Changes in Matter
Chapter Seventeen: Chemical
Reactions
• 17.1 Understanding Chemical Reactions
• 17.2 Energy and Chemical Reactions
Investigation 17A
Chemical Reactions
• How do you know when a chemical
reaction has occurred?
17.1 Understanding chemical
reactions
• Ice melting into water is an example of a
physical change.
• During a physical change, a substance changes
its form but remains the same substance.
• The water can be refrozen into ice again.
17.1 Understanding chemical
reactions
• A chemical change turns one or more
substances into different substances that
usually have different properties.
17.1 Chemical changes rearrange
chemical bonds
• A chemical reaction is a system of
chemical changes that involves the
breaking and reforming of chemical
bonds to create new substances.
17.1 Evidence of chemical change
Four indicators of chemical
change are:
1. Formation of new gas
2. Formation of new solid
3. Release of energy (heat
or light)
4. Color change
17.1 Products and reactants
•
In chemical reactions, you start with
reactants that are combined to make
products.
– The reactants are the starting substances.
– The products are the new substances which
result from the chemical reaction.
17.1 Reactants
•
•
The first step in understanding a reaction
is to see what atoms are in the reactants.
Counting atoms is one way to understand
a reaction.
How many carbon atoms?
How many oxygen atoms?
How many hydrogen atoms?
How many sodium ions?
17.1 Products
•
The chemical reaction rearranges the
same atoms in the reactants to become
new compounds in the products.
How many carbon atoms?
How many oxygen atoms?
How many hydrogen atoms?
How many sodium ions?
17.1 Chemical equations
• A chemical equation is an abbreviated way to
show the exact numbers of atoms and
compounds in a chemical reaction.
• The arrow shows the direction the reaction goes,
from reactants to products.
Reactants
Product
17.1 Chemical equations
• The reaction combines hydrogen and
oxygen molecules.
17.1 Chemical equations
• Count the atoms to see if there are the same
number of each type of atom on the reactant and
product sides of the equation.
• Since there is one more oxygen atom on the
reactant side than there is on the product side,
the reaction equation is not balanced.
17.1 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
hydrogen molecule to the reactant side.
• We can practice balancing equations using CPO
periodic table tiles and pencil and paper.
17.1 Numbers in equations