Writing Chemical Reactions
Chemical reactions, or chemical changes, happen when the atoms in one or
more chemicals split up and join together in new ways.
For example, hydrogen can react with oxygen :
hydrogen + oxygen
water
Before the reaction the oxygen atoms go
round in pairs, and the hydrogen atoms
go round in pairs. When they react we get
two new molecules, of the compound
water.
Each of these molecules has three atoms,
two hydrogen atoms joined to one oxygen
atom.
We can write this as H2O.
 Many
chemical
reactions have
defining
characteristics
which allow
them to be
classified as to
type.

The five types of chemical reactions in this
unit are:
◦ Combination
◦ Decomposition
◦ Single Replacement
◦ Double Replacement
◦ Combustion
The reaction of sugar with
concentrated sulphuric acid.

Two or more substances combine to form
one substance.
◦ The general form is A + X AX

Example:
◦ Magnesium + oxygen  magnesium
oxide
◦ 2Mg + O2  2MgO


Combination reactions may also be called
composition or synthesis reactions.
Some types of combination reactions:
◦ Combination of elements
 K + Cl2 
 One product will be formed

K + Cl2 

Write the ions: K+ Cl-

Balance the charges: KCl

Balance the equation: 2K + Cl2  2KCl
 Some
types of combination
reactions:
◦ Oxide + water 
 Nonmetal oxide + water  acid
 SO2 + H2O  H2SO3
 Metal oxide + water  base
 BaO + H2O  Ba(OH)2
 Some
types of combination
reactions:
◦ Metal oxides + nonmetal oxides
 Na2O + CO2  Na2CO3
 CaO + SO2  CaSO3

One substance reacts to form two or more
substances.
◦ The general form is AX  A + X

Example:
◦ Water can be decomposed by
electrolysis.
◦ 2H2O  2H2 + O2
 Types
of Decomposition
Reactions:
◦ Decomposition of carbonates
 When heated, some carbonates
break down to form an oxide and
carbon dioxide.
 CaCO3  CaO + CO2
 H2CO3  H2O + CO2
 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.
 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.
 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.
 H2SO3  H2O + SO2
 Some
decomposition reactions are
difficult to predict.
 The
decomposition of nitrogen
triiodide, NI3, is an example of an
interesting decomposition
reaction.
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

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)



Not all single replacement reactions that can
be written actually happen.
The metal must be more active than the metal
ion.
Aluminum is more active than iron in Al +
Fe2O3 in the following reaction:
 Al
+ Fe2O3 
 Aluminum will replace iron(III) as
was seen in the video.
 Iron(III) becomes Fe and aluminum
metal becomes Al3+.
 2Al + Fe2O3  2Fe + Al2O3
 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
 Ions
of two compounds exchange
places with each other.
◦ The general form is AX + BY 
AY + BX
 Metathesis
is an alternate name
for double replacement reactions.






NaOH + CuSO4 
The Na+ and Cu2+ switch places.
Na+ combines with SO42- to form Na2SO4.
Cu2+ combines with OH- to form Cu(OH)2
NaOH + CuSO4  Na2SO4 + Cu(OH)2
2NaOH + CuSO4  Na2SO4 + Cu(OH)2
 CuSO4 + Na2CO3 
 Cu2+ combines with
CO32- to form
CuCO3.
 Na+ combines with SO42- to form
Na2SO4.
 CuSO4 + Na2CO3  CuCO3 +
Na2SO4
 Na2CO3
+ HCl 
 Notice that gas bubbles were
produced rather than a
precipitate.
 What was the gas?
 Write the double replacement
reaction first.
 Na2CO3 + HCl
 Na+ combines

with Cl- to form
NaCl.
 H+ combines with CO32- to form
H2CO3.
 Na2CO3 + 2HCl  2NaCl + H2CO3
 H2CO3 breaks up into H2O and
CO2.
 The
gas formed was carbon
dioxide.
 The final balanced reaction is:
Na2CO3 + HCl  NaCl + H2O +
CO2.
 Balance the equation.
 Na2CO3 + 2HCl  2NaCl + H2O +
CO2
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.
 The combustion
reaction may be
burning of a fuel.

 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
 Combustion
reactions involve
light and heat energy released.
 Natural gas, propane, gasoline,
etc. are burned to produce heat
energy.
 Most of these organic reactions
produce water and carbon
dioxide.
 Classify
to type:
each of the following as
 H2
+ Cl2  2HCl
 Ca
+ 2H2O  Ca(OH)2 + H2
◦ Combination
◦ Single replacement
 2CO
+ O2  2CO2
◦ Combination and combustion
 2KClO3
 2KCl + 3O2
◦ Decomposition
 FeS
+ 2HCl  FeCl2 + H2S
◦ Double replacement
 Zn
+ HCl  ?
◦ Single replacement
◦ Zn + 2HCl  ZnCl2 + H2