Intro to Balancing Equations
Q: How could you represent this
reaction (rusting bicycle),
besides just describing it in
words?
A: Scientists use a standard method
to represent a chemical reaction,
called a chemical equation.
Fe + O2  Fe2O3
(Rust)
ANALOGY for a Chemical Equation
WHEEL
FRAME
HANDLEBAR
PEDAL
BYCICLE - New product
Chemical equations are used
to represent
CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
Q: What is the general equation
for the reaction in which iron
rusts?
Iron + Oxygen → Iron Oxide
Or, using symbols:
Fe + O2 → Fe2O3 (rust)
Q: What are the reactants and
products?
Fe + O2 → Fe2O3 (new product - rust)
Reactants Products
Chemical Equations
Because of the principle
of the conservation of
matter, an equation
must be balanced.
Lavoisier, 1788
When is an equation called
balanced?
when an equation has
the same number of
atoms of the same
kind on both sides.
Learning Check:
A chemical reaction is balanced
when
a. The reactants are the same as the products
b. The same atoms and molecules appear on
each side of the equation
c. The same number of each type of atom
appears on both sides of the arrow.
d. None of the above
The substances that begin a
chemical reaction are called
REACTANTS
The substances that form in a
chemical reaction are called
PRODUCTS
Chemical Equations
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
 The numbers in the front are called
coefficients
subscripts
 The letters (s), (g), and (l) are the
physical states of compounds.
(aq) – aqueous solution
 The arrow  means:
to yield = to produce = to form
Learning Check – Cold Call
What does “2“ mean in front of LiOH?
Number of molecules
2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g)
What does “2“ to the right of “H2” mean?
Number of atoms of H
What does “(aq)“ to the right of LiOH mean?
Aqueous solution
What does ““ mean?
Yield, form, produce
1 min – answer in your IAN
Q: What is the chemical equation
for this reaction?
A: The chemical equation is:
C + O 2 → CO 2
Classroom discussion
Q: How have the atoms of the reactants
been rearranged in the products of
the reaction?
C + O 2 → CO 2
A: Carbon atom chemically bonds with
two oxygen atoms.
HOW ? What bonds are broken and
what new bonds have formed?
Classroom discussion
C + O 2 → CO 2
C + O=O → O=C=O
Bonds
Break
New Bonds Form
A: Bonds between the oxygen atoms in
the oxygen molecule have been broken,
and new bonds have formed between the
carbon atom and the two oxygen atoms.
Learning Check 1
30 sec - Turn to your elbow partner…..
Q: What needs to happen for a chemical
reaction (change) to take place?
A: Bonds between atoms of the reactants
break and NEW bonds form, producing
new substance.
Learning Check 2
10 sec  Cold Call
Q: Is this chemical reaction correct?
Explain why.
+

+
A: No. There are no “pink atoms” on the
reactant side – NEW matter (pink) on the
product side. Matter canNOT be created.
Learning Check 2
Why is this reaction correct?
B/C atoms must REARRANGE for a chemical
reaction to take place
+

+
NEW
PRODUCTS
 An ATOM is the smallest particle
of an element.
 Count the number of atoms of
each element below:
O2
H2O
HCl
2x1 =2
Mg(NO3)2
2x3 =6
2 atoms of oxygen (O)
2 atoms of hydrogen &
1 atom of oxygen (O)
1 atom of hydrogen (H) &
1 atom of chlorine (Cl)
1 atom of magnesium (Mg),
2 atoms of nitrogen (N) &
6 atoms of oxygen (O)
 A MOLECULE is a pure substance
composed of two or more atoms.
 Count the number of molecules
below:
1 O2
One molecule of oxygen gas
2H2O
Two molecules of water
17HCl
Seventeen molecules of an acid
Chemical Equations
4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
This equation means:
4 Al atoms + 3 O2 molecules ---> 2 molecules of
Al2O3
Learning Check
NaHCO3
# of “O” atoms?
3
Mg(OH)2
# of “H” atoms?
2
C6H12O6
# of “C” atoms?
6
Complete
Practice – Counting Atoms
5 min
Cold Call
Q: Is this reaction Balanced?
When called on, be ready to
justify your answer
C + O 2 → CO 2
Which of the following equations
is balanced?
a. 3Fe + 3O2  3Fe2O3
b. 2Fe + O2  Fe2O3
c. 4Fe + 2O2  3Fe2O3
d. 4Fe + 3O2  2Fe2O3
When called
on, be ready to
justify your
answer
Does this equation obey the Law
of Conservation of Mass?
(Is it balanced?)
2H2O2  2H2O + O2
YES
What is the coefficient ratio?
2:2:1
Does this equation obey the Law
of Conservation of Mass?
(Is it balanced?)
NO
2Al + O2  Al2O3
Why?
2 atoms of Al of left side
Al is balanced
2 atoms of Al on right side
2 atoms of O on left side
3 atoms of O on right side
O is NOT
balanced
REMEMBER:
To balance chemical equation, you
must add COEFFICIENTS (big
numbers) in front of the chemical
formulas in the equation. You cannot
add or change subscripts!!!
Why?
Because it will alter the identity of the substance otherwise.
Ex. O2 – oxygen, if a subscript changed to 3, O3 = ozone
Balancing using shapes
2
1
2
____H
+
____O

____H
2
2
2O
Is this reaction balanced?
Now fix the H on the left.
Double Hydrogen too.
Coefficient ratio: 2:1:2
BALANCED !
Balancing using shapes
You try!
3
4
2
__Al
O
___Al
+
___O
2 3
2
+
Now fix the Al on the right.
Coefficient ratio: 2:4:3
BALANCED !
Balancing Equations using T-chart
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
1. Count the
number of
atoms of each
element found in
the reaction.
R
P
2H
2H
2O
1O
What Happened to the Other Oxygen Atom?????
This equation is NOT balanced!
Balancing Equations using T-chart
2 H2(g) + ___
1 O2(g) ---> ___
___
2 H2O(l)
2. Add coefficients
to balance the atoms
R
4 2H
2H 4
2O
1O 2
3. Don’t forget
to recount.
Now fix the H on the left.
Don’t forget there is
a 1 in front of O2.
P
Now it is
balanced!
Easy Balancing
RULE: Balance H and O last
2 Al 1
+
3
O2

R
P
2
Al2O3
4 2 1 Al
2 Al 2 x 2 = 4
2x3=6 2O
3O 3x2=6
2:3 – find a common multiple
Coefficient ratio: 4:3:2
The charcoal used in a grill is
basically carbon. The carbon
reacts with oxygen to yield
carbon dioxide. The chemical
equation for this reaction,
C + O2  CO2,
R
P
1C
1C
2O
2O
Is this a balanced
equation?
Count the number of
atoms of each element
found in the reaction.
YES
You Try!
Use shapes or T-cart method
Fe + O2  Fe2O3
2x2 = 4
2
3 2  ____Fe
2
_____Fe
+ ____O
2O3
3x2 = 6
Common multiple
Coefficient ratio: 4:3:2