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

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

3. 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

4. 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

5. 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.

6. How do we balance the chemical equation:
Steps:
Write all reactants on the left and all products on the right side of the equation arrow. Make sure you write the correct formula for each element.
Use coefficients in front of each formula to balance the number of atoms on each side.
Multiply the coefficient of each element by the subscript of the element to count the atoms. Then list the number of atoms of each element on each side.
It is often easiest to start balancing with an element that appears only once on each side of the arrow. These elements must have the same coefficient. Next balance elements that appear only once on each side but have different numbers of atoms. Finally balance elements that are in two formulas in the same side.
Remember:
Balance equations by changing coefficients
Never by changing subscripts