Ch. 8 – Chemical Reactions
I. Intro to Reactions
II. Balancing Equations
(p. 241 – 254)
A.Signs of a Chemical Reaction
Evolution of heat and light
- large amounts relative to physical
 Formation of a gas
- effervescence, new smell
 Formation of a precipitate
- obvious solid
 Color change
- sometimes...

B.Law of Conservation of Mass

mass is neither created nor destroyed
in a chemical reaction
total mass stays the same
 atoms can only rearrange

4H
36 g
2O
4H
2O
4g
32 g
C. Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS
C. Chemical Equations
p. 246
D. Writing Equations
2H2(g) + O2(g)  2H2O(g)
Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
 In what state? - physical state

Remember the diatomic elements.
D. Writing Equations
Aluminum reacts with aqueous copper(II)
chloride to produce copper and aqueous
aluminum chloride.
2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq)
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
D. Writing Equations

Coefficients can mean:
 individual atom = “atom”
 covalent substance = “molecule”
 ionic substance = “formula unit”
3CO2  3 molecules
2Mg
 2 atoms
4MgO  4 formula units
REACTING
MOLAR
RATIOS!!
E. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Double check atom balance!!!
E. Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
E. Balancing - Helpful Tips
Balance one element at a time.
 Update ALL atom counts after adding
a coefficient.
 If an element appears more than
once per side, balance it last.
 Balance polyatomic ions as single
units.
 “1 SO4” instead of “1 S” and “4 O”

E. Balancing Examples
Fe +
H2O -->
Na +
O2 -->
CaO +
S+
Fe3O4 +
Na2O
H2O -->
O2 -->
H2
SO3
Ca(OH)2
F. Algebraic Method
equation:
Al2(SO4)3 + CaCl2 --> AlCl3 + CaSO4
add letters:
a Al2(SO4)3 + b CaCl2 --> c AlCl3 + d CaSO4
list atoms:
Al
S
O
Ca
Cl
assign variables:
2a = c
3a = d
12a = 4d
b=d
2b = 3c
F. Algebraic Method
calculate:
a=1 c=2
d=3
b=3
finish with coefficients:
1Al2(SO4)3 + 3CaCl2 --> 2AlCl3 + 3CaSO4
CHECK YOUR WORK!!
F. Algebraic Method
BCl3 +
P4 +
H2 
BP +
HCl