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
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