World’s Most
Dangerous Bellringer
If the 4 young ladies in the front
row have a combined weight of
2.15x1011micrograms. What is
the average weight of each?
Convert their weight to Pounds!
I
II III IV V
Ch. 9 – 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

A. Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS
A. Chemical Equations
B. Writing Equations
2H2(g) + O2(g)  2H2O(g)

Identify the substances involved.

Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
Remember the diatomic elements.
 In what state? - physical state
B. 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)
C. 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
D. Balancing Equations

An equation is balanced when it has
the same # and kind of atoms.
D. Balancing Equations
Law of Conservation of Mass: mass
is neither created nor destroyed in a
chemical reaction (Lavoisier)
 total mass stays the same; atoms can
only rearrange.

4H
36 g
2O
4H
2O
4g
32 g
D. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # atoms
3MgCl2
4Ca(OH)2
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Double check atom balance!!!
D. Helpful Tips
Balance one element at a time.
 Update ALL atom counts after adding
a coefficient.
 If elements appear more than once
per side, balance them last.
 Balance polyatomic ions as single
units.
 “1 SO4” instead of “1 S” and “4 O”
