Download 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
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
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.
•
→ H2 O
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 ?
Ca= 1
N= 2
O=6
 Example 2: How many atoms of each element are in one
formula unit of Magnesium Oxide: MgO ?
Mg= 1
O= 1
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 ?


Ca= 3
N= 6
O= 18
Example 2: How many atoms of each element are there in 2 formula units
of Magnesium Oxide:
2MgO
Mg = 2
O=2
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.
How do we balance the chemical
equation:
 Balance equations by changing coefficients
 Never by changing subscripts
Practice Problems:
Circle and label the reactants and the product(s)
 _____Zn + ______HCl  _____ZnCl2 + _____H2
 _____KClO3  _____KCl + _____O2
 _____S8 + _____F2  _____SF6
 _____Fe + _____O2  _____Fe2O3