1. The Law of Conservation of Mass:
The total mass of the reactants must equal the total mass of the products
Total mass of reactants = Total mass of products

2. This makes sense because atoms cannot be created or destroyed
Atoms present at the beginning of the reaction must still be present after the reaction has taken place
How??
Atoms rearrange themselves, bonding to new atoms to make a different product
Fruit salad game example,
Square dancing

3. So when you write a chemical equation…
The number of atoms of each type must be the same on each side
Equations must be balanced!
reactants
products

4. Writing Chemical Equations
A CHEMICAL EQUATION is used to represent a chemical reaction
E.g. oxygen and hydrogen in a balloon react to produce water
There are 3 forms of chemical equations that we can use to represent a reaction

5. 1. Word Equations hydrogen + oxygen water reactant reactant product
and reacts to produce

6. 2. Skeleton Equations
H O H2O

7. 3. Balanced Chemical Equations
2H O H2O
A COEFFICIENT is a number that is placed in front of a chemical formula in a balanced chemical equation

8. H2 + O2 H2O2 hydrogen peroxide
The only way to balance a chemical equation is to change the coefficients.
If you change a subscript, you will change the identity of the substance
H O H2O2
hydrogen peroxide

9. Tips for Balancing Equations
Remember the diatomic molecules: H2, N2, F2, Cl2, Br2, I2 and O2 (Remember “HOFBrINCl”)
Make sure your chemical formulas are correct
Do a final check by counting atoms of each element

10. Example
A Bunsen burner works when methane gas burns in oxygen to produce carbon dioxide and water.
Write the balanced chemical equation for this reaction

11. Step 1: Write the word equation
products
Methane + oxygen  carbon dioxide + water
reactants

12. Step 2: Write the skeleton equation
Replace each name with its formula
Methane + oxygen  carbon dioxide + water
CH4 + O2  CO2 + H2O

13. Step 3: Count atoms
Count the number of atoms of each type in reactants (left side) and products (right side)
CH4 + O2  CO2 + H2O
Type of Atom
Reactants
Products
Balanced? C 1  H 4 2  O
(2+1) = 3

14. Step 4: Balancing
Multiply each compound by the appropriate coefficients to balance the number of atoms (do NOT change subscripts)
Balance compounds first and elements last
Balance hydrogen and oxygen last
If a polyatomic ion appears in both a reactant and a product, think of it as a single unit
Trial and error (be patient )

15. Step 4: Balancing CH4 + O2 -- CO2 + 2H2O Type of Atom Reactants
Products
Balanced? C 1  H 4 O 2
(2+2) = 4 
Remember when you put a coefficient in front of a compound, ALL atoms are multiplied!!

16. Step 4: Balance CH4 + 2O2  CO2 + 2H2O Type of Atom Reactants Products
Balanced? C 1  H 4 O

17. Human Balancing Act Sodium + Chlorine Gas  Table Salt
Na (s) + Cl2 (g)  NaCl (s)
2 Na (s) + Cl2 (g)  2 NaCl (s)