1. Unit 4: Chemical Equations
Counting Atoms

2. H2O Counting Atoms How many H? How many O?
subscript
How many H?
How many O?
Subscripts only apply to the atom that they immediately follow.

3. Counting Atoms
If elements or compounds are inside of parentheses, then the subscript that follows the parentheses applies to everything inside (just like in math class).
Ba(OH)2
Atoms of Barium:
Atoms of Oxygen:
Atoms of Hydrogen:

4. Counting Atoms Practice
Ca3(PO4)2
Atoms of Calcium:
Atoms of Phosphorus:
Atoms of Oxygen:
Al2(SO4)3
Atoms of Aluminum:
Atoms of Sulfur:
Atoms of Oxygen:

5. Unit 4: Chemical Equations
Balancing Equations

6. Balancing Equations reactants  products
Remember the law of conservation of mass?
When a chemical reaction occurs, no atoms are created or destroyed – they are just rearranged
Amount of each element on the left side (reactants) must equal the amount of the same element on the right side (products)
reactants  products

7. Balancing Equations – The Steps!
Put coefficients in front of each substance to get the quantities on the left side = the right side
2Na + Cl2  2NaCl
Use the smallest whole number coefficients possible.

8. Balancing Equations - Hints
Start with one of each reactant and product.
Save hydrogen for 2nd to last.
Save oxygen for last.
Diatomic elements: HOFBrINCl (only when alone) Example: hydrogen gas is written H2
Phases:
Solid = (s)
Liquid = (l)
Gas = (g)
Aqueous (dissolved in water or solution) = (aq)

9. Balancing Equations Example #1
Now let’s balance this equation…
__C3H8 (g) + __O2 (g)  __CO2 (g) + __H2O (l)

10. Balancing Equations Practice #1
Balance the following chemical equation:
__PbS2 (s) + __O2 (g)  __PbO2 (s) + __SO2 (g)