1. Balancing Chemical Equations

2. Overview Brief Review Law of Conservation of Mass
Balancing Chemical Equations

3. Review
Atoms combine chemically by sharing, accepting or donating electrons.
Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

4. An equation is a short hand way of writing a chemical reaction.
Reactants go on the left, products are on the right
Mg + 2HCl  MgCl2 + H2

5. Law of Conservation of Mass
Credited to Antoine Lavoisier (1788)
Was not the first to demonstrate philosophically or experimentally
First comprehensive theory that was generally accepted

6. "Nothing is created, either in the operations of art or in those of nature, and it may be considered as a general principle that in every operation there exists an equal quantity of matter before and after the operation; that the quality and quantity of the constituents is the same, and that what happens is only changes, modifications. It is on this principle that is founded all the art of performing chemical experiments; in all such must be assumed a true equality or equation between constituents of the substances examined, and those resulting from their analysis."
Word Count: 96

7. More Simply Stated . . .
In a chemical reaction, matter is neither created nor destroyed.
There is always balance between reactants (what you put in) and products (what comes out)

8. Because of the principle of the conservation of matter,
an equation must be balanced.
It must have the same number of the same kind of atoms on both sides.
Lavoisier, 1788

11. Subscripts vs. Coefficients
The subscripts tell you how many atoms of a particular element are in a compound.
The coefficient tells you about the number of molecules of the compound.

12. Subscripts Mg + 2HCl  MgCl2 + H2
Subscript: Number after a chemical symbol
Mg + 2HCl  MgCl2 + H2

13. Coefficients Mg + 2HCl  MgCl2 + H2 Applies to the entire formula
Number before a chemical symbol or formula
Mg + 2HCl  MgCl2 + H2
Applies to the entire formula
2HCl tells us that the reactants include 2 atoms of both H and Cl

14. If a formula contains both a coefficient and subscript, they multiply
2MgCl2
2 atoms of Mg (coefficient of 2 * subscript of 1)
4 atoms of Cl (coefficient of 2 * subscript of 2)

15. Balancing Equations
When balancing a chemical reaction you may place coefficients in front of the compounds to balance the reaction, but you may not change the subscripts.
Changing the subscripts changes the compound.

16. H2O

17. A Brief Intermission… Q: What do you do when you find a dead chemist?
A:Barium

18. (what is produced by the reaction)
Example 1
Products
(what is produced by the reaction)
Reactants
(what you put in) =
Fe + S  FeS
Reactants
Fe: 1
S: 1
Product
Fe: 1
S: 1
This equation is already balanced. It is not usually this easy, however . . .

19. H2 + Cl2  HCl Example 2 Reactants Product H: 2 Cl: 2 H: 1 Cl: 1
We need to balance the equation using coefficients