Balancing Chemical Equations
Chemistry Unit
Science 10
What is a chemical equation?
• When a chemical reaction occurs, it can be
described by an equation.
• This shows the chemicals that react (called the
reactants) on the left-hand side, and the
chemicals that they produce (called the
products) on the right-hand side.
• The chemicals can be represented by their
names or by their chemical symbols.
What is a chemical equation?
• For example:
• Aluminium + Oxygen
Reactants
Aluminium Oxide
Product
• Numbers found in front of the elements
are called coefficients.
ex: 2Aluminium
To balance a chemical equation
Step 1: Write the skeleton equation
(aka, no numbers)
• Be sure you have the correct
formulas for all the compounds.
• Watch especially for polyatomics
Ex: Al + NO3
Al2(NO3)3
To balance a chemical equation
Step 2: Balance the elements or
polyatomic ions that appear just once
on each side first.
Ex: Al + NO3
2Al + 3(NO3)
Al2(NO3)3
Al2(NO3)3
To balance a chemical equation
Step 3: Check for any terms you
haven’t looked at and balance them
so the total on the reactants side =
the total on the products side.
Ex: Al + NO3
2Al + 3NO3
6Al + __NO3
Al2(NO3)3
Al2(NO3)3
__Al2(NO3)3
To balance a chemical equation
Step 4: Double check the equation
• A table can be very helpful
Ex: 2Al + 3NO3
Al = 2
NO3 = 3
Al2(NO3)3
Al = 2
NO3 = 3
Example - Photosynthesis
• Word Equation:
water + carbon dioxide + sun energy produces carbohydrate + oxygen
• Skeleton Equation:
H2O + CO2 + sun energy  C6H12O6 + O2
Example - Photosynthesis
H2O + CO2 + sun energy  C6H12O6 + O2
• H appears only once on each side. Choose a
coefficient 6 for H2O (to put 12 hydrogen on each
side) (6x2 = 12)
6H2O + __CO2 + energy __C6H12O6 + __O2
• C appears only once on each side. Choose a
coefficient 6 for CO2 (6x1 = 6)
6H2O + 6CO2 + energy __C6H12O6 + __O2
Example - Photosynthesis
6H2O + 6CO2 + sun energy  C6H12O6 + O2
• Oxygen appears more than once on both sides.
Count the total on each side and make the
adjustment as needed.
6O + 12O = 18O
6O + 6x2O = 18O
6H2O + 6CO2 + energy  C6H12O6 + 6O2
H = 12
H = 12
C=6
C=6
O = 18
O = 18
Diatomic Elements
Diatomic means that they exist as X2 instead
of just X in the natural environment.
There are 7 diatomic elements:
•
•
•
•
•
•
•
Hydrogen (H2)
Nitrogen (N2)
Oxygen (O2)
Fluorine (F2)
Chlorine (Cl2)
Iodine (I2)
Bromine (Br2)
Types of Reactions
• Reactions can be either Exothermic or
Endothermic.
• Exothermic reactions: Reactions where
energy is…
released.
• Endothermic reactions: Reactions where
energy is…
absorbed
Types of Reactions
• In Photosynthesis, energy is required. It is on the
reactant side of the equation.
Therefore it is…
Endothermic
• In Cellular Respiration, energy is given off. It is on
the product side of the equation.
Therefore it is…
Exothermic
*Even if a small amount of energy is needed to start the reaction
(like a match), if there is energy generated by the reaction
itself (like an explosion), it is exothermic
Types of Reactions: Synthesis
• When a reaction has two or more reactants
that synthesize to form a new product.
X + Y  XY
ex: water is synthesised from hydrogen and
oxygen gas:
2H2(g) + O2(g)  2H2O(l) + thermal energy
Types of Reactions: Decomposition
• A compound decays into two or more
simpler compounds or elements.
