1. Law of Conservation of Mass & Balancing Equations
LG: I can use the law of conservation to explain why chemical reactions need to be balanced

2. Law of Conservation of Mass
During a chemical reaction, the atoms of the reactants are rearranged to form the products.
H2 + Cl2  2 HCl
No atoms are gained or lost; therefore no mass is gained or lost. This principal is known as the law of conservation of mass.

3. Law of Conservation of Mass
Some reactions appear ‘unbalanced’ at first: 
Chemical Equation: Mg HCl  MgCl H2
Particle Picture:
Chemists fix this problem by ‘balancing’ the equation to show that all atoms in the reactants are still present in the products:
Chemical Equation: Mg HCl  MgCl H2
Unbalanced equations are called skeleton equation “because thy show the bare bones” of what’s happening

4. Balancing Equations
Example 1 Skeleton NaCl + BeF2 → NaF + BeCl2 Equation Balanced Particle Picture Balanced Equation
Discuss rules of balancing – if you need more of a particular atom, you have to add more reactant or product MOLECULES. Can only change coefficients, not subscripts.

5. Balancing Equations Strategies:
Balancing by inspection - Balance particle in this order:
Atoms that are not part of a polyatomic ion, and not H or O
Next, balance polyatomic ions
Last, balance H and O
Example 2 Cu    O2  →  CuO
Example 3 Al(NO3) H2SO → HNO Al2(SO4)3

6. Balancing Equations Example 4
Sodium phosphate reacts with calcium chloride to produce calcium phosphate and sodium chloride
Word
Equation
Chemical Equation
(& balance it!)

7. Balancing Equations Practice!
H2 + N2  NH3 Cu + AgNO3  Ag + Cu(NO3)2 NH4 + O2  H2O + NO2 C3H6 + O2  CO2 + H2O CaCO3 + HCl  CaCl2 + H2O + CO2
Show how to balance using ‘half an oxygen’

8. Homework! Old Book: Pg. 215 # 3, 4 (don’t worry about ‘mole ratio’)
New Book: Pg. 155 # 3ab, 5 (don’t worry about states in #3)