Writing Chemical
Equations
Parts of a Reaction
► Reactants
 The starting materials in a chemical reaction
► Products
 The substances that are formed in a chemical
reaction
Parts of a Reaction
►A
chemical equation shows how atoms
are rearranged during a reaction
 Reactants appear on the LEFT
 Products appear on the RIGHT
 An arrow separates the reactants from the
products ()
Reactants  Products
Special Notations
► Descriptions
of the compounds appear as
subscripts after the compound.
► Gas:
(g)
► Liquid:
► Solid:
(l)
(s)
► Aqueous:
Dissolved in water (aq)
Special Notations
► Special
conditions appear with the arrow.
► Heat:
Heat is added to make the reaction
happen
D
► Catalyst:
A catalyst is added in order for
the reaction to happen – The compound is
listed
 Pt
Writing a Chemical Equation
Potassium oxide (K2O) and water react to
form potassium hydroxide (KOH).
 Reactants: K2O, H2O
 Products: KOH
K2O + H2O  KOH
Writing a Chemical Equation
Aluminum and iron oxide (FeO) react to form
aluminum oxide (Al2O3) and iron.
 Reactants: Al, FeO
 Products: Al2O3, Fe
Al + FeO  Al2O3 + Fe
Writing a Chemical Equation
Silicon dioxide (SiO2) and hydrogen fluoride
(HF) react to form silicon tetrafluoride (SiF4)
and water.
 Reactants: SiO2, HF
 Products: SiF4, H2O
SiO2 + HF  SiF4 + H2O
Writing a Chemical Equation
Solid sodium carbonate (Na2CO3) and
aqueous hydrochloric acid (HCl) react to form
carbon dioxide gas (CO2), liquid water, and
aqueous sodium chloride (NaCl)
 Reactants: Na2CO3(s), HCl(aq)
 Products: CO2(g), H2O(l), NaCl(aq)
Na2CO3(s) + HCl(aq)  CO2(g) + H2O(l) + NaCl(aq)
Writing a Chemical Equation
Heating solid sodium nitrate (NaNO3) results
in solid sodium nitrite (NaNO2) and gaseous
oxygen (O2)
 Reactants: NaNO3(s)
 Products: NaNO2(s), O2(g)
 Special Conditions: Heat
D
NaNO3(s)  NaNO2(s) + O2(g)
Writing a Chemical Equation
Aqueous hydrogen peroxide (H2O2) reacts to
form liquid water (H2O) and oxygen gas (O2)
in the presence of a platinum catalyst (Pt).
 Reactants: H2O2(aq)
 Products: H2O (l), O2(g)
 Special Conditions: Platinum Catalyst
Pt
H2O2(aq)  H2O
(l)+
O2(g)
Balancing an Equation
► In
a chemical reaction, atoms are
rearranged, but they cannot be destroyed or
changed.
► If an atom appears on the left side of the
reaction, it must appear on the right side.
Balancing an Equation
K2O + H2O  KOH
► In the reaction above, there are unequal
numbers of atoms on either side
 Potassium (K): 2 atoms on the left, 1 atom on
the right
 Oxygen (O): 2 total on the left, 1 atom on the
right
 Hydrogen (H): 2 atoms on the left, 1 atom on
the right.
Balancing an Equation
K2O + H2O  KOH
► The reaction must be modified in order to
make the atoms equal
► We CANNOT add subscripts to KOH,
because that would change the compound.
► Instead, we add coefficients in front of
compounds.
Balancing an Equation
Determine how many of each atom are
present.
___ K2O + ___ H2O  ___ KOH
K O K
K: 2
H: 2
O: 2
H O H
K O H
K: 1
H: 1
O: 1
Balancing an Equation
There are different numbers of atoms on both
sides. In order to match them up, we can
add more compounds, but not atoms alone.
