Q: How could you chemically
describe this reaction?
A: Scientists use a standard method
to represent a chemical reaction,
called a chemical equation.
Chemical equations are
used to represent
CHEMICAL REACTIONS
Chemical equations are
used to represent
CHEMICAL
REACTIONS
General equation for
reaction I which iron rusts:
Fe + O2 → Fe2O3
• Iron + Oxygen → Iron Oxide (RUST)
2Al + 3I2  Al2I6
VIDEO: Aluminum and Iodine
Reaction
Q: What are the reactants and
products?
2Al + 3I2  Al2I6
Reactant(s)
The atoms/compounds
you have BEFORE the
reaction occurs.
Product(s)
The atoms/compounds
you have AFTER the
reaction occurs.
When is an equation balanced?
When there is the same the
number of each type of atom
on both sides
BEFORE:
4 White (H)
1 Black (C)
4 Red (O)
AFTER:
4 White (H)
1 Black (C)
4 Red (O)
Chemical Equations
coefficients
4 Al (s) + 3 O2 (g)  2 Al2O3 (s)
Coefficient: numbers in front of the
letters, represents how many molecules
you have
Chemical Equations
4 Al (s) + 3 O2 (g)  2 Al2O3 (s)
subscripts
Subscript: numbers behind the letters,
represent how many atoms are in that
molecule
Chemical Equations
4 Al (s) + 3 O2 (g)  2 Al2O3 (s)
The letters (s), (g), and (l) are the physical states
(aq) means aqueous
The arrow means : yield/create/produce/form
Learning Check
2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g)
What does “2“ mean in front of LiOH?
Number of molecules
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
Q: What is the chemical equation
for this reaction?
A: The chemical equation is:
C + O 2 → CO 2
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.
What bonds are broken and what new
bonds have formed?
C + O 2 → CO 2
C + O=O → O=C=O
Bonds
Break
New Bonds Form
A: Bonds between the Oxygen atoms
have broken
New bonds have formed between Carbon
and the 2 Oxygens
C + O 2 → CO 2
C + O=O → O=C=O
Bonds
Break
New Bonds Form
Q: What type of bond has formed between the
Carbon and 2 Oxygens?
A: COVALENT Bond (Nonmetal to nonmetal)
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
Learning Check
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 to create
the products
Learning Check
Q: Is this chemical reaction correct?
Explain why.
+

+
A: No. The numbers of each type of square on
both sides are not equal
Learning Check
Why is this reaction correct?
The numbers of each type of atom on both
sides are equal.
(so the atoms rearranged)
+

+
NEW
PRODUCTS
To Balance and equation, you are
only allowed to add Coefficients (the numbers in
front of chemical formulas in the equation)
You CANNOT add or change the subscripts
because this would change the identity of the
molecules
For example: O2 is Oxygen but O3 is Ozone
The order of the Coefficients in the Chemical Equation is
important because this is how you write the COEFFICIENT
RATIO.
Balancing using shapes
2
1
2
____H
+
____O

____H
2
2
2O
Is this reaction balanced?
Now fix the H on the left.
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
H-2
O-2
P
H-2
O-1
This equation is NOT balanced!
Balancing Equations using T-chart
2 H2(g) + ___
1 O2(g) ---> ___
___
2 H2O(l)
R
2. Add coefficients
to balance the atoms
3. Don’t forget
to recount.
H-2
H-2 4
O-2
O-1 2
Now fix the H on the left.
Don’t forget there is
a 1 in front of O2.
P
4
Now it is
balanced!
Now You Try!
Now fix the Fe atoms
2x2=4
4
3
2
_____Fe + ____O2  ____Fe2O3
Find the common denominator:
6
6/3 = 2
6/2 = 3
Coefficient ratio: 4:3:2
Now You Try Again!
RULE: Balance O and H Last!
2 Al
+
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