Ch. 8 – Chemical Reactions
I.
Intro to Reactions
I
II III IV V
A.Signs of a Chemical Reaction
Evolution of heat and light
 Formation of a gas
 Formation of a precipitate
 Color change

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)

Identify the substances involved.

Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
 In what state? - physical state

Remember the diatomic elements.
D. Writing Equations
Two atoms of aluminum react with
three units of aqueous copper(II)
chloride to produce three atoms of
copper and two units of aqueous
aluminum chloride.
• How many?
• Of what?
• In what state?
2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq)
E. Describing Equations

Describing Coefficients:
 individual atom = “atom”
 covalent substance = “molecule”
 ionic substance = “unit”
3CO2  3 molecules of carbon dioxide
2Mg
 2 atoms of magnesium
4MgO  4 units of magnesium oxide
E. Describing Equations
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
• How many?
• Of what?
• In what state?
One atom of solid zinc reacts with
two molecules of aqueous
hydrochloric acid to produce one unit
of aqueous zinc chloride and one
molecule of hydrogen gas.
Ch. 8 – Chemical Reactions
II. Balancing Equations
(p. 250-254)
I
II III IV V
A. 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!!!
B. 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”

C. Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6