1. Balancing Chemical Equations

2. Balancing Chemical Equations

3. 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?

4. 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.

5. But Before We Balance Equations….
we must learn how to count the number of atoms in a given formula!

6. Multiply the total number of atoms inside the parentheses by the
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

7. Examples Parentheses Subscripts Extras Coefficients 2Ba(NO3)2
4(NH4)3PO4
P – 4 atoms total
N 1
O 3
4 atoms
Parentheses
P – 5 atoms total
N 1
H 4
5 atoms
Subscripts
S – 8 atoms total
(4)(2) = 8 atoms
S – 15 atoms total
(5)(3) = 15 atoms
Extras
E – 9 atoms total
8 + 1 = 9 atoms
E – 20 atoms total
= 20 atoms
C – 18 atoms total
(9)(2) = 18 atoms
Coefficients
C – 80 atoms total
(20)(4) = 80 atoms

8. 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

10. Example: Iron (II) Sulfide
iron + sulfur iron (II) sulfide
Fe + S FeS
Note that in a chemical equation, by convention, we use the arrow “ " instead of the equals “ = ".

11. Balancing Equations 2 3
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

12. 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

13. Mg + O2  MgO Mg + O2  MgO 2 Then start balancing:
[1] Just count up the atoms on each side
Mg + O2  MgO Mg O 1 1 2 1
[2] The numbers aren’t balanced so then add “BIG” numbers to make up for any shortages
Mg + O2  MgO Mg O 1 2 2 2 2
And adjust totals

14. WE SAY THAT THE EQUATION IS BALANCED!!
But the numbers still aren’t equal, so add another “BIG” number 2
Mg + O2  MgO Mg O 1 2 2
And adjust totals again
NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS
WE SAY THAT THE EQUATION IS BALANCED!!

15. 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

16. How did you do??
Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH O2  CO H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3