1. Law of Conservation of Mass
Balancing Equations:
Law of Conservation of Mass

2. Law of Conservation of Mass:
Mass is neither created nor destroyed during a chemical reaction- it is conserved
Mass reactants = Mass products

3. Chemical Equations/ Law of Conservation of Mass
For an equation to follow the Law of Conservation of Mass, the equation must be balanced.
Atoms can neither be created nor destroyed in an ordinary chemical reaction
There must be the same number of atoms on both sides of the equation

4. Parts of a Chemical Equation:
Chemical equations tell us what substances are reacting and what substances are being produced.
Reactants  Products
Reactants- starting substances
H2 + O2 →
Products- substances formed during the reaction.
→ H2O

5. Numbers found in a chemical reaction
Subscripts-
The small numbers to the lower right of chemical symbols
Represent the number of atoms of each element in the molecule
Example 1: How many atoms of each element are in one formula unit of Calcium Nitrate: Ca(NO3)2 ?
Example 2: How many atoms of each element are in one formula unit of Magnesium Oxide: MgO ?
Ca= 1
N= 2
O = 6
Mg= 1
O= 1

6. Numbers found in a chemical reaction
Coefficients
The large numbers in front of chemical formulas
Coefficients represent the number of molecules of the substance in the reaction.
Example 1: How many atoms of each element are there in 3 formula units of Calcium Nitrate: 3Ca(NO3)2 ?
Example 2: How many atoms of each element are there in 2 formula units of Magnesium Oxide: MgO
Ca= 3
N= 6
O= 18
Mg = 2
O = 2

7. Writing Chemical Equations
Check for diatomic molecules:
H2 , N2 , O2 , Cl2 , F2 , Br2 , I2
If these elements appear by themselves in an equation, they must be with the subscript 2.

8. How do we balance the chemical equation:
Balance equations by changing coefficients
Never by changing subscripts

9. Practice Problems: Circle and label the reactants and the product(s)
_____Zn + ______HCl  _____ZnCl2 + _____H2
_____KClO3  _____KCl + _____O2
_____S8 + _____F2  _____SF6
_____Fe + _____O2  _____Fe2O3