oxygen gas.
3) Many car airbags rely on decomposition of the compound sodium azide (NaN3) to produce nitrogen
gas that rapidly inflates the bag. Sodium metal is also produced in the reaction. Write the balanced
chemical equation for this decomposition reaction.
SINGLE AND DOUBLE DISPLACEMENT REACTIONS
Single Displacement Reactions

A chemical reaction in which one element takes the place of another element in a compound.
A + BC → AC + B (for a single displacement where A is a metal. If the single element
is a metal, it will replace the metal in the compound – then that metal will become the single
element in the products).
OR
A + BC → BA + C (for a single displacement where A is a non-metal. If the single
element is a non-metal, it will replace the non-metal in the compound – then that non-metal will
become the single element in the products).

Example) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)

Example) Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
[copper wire in silver nitrate producing copper nitrate and silver]
The Activity Series

Is a list of elements organized according to their chemical reactivity. The most reactive element
appears at the top of the list, and the least reactive element appears at the bottom.

Alkali metals (group #1) are the most reactive metals.

A reactive metal will displace, or replace, a metal in a compound that is below it in the activity
series. That is why (in the example above) copper is able to displace silver (because it is higher
on activity series). If you were to place a piece of silver into a solution of a copper compound,
there would not be a reaction.
Activity Series for Metals
Lithium
Most reactive
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Iron
Nickel
Tin
Hydrogen (*)
Copper
Silver
Platinum
Gold
Least reactive
(*) Hydrogen ions can be positively charged, so metals can take the place of hydrogen in a compound. The element formed in these
single displacement reactions is hydrogen gas. Example) Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Practice Problems
1) Complete and balance the following single displacement reactions. If the reaction will not happen
(based on the activity series), write “no reaction”. (all metals are solids, and the compounds are in aqueous solutions).
a) SnCl4(aq) + Al(s) →
b) CuF2(aq) + Mg(s) →
c) Cu(s) + HCl(aq) →
d) Au(NO3)3(aq) + Ag(s)
→
e) Al(s) + Fe2O3(aq) →
f) Li(s) + HCl(aq)
→
2) When magnesium metal is added to a blue copper (II) sulfate solution, the blue colour fades as
colourless magnesium sulfate and brown pieces of copper form. Write a balanced chemical equation for
this reaction.
Replacing Non-Metals with Non-Metals
The ion of a non-metal is negatively charged, so it can be replaced only with another non-metal.
There is a separate activity series for non-metals – the most reactive elements are at the top of the
list.
Example) F2(g) + 2NaI(s) → I2(s) + 2NaF(s)
Activity: How Active Are the Non-Metals?
A scientist wished to determine the reactivity of 5 elements (oxygen, bromine, iodine, chlorine, and
fluorine) in order of most active to least active. The scientist conducted six tests, putting one element
and one compound together in each test. The results of the six chemical tests are expressed below as
balanced equations.
Examine each equation, and infer which non-metal element was more active for that equation. Write the
names of the five elements in a list, arranged from most active to least active. Explain how you decided
on the order of your list.
a) Cl2(g) + MgBr2(aq) → Br2(l) + MgCl2(aq)
b) I2(s) + MgCl2(aq) → no reaction
c) Br2(l) + MgI2(aq) → I2(s) + MgBr2(aq)
d) 2F2(g) + 2MgO(aq) → O2(g) + 2MgF2(aq)
e) O2(g) + 2MgBr2(aq) → 2Br2(l) + 2MgO(aq)
f) Cl2(g) + MgO(aq) → no reaction
Double Displacement Reactions

A chemical reaction in which the positive ions of two different compounds exchange places,
resulting in the formation of two new compounds – one of which may be a precipitate.

The reactants of a double displacement reaction are often aqueous solutions of ionic compounds.
AB + CD → AD + CB
(A and C are cations, and B and D are anions)

Example) 2AgNO3(aq) + K2CrO4(aq) → Ag2CrO4(s) + 2KNO3(aq)
Practice Problems
1) Complete and balance the chemical equations.
a) Pb(NO3)2(aq) + KI(aq) →
b) MgCl2(aq) + Pb(NO3)2(aq) →
c) AlCl3(aq) + CuNO3(aq) →
d) KCl(aq) + AgNO3(aq) →
e) CaI2(aq) + Na2CO3(aq) →
2) Aqueous solutions of potassium phosphate and magnesium iodide are known to undergo a double
displacement reaction and produce a precipitate. Write a balanced chemical equation for this reaction.
Acid-Base Reactions occur when an acid and a base are placed together, and react to neutralize
the acid and base properties, producing a salt. The H(+) cation of the acid combines with the OH(-) anion
of the base to form water. Example)
(
look for the “H” ion in one compound, and the “OH” ion in the other compound of
the reactants – there will always be a product of H2O)