XY  X + Y
ex: water + energy decays into hydrogen and
oxygen gas (boiling water):
2H2O(l) + thermal energy  2H2(g) + O2(g)
Types of Reactions: Single Displacement
• One element takes the place of (displaces)
another element in a compound
A + BX  B + AX or
AX+ Y  AY + X
ex: 2Mg(s) + 2 HCl(aq)  2MgCl(aq) + H2(g)
2AgNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2Ag(s)
One element takes the place of another 
Types of Reactions: Single Displacement
• This is like someone cutting in on a
couple dancing.
• Before the ‘cut-in’ there are two people
dancing and one single.
• After the ‘cut-in’ there are still two people
dancing but one has changed and there is
a different single.
Types of Reactions: Single Displacement
• Predict the products of the following
reaction:
• cobalt + aluminum chloride 
• The metal element (cobalt) replaces
the positive ion (aluminium) in the
compound.
•
cobalt + aluminum chloride  aluminum + cobalt(II) chloride
Types of Reactions: Single Displacement
• How did we know the charge on
cobalt was +2 since it is multi-valent
(has more than 1 charge)?
• When predicting single displacement
reaction products, we choose the most
common charge for the metal ion.
• The most common charge is listed first on
your periodic table of ions.
Types of Reactions: Double Displacement
• The positive ions of two different compounds
change (displace) places, forming 2 new
compounds.
WX + YZ  WZ + YX
ex: NaOH(aq) + HCl(aq)  NaCl(aq) + HOH(l)
Types of Reactions: Double Displacement
• This is like a dance where two
couples are dancing and they
exchange partners.
• Before the exchange everybody is
dancing.
• After the exchange, everybody is still
dancing but with a different partner.
Types of Reactions: Double Displacement
• Predict the products of the following reaction:
– cobalt(II) nitrate + aluminum chloride 
• The metal ions (cobalt(II) and aluminium) switch
places.
cobalt(II) nitrate + aluminum chloride  cobalt(II) chloride + aluminum nitrate
• Notice that the charge on each metal ion remains
the same on both sides of the reaction arrow.
• Remember to balance the reaction after you have
predicted the products
– 3Co(NO3)2 + 2AlCl3  3CoCl2 + 2Al(NO3)3
Types of Reactions: Combustion
• The reaction of a substance with a
diatomic oxygen molecule is said to be a
combustion reaction.
• AB + O2  AO + BO
• Combustion reactions are almost always
exothermic (they give off heat).
• For example when wood burns, it must
do so in the presence of O2 and a lot of
heat is produced:
Types of Reactions: Combustion
• Wood as well as many common items that
combust are organic (i.e., they are made up of
carbon, hydrogen and oxygen).
• Organic chemistry is chemistry involving these
3 elements and has a lot to do with biological
make up.
• When organic molecules combust the reaction
products are carbon dioxide and water (as well
as heat).
Which is the Which of Which one?
___ Pb(NO
___
PbO ++ 4___
O2 Decomposition
2 Pb(NO
2 PbO
NONO
3)23)
2 
2 + 21+O___
2
3 Ca
+ 2 SO
Ca3Ca
(SO
___ Ca
+ ___
SO33 
 1___
3)2 3)2
3(SO
Synthesis
1 Ca
+2H
Ca(OH)
___ Ca
+ ___
H22O
O
 1___
Ca(OH)
H2
2 + 21+H___
2
Single
Displacement
2 CrCl
+3H
Cr2Cr
(SO
+)36+HCl
___ CrCl
H22SO
SO44 
 1___
___ HCl
4 )3 4
3 +3 ___
2(SO
Double
Displacement
1 Fe(NO
+ 3 NH
Fe(OH)
___ Fe(NO
NH44OH
OH 
 1___
Fe(OH)
___
NH
3 ___
3 + 33+NH
4NO
3 4NO3 Double
3)33)+
Displacement
Single
___ Al
++
___
PO44
1___
AlPO
+
___
K
1 Al
1 KK33PO
AlPO
+
3
K
4
4
Displacement
___ Fe
++
___
O22
2___
4 Fe
3O
Fe2Fe
O32O3
Combustion