1. Balancing Chemical Equations

2. The Law of Conservation of Mass
Rationale for balancing chemical equations
Discovered by Antoine Lavoisier
“Matter is neither created nor destroyed”
French chemist, Antoine Lavoisier
circa 1700’s

3. Coefficient & Subscript
The coefficient times the subscript gives the total number of atoms. When no coefficient or subscript is shown, a one is understood.

4. Examples of Coefficients & Subscripts
2H2 – there are 2 x 2 atoms of hydrogen
(a total of 4)
2(NH4)2S – there are 2 x 4 x 2 atoms of hydrogen (a total of 16), and 2 x 1 atoms of sulfur ( a total of 2)

5. Equations Zn + 2HCl ZnCl2 + H2
(reactant) (reactant) yields (product) (product)
produces
equals

6. Law of Conservation of Mass
We must finish our chemical reaction with as
many atoms of each element as we started.
2H2+ O H2O
The equation above is balanced. It has the same number of each atom (mass) on each side of the arrow.

7. Steps for Balancing Equations
Write out the equation
List # of atoms on each side
Change the coefficients to balance the equation

8. Balancing Equations Mg + O2 MgO Mg = 1 Mg = 1 O = 2 O = 1