Download Balancing Chemical Equations

Balancing Chemical Equations
Balancing Chemical Equations
Law of Conservation of Mass
Do you remember this law?

The Law of Conservation of Mass
states: that mass is neither created
nor destroyed in any chemical
reaction.

Can you provide an example that
illustrates this law?
Law of Conservation of Mass

Because mass can be neither created
nor destroyed during a chemical
change…

the balancing of equations requires the
same number of atoms on both sides of
a chemical reaction.
But Before We Balance Equations….
we must learn how to count
the number of atoms in a
given formula!
Totaling Atoms Using PSEC
Parentheses
Count up the total number of atoms inside the parentheses
Subscripts
Multiply the total number of atoms inside the parentheses by the
subscript value
Extras
Add any extra atoms that were not yet accounted for to the total
Coefficients
Multiply the total number of atoms (from PSE) by the coefficient
Examples
2Ba(NO3)2
P – 4 atoms total
N 1
O 3
4 atoms
4(NH4)3PO4
Parentheses
P – 5 atoms total
N 1
H 4
5 atoms
S – 8 atoms total
Subscripts
E – 9 atoms total
Extras
E – 20 atoms total
C – 18 atoms total
Coefficients
C – 80 atoms total
(4)(2) = 8 atoms
8 + 1 = 9 atoms
(9)(2) = 18 atoms
S – 15 atoms total
(5)(3) = 15 atoms
15 + 5 = 20 atoms
(20)(4) = 80 atoms
Law of Conservation of Mass

The mass of all the reactants (the
substances going into a reaction) must
equal the mass of the products (the
substances produced by the reaction).

Reactant + Reactant = Product
Example: Iron (II) Sulfide



iron + sulfur
Fe + S
iron (II) sulfide
FeS
Note that in a chemical equation, by
convention, we use the arrow “
" instead
of the equals “ = ".
Balancing
Equations
2
3
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)
Learning Through Example
Magnesium Oxide
Mg + O2  MgO
Then all you do is list the atoms that are involved
on each side of the arrow
Mg + O2  MgO
Mg
O
Mg
O
Then start balancing:
[1] Just count up the atoms on each side
Mg + O2 
MgO
1
Mg
1
2
O
1
[2] The numbers aren’t balanced so then add “BIG”
numbers to make up for any shortages
Mg + O2  2 MgO
And adjust totals
1
Mg
1
2
2
O
1
2
But the numbers still aren’t equal, so add
another “BIG” number
2 Mg + O2  2 MgO
2
1
Mg
2
2
O
2
And adjust totals again
NOW BOTH SIDES HAVE EQUAL
NUMBERS OF ATOMS
WE SAY THAT THE
EQUATION IS BALANCED!!
Try to balance these equations using the same
method:
[1] Na + Cl2  NaCl
[2] CH4 + O2  CO2 + H2O
[3] Li + HNO3  LiNO3 + H2
[4] Al + O2  Al2O3
How did you do??
Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH4 + 2 O2  CO2 + 2 H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3