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CHEMICAL CHANGE 17.2
Chapter Seventeen: Chemical
Change
17.1 Chemical Reactions
17.2 Balancing Equations
17.3 Classifying Reactions
Chapter 17.2 Learning Goals
Relate a balanced chemical
equation to the law of
conservation of mass.
Determine the formula and molar
masses of chemical compounds.
Write and balance chemical
equations.
Investigation 17B
Conservation of Mass
Key Question:
How do scientists describe what happens in a
chemical reaction?
17.2 Balancing Equations
Antoine Laurent Lavoisier, established an
important principal based on his
experiments with chemical reactions.
He stated that the total mass of the
products of a reaction is equal to the total
mass of the reactants.
The law of conservation of mass holds true
for even a burning mass of wood.
17.2 Balancing Equations
The combined mass of the burning
wood and oxygen is converted into
carbon dioxide and water.
17.2 Conservation of mass
Lavoisier showed
that a closed system
must be used when
studying chemical
reactions.
When chemicals are
reacted in a closed
container, you can
show that the mass
before and after the
reaction is the same.
17.2 Formula mass
The sum of the atomic mass values of
the atoms in a chemical formula is
called the formula mass.
17.2 Avogadro’s Number
The Avogadro number was named in honor
of Amedeo Avogadro who discovered that a
mole of any gas under the same conditions
has the same number of molecules.
Johann Josef Loschmidt, a German
physicist, named and discovered the
Avogadro number.
Loschmidt realized that a mole of any
substance—be it a gas, liquid, or solid—
contains 6.02 x 1023 atoms or molecules.
17.2 Molar Mass
The mass (in grams) of one mole of a
compound is called its molar mass.
Solving Problems
What is the molar mass of one mole of
CaCO3?
1. Looking for:
 … molar mass of CaCO3
2. Given
 … chemical formula
3. Relationships:
 no. amu in formula = molar mass in grams
Solving Problems
4. Solution
 Use periodic table and round values as
needed.
Formula mass CaC03 = 100.19 g
1 mole CaC03 = 100.19 g CaCO3
17.2 Chemical Equations
When a chemical reaction is written
using chemical formulas and symbols,
it is called a chemical equation.
17.2 Chemical equations
An arrow is always included between
reactants and products.
It means “to produce” or “to yield.”
to produce
Reactants
Products
“Methane combines with oxygen gas to produce
carbon dioxide gas and water vapor.”
17.2 Numbers in equations
17.2 Balancing equations
The law conservation of mass is applied by
balancing the number and type of atoms on
either side of the equation.
17.2 Balancing equations
 Counting atoms is necessary to
balance an equation.
How many carbon atoms?
How many hydrogen atoms?
How many oxygen atoms?
17.2 Balancing chemical
equations
A balanced chemical equation has the same
number of each type of atom on the product
side and the reactant side.
To balance the equation, we add another
water molecule to the product side and add
another oxygen molecule to the reactant
side.
We can practice balancing equations using
CPO periodic table tiles and pencil and
paper.
17.2 Steps for balancing
1. If not provided, write the word form of the
equation.
 Calcium carbonate reacts with hydrochloric acid
to produce calcium chloride, carbon dioxide and
water.
17.2 Steps for balancing
2. If not provided, write the chemical
equation from the word form.
17.2 Steps for balancing
3. Count the number of each type of
atom on both sides.
17.2 Steps for balancing
4. Add coefficients to balance the
equation.
Solving Problems
 In this reaction, chalcocite (a
mineral) reacts with oxygen in the
presence of heat. The products
are a type of copper oxide and
sulfur dioxide. Balance this
equation:
Cu2S + O2 → Cu2O + SO2
Solving Problems
1. Looking for:
 …the coefficients for each molecule
2. Given
 … chemical formulas which show types
and no. of atoms
Solving Problems
3. Relationships
 Coefficients can be added in front of any
chemical formula in a chemical equation.
 When a coefficient is added in front of a
chemical formula, all atoms in that
formula are multiplied by that number.
 Use common denominators to help choose
coefficients to try.
Solving Problems
4. Solution- Trial and error