1. Section 2.7—Balancing Equations
We need to finish writing those equations we started!

2. Law of Conservation of Matter/Mass
Law of Conservation of Matter – Matter cannot be created nor destroyed during chemical or physical changes
Also called the Law of Conservation of Mass (since all matter has mass)

3. How Does the Law Lead to Balancing?
Law of
Conservation
of Matter
Therefore…
So we must…
Matter cannot be created nor destroyed during a chemical or physical change
The matter on the reactants side and the matter one the products side must be the same
Ensure the numbers of each type of atom are the same on both sides of the equation…by balancing!

4. How do we Balance Equations?
Number of compounds in the reaction
Coefficients
2 H2 + O2  2 H2O
Subscripts
# of atoms in a compound
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation

5. What do Coefficients Really Mean?
CH4 + 2 O2  CO2 + 2 H2O
Total:
1 C
4 H
4 O
Total:
1 C
4 H
4 O
The equation is balanced.

6. How to Balance Chemical Equations

7. How to Balance By Inspection:1
Make a table of elements
_____ C H4 +
_____ O2 
_____ H2 O +
_____ C O2
Reactants
Products C H O

8. How to Balance By Inspection:2
Count the number of each element or ion on the reactants and products side.
Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound!
_____ C H4 +
_____ O2 
_____ H2 O +
_____ C O2
Reactants
Products C 1 1 H 4 2 O 2 3

9. How to Balance By Inspection:3
Add coefficients to balance the numbers
Each time you add a coefficient, update your table with the new quantities of each atom.
_____ C H4 +
_____ 2 O2 
_____ 2 H2 O +
_____ C O2
Reactants
Products C 1 1 H 4 2 4 O 2 4 3 4

10. How to Balance By Inspection:4
Place a “1” in any empty coefficient location
Filling each coefficient location lets you and the grader know that you finished the problem rather than you left some blank because you weren’t done!
_____ 1 C H4 +
_____ 2 O2 
_____ 2 H2 O +
_____ 1 C O2
Reactants
Products C 1 1 H 4 2 4 O 2 4 3 4

11. Choosing the Order of Balancing
How do you know what order to balance in?
Start Elements that appear only 1 time per side
Save for later
Elements that are uncombined
Pb + PbO2 + H+  Pb2+ + H2O
Start Elements in most complicated molecules
Save for later
Elements that appear more than 1 time per side
To balance this equation, use the order: O, H, Pb

12. How is Balancing Affected by Order?
What happens if we balance in the order determined in the last slide?
O, H, Pb
_____ 1 Pb +
_____ 1 Pb O2 +
_____ 4 H+ 
_____ 2 H2 O +
_____ 2
Pb2+
Reactants
Products O 2 1 2 H 1 4 2 4 Pb 2 1 2

13. What about a different order?
How is it different if we balance in a different order?
H, O, Pb 4
_____ 1 Pb +
_____ 1 Pb O2 +
_____ 2 H+ 
_____ 2 H2 O +
_____ 2
Pb2+
Reactants
Products  H 1 2 4 2 4 O 2 1 2 Pb 2 1 2
You’ll still get to the correct answer, but it will take longer and be more complicated!

14. Polyatomic Ions
Polyatomic ion – Group of atoms that together has a net charge
e.g Nitrate NO31-
Carbonate CO32-
A polyatomic ion is a group of atoms that together have a charge.

15. Balancing with Polyatomic Ions:1
Make a table of elements
You may leave polyatomic ions together—IF they appear intact on both sides of the reaction.
OH is a polyatomic ion that is sometimes “hidden” in H2O.
Re-write H2O as HOH to “see” the OH polyatomic ion. H OH
_____ H3
PO4 +
_____ Ca
(OH)2 
_____
Ca3
(PO4)2 +
_____ H2 O
Reactants
Products H
PO4 Ca OH

16. Balancing with Polyatomic Ions:
Count the number of each element or ion on the reactants and products side. 2 H OH
_____ H3
PO4 +
_____ Ca
(OH)2 
_____
Ca3
(PO4)2 +
_____ H2 O
Reactants
Products H 3 1
PO4 1 2 Ca 1 3 OH 2 1

17. Balancing with Polyatomic Ions:3
Add coefficients to balance the numbers H OH
_____ 2 H3
PO4 +
_____ 3 Ca
(OH)2 
_____
Ca3
(PO4)2 +
_____ 6 H2 O
Reactants
Products H 3 6 1 6
PO4 1 2 2 Ca 1 3 3 OH 2 6 1 6

18. Balancing with Polyatomic Ions:5
Place a “1” in any empty coefficient location H OH
_____ 2 H3
PO4 +
_____ 3 Ca
(OH)2 
_____ 1
Ca3
(PO4)2 +
_____ 6 H2 O
Reactants
Products H 3 6 1 6
PO4 1 2 2 Ca 1 3 3 OH 2 6 1 6

19. Balance the following equation
Let’s Practice #1
Example:
Balance the following equation
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O

20. Balance the following equation
Let’s Practice #1
Example:
Balance the following equation
Did you see the “OH” polyatomic ion & change H2O to HOH? 2 1 1 2
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O
HOH

21. Balance the following equation
Let’s Practice #2
Example:
Balance the following equation
__ H2 + __ O2  __ H2O

22. Balance the following equation
Let’s Practice #2
Example:
Balance the following equation 2 1 2
__ H2 + __ O2  __ H2O

23. Balance the following equation
Let’s Practice #3
Example:
Balance the following equation
__ Fe + __ O2  ___ Fe2O3

24. Balance the following equation
Let’s Practice #3
Example:
Balance the following equation 4 3 2
__ Fe + __ O2  ___ Fe2O3