1. Section 2.7—Balancing Equations
Objectives:
Use the Law of Conservation of Mass to explain the need for balanced equations
Convert word equations to balanced chemical equations

2. Review How to write chemical formulas For ionic compounds:
1) write the symbol and charge for the cation
2) write the symbol and charge for the anion
3) criss-cross the NUMBER only of the charge
to obtain a subscript
4) Reduce the subscripts, if necessary
5) Use parentheses when there is more than
one polyatomic ion needed.

3. Review
b. For binary covalent compounds: 1) Write the symbols of the first and second elements 2) Translate any PREFIXES into a subscript of that element Example: dinitrogen pentoxide N2O5

4. Review
c. For acids without oxygen (the acid will be named with the prefix “hydro”): 1) The cation will be H+ 2) The anion will be found in the root of the acid’s name (following the “hydro”) – determine its symbol & charge 3) Criss-cross the numbers of the charges to get the formula

5. Review d. For acids with oxygen (acid name will end in “ic” or “ous”):
1) The cation is H+
2) If the acid ends in “ic”, look for a
polyatomic anion that ends in “ate”
3) If the acid ends in “ous”, look for a
polyatomic anion that ends in “ite”
4) Criss-cross the numbers of the
charges to get the formula
REMEMBER: Bases are just ionic compounds with the anion “hydroxide”. NH3 (ammonia) is an exception.

6. Review
e. There are seven elements that are found in nature as diatomic molecules. They must be written that way in a formula. H2 N2 O2 F2 Cl2 Br2 I2

7. Review
2. How to write formula equations a. Locate each name of a compound and write its formula (using those rules reviewed in previous slides.) b. Separate all reactants and products with a “+” sign c. Separate reactants ffrom products with an 

8. Practice
Hydrogen and oxygen react to form water.

9. Law of Conservation of Matter/Mass
Notice that, in this practice equation, the number of atoms that we start with does not equal the number of atoms we finish with.
2 H atoms + 2 O atoms  2 H atoms + 1 O atom
This violates a scientific law that says matter cannot be destroyed (or created.)

10. 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)

11. 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 on 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!

12. 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

13. 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.

14. How to Balance Chemical Equations

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

16. How to Balance By Inspection:2
Count the number of each element 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
1 C
1 C
4 H
2 H
2 O
3 O

17. 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
1 C
1 C
4 H
2 4 H
2 4 O
3 4 O

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

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

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

21. 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 2
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O

22. Write and balance the following equation
Let’s Practice #3
Example:
Write and balance the following equation
Iron and oxygen combine to produce iron (III) oxide.

23. Write and balance the following equation
Let’s Practice #3
Example:
Write and balance the following equation 4 3 2
__ Fe + __ O2  ___ Fe2O3