1. What is a Chemical Reaction?
Two atoms can be mixed together to create a new compound (sodium & chlorine)
Can also mix two compounds to create new compounds (baking soda & vinegar)
New substances are produced in each case with properties different from the original substances.

2. Dalton’s Atomic Theory
All matter is made up of small particles called atoms
Atoms cannot be created, destroyed, or divided into smaller particles
All atoms of the same element are identical in mass and size, but different in mass and size from atoms of other elements
John Dalton

3. Conservation of Mass Law of conservation of mass states:
The total mass of the reacting substances (the reactants) is always equal to the total mass of the resulting substances (the products).
Matter is neither created or destroyed in a chemical reaction.
Atoms in the reactants are present in the products.

4. Writing Chemical Equations
Lets look at how water is formed.
First we can look at the word equation.
Hydrogen + Oxygen  Water
Reactants on the left side of the arrow
Products on the right side of the arrow
Plus sign means “reacts with”
Arrow means “produce”

5. The Hindenburg Zeppelin
Hydrogen + Oxygen  Water
Video on File

6. Diatomic Molecules Hydrogen - H2 Chlorine - Cl2 Oxygen - O2
Atoms can share electrons with the same atom.
These molecules have two of the same atoms joined by a covalent bond.
Since there are two of the same atoms the word diatomic is used. (“di” meaning two)
Seven elements are diatomic:
Hydrogen - H2
Chlorine - Cl2
Oxygen - O2
Bromine - Br2
Nitrogen - N2
Iodine - I2
Fluorine - F2

7. Skeleton Equations For water the skeleton equation is: H2 + O2  H2O
According to Conservation of Mass the reactants and products must have the same number of atoms.
We must balance the chemical equation.
This is done by adding coefficients in front of the compounds.

8. Balancing the Reaction
Important: Subscripts CANNOT be changed!!!!!!
Hydrogen is balanced so you must balance the oxygen by adding another water molecule
H2 + O2  2H2O
Now hydrogen is not balanced so we must add another hydrogen molecule
2H2 + O2  2H2O

9. Check The Atoms
Add up all the atoms of both reactants and products to make sure the amounts are the same.
2H2 + O2  2H2O
Reactants
2 H2 Molecules
4 H atoms
1 O2 Molecule
2 O atoms
Products
2 H2O molecules
4 H atoms
2 O atoms

10. Tips for Balancing
Adjust co-efficients, not chemical formulas (subscripts).
Balance metals first
Add co-efficients to individual elements last
If a polyatomic ion appears in in both reactant and product, treat it as a single unit.
Balance oxygen and hydrogen last.
Don’t forget diatomic molecules.
Count your atoms as a check

11. Let’s Try Some Together
Mg + O2  MgO
AgI + Na2S  Ag2S + NaI
NaOH + H2SO4  Na2SO4 + H2O
CaCl2 + Na3PO4  Ca3(PO4)2 + NaCl
FeS + O2  Fe2O3 + SO2

12. Your Turn 1. H2 + O2  H2O 2. S8 + O2  SO3 3. HgO  Hg + O2
4. Zn + HCl  ZnCl2 + H2
5. Na + H2O  NaOH + H2

13. Chemical States
The state of the chemicals is important to the chemical reaction.
State can be added by putting an abbreviation in parentheses after each chemical.
State
Abbreviation
Example
Solid
(s)
NaCl(s)
Liquid
(l)
H2O(l)
Gas
(g)
H2(g), O2(g)
Aqueous
(aq)
NaCl(aq) (in water)

14. Water: Final Balanced Equation
Hydrogen and Oxygen are both gases
Water is a liquid
Final balanced equation with the states of all components is:
2H2(g) + O2(g)  2H2O(l)
You will not be required to give the states for an equation

15. Resources - Links Khan Academy Balancing Equations
pHET Balancing Applet
Balancing Complex Equations with the Table Method Video

16. Resources – Table Example
Step 1: Start with an unbalanced equation

17. Resources – Table Example
Step 2: Draw boxes around all the chemical formulas

18. Resources – Table Example
Step 3: Make an element inventory
Before = Reactant
After = Product

19. Resources – Table Example
Step 4a: Balance the Equation and the Inventory
Before = Reactant
After = Product

20. Resources – Table Example
Step 4b: Final Steps…
Before = Reactant
After = Product

21. Resources – Extra Practice
The following are Word Based Balancing Problems
Page 137: Q14
Page139: Q22, Q23, Q26

22. Answers - Let’s Try Some Together
2Mg + O2  2MgO
2AgI + Na2S  Ag2S + 2NaI
2NaOH + H2SO4  Na2SO H2O
3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl
4FeS + 7O2  2Fe2O3 + 4SO2

23. Answers - Your Turn 1. 2H2 + O2  2H2O 2. S8 + 12O2  8SO3
3. 2HgO  2Hg + O2
4. Zn + 2HCl  ZnCl2 + H2
5. 2Na + 2H2O  2NaOH + H2