CHEMICAL
EQUATIONS &
REACTIONS
Describing Reactions
What is a Chemical Reaction?
Occurs when compounds are mixed
(sometimes with help)
Chemical structures and properties are
changed
Describing Reactions

Chemical equations: chemical
formulas are used to express
chemical and physical changes
 Reactants
 Products
are written on the left
written on right
 Separate
multiple reactants/products
with plus sign(s)
 Separate
an arrow
reactants from products with
Describing Reactions
X+Y  Z
Reactants
Products
W+X  Y+Z
Reactants
Products
Chemical Equations
 Chemical
equations follow a set of rules
 Matter
cannot be created or destroyed
during a reaction, only changed
 Law
of Conservation of Matter!
 Chemical
 Use
equations will be balanced
coefficients (mole-mole ratio) so that
atoms present in reactants are also found
in products
Law of Conservation of
Matter
C + O2  CO2
Why is Oxygen written as “O2”??
Some elements will only be stable
when they can group in pairs (H2, N2,
O2, F2, Cl2, Br2, I2)
Diatomic
“Two atoms”
Are they balanced?
H2 O  H 2 + O 2
NaCl + H2SO4  HCl + Na2SO4
C3H8 + O2  H2O + CO2
Rules for Balancing Equations
1. Count
up starting atoms
2. Balance

all non-H’s and O’s first
Look for element(s) that only appear in one reactant
and one product!
3. Add
a coefficient to the front of a substance
with the fewer number of atoms.

NEVER change a subscript!
4. Re-count
write
5. Balance
atoms after each coefficient you
H’s, then O’s
6. Re-check
the count
Balancing Equations
H 2 O  H2 + O 2
1.Count up starting atoms
Reactants
Products
H
2
2
O
1
2
Balancing Equations
H 2 O  H2 + O 2
2. Balance all non-H’s and O’s first
There aren’t any! Go to the next step!
Balancing Equations
H2 O  H 2 + O 2
3.Add a coefficient to the front of a substance
with the fewer number of atoms.
_2_H2O  ___H2 + ___O2
Balancing Equations
H2 O  H 2 + O 2
4. Re-count atoms after each coefficient you write
_2_H2O  ___H2 + ___O2
Reactants
Products
H
2
4
O
2
2
Balancing Equations
H2 O  H 2 + O 2
5. Balance H’s, then O’s
_2_H2O  ___H2 + _2_O2
Balancing Equations
H2 O  H 2 + O 2
6. Re-check the count
_2_H2O  ___H2 + _2_O2
Reactants
Products
H
4
4
O
2
2
More Balancing Tips
 NEVER
change a subscript!
 The final equation only has whole
numbers!
 Leave single atoms and diatomic
molecules until the end
 If a polyatomic ion is on both sides,
you don’t have to split up the
atoms in your tally
Learning Check

Which of the following sets of coefficients correctly
balances the following chemical equation?
Mg + AgNO3  Ag + Mg(NO3)2
•
•
•
•
1, 1, 1, 1
1, 3, 1, 2
2, 1, 1, 2
1, 2, 2, 1
Balancing Equations
NaCl +
H2SO4 
HCl +
Na2SO4
Reactants
Products
Na
1
2
Cl
1
1
H
2
1
SO4
1
1
Balancing Equations
2 NaCl +
H2SO4 
2 HCl +
Reactants
Products
Na
2
2
Cl
2
2
H
2
2
SO4
1
1
Na2SO4
Balancing Equations
C3H8 +
O2 
H2O +
CO2
Balancing Equations
C3H8 +
O2 
H2O +
Reactants
Products
C
3
1
H
8
2
O
2
3
CO2
Balancing Equations
C3H8 + 5 O2  4 H2O + 3 CO2
Reactants
Products
C
3
3
H
8
8
O
10
10
Balancing Equations
Zinc + hydrogen sulfate (sulfuric acid)
 zinc sulfate + hydrogen gas
Zn + H2SO4  ZnSO4 + H2
Classifying Equations
Reaction Type
General Formula
Combustion:
A+ O2  CO2 + H2O
Single
A + BY  B + AY
Displacement:
AX + Y  X + AY
Double
Displacement: AX + BY  BX + AY
Composition:
A + B  AB
Decomposition:
AB  A + B