___ K2O + ___ H2O  ___ KOH
K O K
K: 2
H: 2
O: 2
H O H
K O H
K O H
K: 1
H: 1
O: 1
Balancing an Equation
The number of each atom in the compound
changes.
___ K2O + ___ H2O  ___ KOH
K O K
K: 2
H: 2
O: 2
H O H
K O
K O
K: 1
H: 1
O: 1
H
H
2
2
2
Balancing an Equation
Now that all the atoms are balanced, we can
add up all the compounds.
This is
called a
coefficient
1 K2O + 1 H2O  2 KOH
K O K
K: 2
H: 2
O: 2
H O H
K O
K O
K: 1
H: 1
O: 1
H
H
2
2
2
Balancing an Equation
Determine how many of each atom are
present.
___ H2 + ___ O2  ___ H2O
H H
H: 2
O: 2
O O
H O H
H: 2
O: 1
Balancing an Equation
The hydrogen is okay, but the oxygen is not.
Add another H2O in order to balance the
oxygen.
___ H2 + ___ O2  ___ H2O
H H
H: 2
O: 2
O O
H O H
H O H
H: 2
O: 1
Balancing an Equation
The hydrogen is okay, but the oxygen is not.
___ H2 + ___ O2  ___ H2O
H H
K: 2
H: 2
O: 2
O O
H O
K O
K: 1
H: 1
O: 1
H
H
2
2
2
Balancing an Equation
___ Al + ___ FeO  ___ Al2O3 + ___ Fe
Al: 1
O: 1
Fe: 1
Al: 2
O: 3
Fe: 1
Balancing an Equation
Begin balancing the aluminum by adding a 2
_2_ Al + ___ FeO  ___ Al2O3 + ___ Fe
Al: 1
O: 1
Fe: 1
Al: 2
O: 3
Fe: 1
Balancing an Equation
Recount the aluminum atoms
_2_ Al + ___ FeO  ___ Al2O3 + ___ Fe
Al: 2
O: 1
Fe: 1
Al: 2
O: 3
Fe: 1
Balancing an Equation
Move on to oxygen – Balance it by placing a
3 in front of FeO, and then recount the
atoms
_2_ Al + _3_ FeO  ___ Al2O3 + ___ Fe
Al: 2
O: 3
Fe: 3
Al: 2
O: 3
Fe: 1
Balancing an Equation
By adding the 3, we also changed the iron –
this is the only way to balance, so we now
fix the iron by adding another coefficient
_2_ Al + _3_ FeO  ___ Al2O3 + _3_ Fe
Al: 2
O: 3
Fe: 3
Al: 2
O: 3
Fe: 3
Balancing an Equation
Now everything is balanced
_2_ Al + _3_ FeO  ___ Al2O3 + _3_ Fe
Al: 2
O: 3
Fe: 3
Al: 2
O: 3
Fe: 3
Balancing an Equation
Identify the number of each atom present –
Start with atoms in the fewest compounds.
We tend to keep H and O last.
___ SiO2 + ___ HF  ___ SiF4 + ___ H2O
Si: 1
F: 1
H: 1
O: 2
Si: 1
F: 4
H: 2
O: 1
Balancing an Equation
Silicon is done, so move on to fluorine.
___ SiO2 + _4_ HF  ___ SiF4 + ___ H2O
Si: 1
F: 4
H: 4
O: 2
Si: 1
F: 4
H: 2
O: 1
Balancing an Equation
Now move on to hydrogen. Since the
compound has 2 hydrogen atoms in it, we
multiply to get the correct number.
___ SiO2 + _4_ HF  ___ SiF4 + _2_ H2O
Si: 1
F: 4
H: 4
O: 2
Si: 1
F: 4
H: 4
O: 2
Balancing an Equation
Oxygen is already done. Everything is
balanced.
___ SiO2 + _4_ HF  ___ SiF4 + _2_ H2O
Si: 1
F: 4
H: 4
O: 2
Si: 1
F: 4
H: 4
O: 2