Chemical Formulas
and
Equations
Getting started with some definitions…
ATOM - is the smallest whole part of an element
ELEMENT - matter made of only one kind of atom
MOLECULE - two or more elements that have
chemically combined
Each new element is identified by a capital letter
 Example: H2SO4

 There



are three ELEMENTS in sulfuric acid
Hydrogen
Sulfur
Oxygen
CHEMICAL FORMULA - contains chemical
symbols and subscripts to show the number
and types of atoms in a compound
SUBSCRIPT – after a chemical symbol and
shows the number of each ATOM in a chemical
formula
• Example: C12H22O11
•
•
•
•
12 atoms of Carbon
22 atoms of Hydrogen
11 atoms of Oxygen
If there is no subscript, it is understood to be 1
•
Example: NaCl
•
•
1 atom of Sodium
1 atom of Chlorine
You Practice!
NaHCO3
K2Cr2O7
Na = 1
H=1
C=1
O=3
K=2
Cr = 2
O=7
What about Formulas with Parentheses???
EX:
Pb(NO3)2
• The “2” after the parenthesis indicates there are
two of what’s in the parentheses
•
You could think of it this way:
(NO3)
and
(NO3)
You MULTIPLY everything INSIDE the
parentheses by the subscript at the end:
Pb = 1
N = 2 (1 N x 2)
O = 6 (3 O x 2)
You Practice!!
(NH4)3PO4
Mg(OH)2
N=3
H = 12
P=1
O=4
Mg = 1
O=2
(2 x 1 O)
H=2
(2 x 1 H)
(3 x 1 N)
(3 x 4 H)
(1 x 1 P)
(1 x 4 O)
COEFFICIENT - in front of the chemical formula
and shows the number of MOLECULES
2 H2(SO4)
•
This means there are 2 molecules of H2(SO4)
•
You could think of it this way –
H2(SO4) and H2(SO4)
•
MULTIPLY coefficients by ALL of the subscripts
in the chemical formula:
2H2SO4
H = 4 (2 x 2 H)
S = 2 (2 x 1 S)
O = 8 (2 x 4 O)
You Practice!!
2 H2O
H=4
O=2
3 Ca3(PO4)2
(2 x 2 H)
(2 x 1 O)
Ca = 9 (3 x 3 Ca)
P = 6 (3 x 1 P x 2)
O = 24 (3 x 4 O x 2)
Got It????
Homework Time…
Counting Atoms WS
Chemical Equations
A CHEMICAL EQUATION shows what happens in a
chemical reaction
2 K + Cl2
2 KCl
Reactants
Products
“yields”
•
The + separates molecules on the same side
Law of Conservation of Mass
•
States that mass cannot be
created or destroyed
•
•
In other words, the mass of the
reactants is equal to the mass of
the products
Proved by French chemist
Antoine Lavoisier (1742-1794)
•
known as the “Father of Modern
Chemistry”
Lavoisier, 1788
So what does that have to do with anything??
Well, consider the following equation:
•
H 2 + O2
H2O
Reactants:
H=2
O=2
Products:
H=2
O=1
This cannot happen….an oxygen
atom cannot be destroyed!!!
So what do we do?
The chemical equation must be written
according to the law of conservation of mass
How it actually happens in nature is like this….
2 H 2 + O2
2 H 2O

Coefficents are added to balance on paper…
Reactants:
H=4
O=2
Products:
H=4
O=2
There MUST be the same number AND types of
atoms on both sides of the equation!
Are These Equations Balanced?
Na + O2
Na = 1
O=2
Mg + 2 HCl
Mg = 1
H=2
Cl = 2
Na2O
Na = 2
O=1
NOPE
MgCl2 + H2
Mg = 1
YEP!!
H=2
Cl = 2
You Practice!!!
MgO
Mg = 1
O= 1
N2 + 3 H2
N= 2
H= 6
Mg + O2
Mg = 1
O= 2
2 NH3
N= 2
H= 6
NOPE
YEP!!
Zn + 2 HCl
ZnCl2 + H2
Zn = 1
H= 2
Cl = 2
Zn = 1
H = 2
Cl = 2
YEP!!
More Practice
K + Br2
K= 1
Br = 2
KBr
K= 1
Br = 1
Fe + O2
Fe = 1
O= 2
2Na + 2H2O
Na = 2
O= 2
H= 4
6Fe2O3
NOPE
Fe = 12
O = 18
2NaOH + H2
Na = 2
O=2
YEP!!
H= 4
NOPE
So What About the Unbalanced
Equations???...
WE MUST BALANCE THEM!!
2 K + Br2
K= 1 2
Br = 2
2 KBr
K=1 2
Br = 1 2
Add COEFFICIENTS and recount
More Practice
6 Fe + 9 O2
Fe = 1 12
O = 2 18
6Fe2O3
Fe = 12
O = 18
Add COEFFICIENTS and recount
One More!!!
4 Na + O2
Na = 1 4
O=2
2 Na2O
Na = 2 4
O=1 2
Add COEFFICIENTS and recount
Got It???
Your turn… Homework time!!
Balanced or Not?