Chapter 2: Chemical Reactions Section 1: Observing Chemical
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Transcript Chapter 2: Chemical Reactions Section 1: Observing Chemical
Chapter 2: Chemical Reactions
Section 1: Observing Chemical Changes
How can matter and changes in matter be
described?
In terms of two
kinds of propertiesphysical properties
and chemical
properties
Changes in matter
can be described in
terms of physical or
chemical changes
What is a physical property?
A characteristic of
a substance that
can be observed
without changing
the substance into
another substance
Examples: ice
melting, color,
hardness, texture,
shine, etc.
What is a chemical property?
A characteristic
of a substance
that describes its
ability to change
into other
substances
Examples:
burning
magnesium,
rusting
What is the difference between a
physical and a chemical
reaction?
A change in matter
that produces one
or more new
substances is a
chemical change, or
chemical reaction
Physical change
examples: braiding
your hair,
squashing a
marshmallow
Chemical change
examples: burning
of gasoline, burning
a marshmallow
How can you tell when a
chemical reaction occurs?
Chemical changes
occur when bonds
break and new
bonds form
Chemical reactions
involve two main
kinds of changes
that you can
observe-formation
of new substances
and changes in
energy
What are some changes in
properties that indicate a
chemical change?
Precipitate – a
solid that forms
from a solution
during a
chemical
reaction
Color change
may indicate a
chemical
reaction
Gas production –
bubbles
How do you categorize changes
in energy?
Endothermic – A
reaction in which
energy is absorbed
– Examples: baking
soda and vinegar
gets cooler when
combined
Exothermic – A
reaction that
releases energy in
the form of heat
– Examples: burning
of airplane fuel
Section 2: Describing chemical reactions
What information does a chemical equation
contain?
Chemical equations
use chemical
formulas and other
symbols instead of
words to summarize
a reaction
Reactants –
substances you
have at the
beginning
Products – new
substances
produced when the
reaction is complete
What does the principle of
conservation of mass state?
That in a
chemical
reaction, the
total mass of the
reactants must
equal the total
mass of the
products
Matter is neither
created nor
destroyed
What does open or closed
system mean?
Open system –
matter can enter
from or escape
to the
surroundings
Closed system –
matter is not
allowed to enter
or leave
What must a balanced chemical
equation show?
The same number of
each type of atom on
both sides of the
equation
Represents the
conservation of mass
Coefficients – a
number placed in front
of a chemical formula
in an equation that
tells you how many
atoms or molecules of
a reactant or product
take place in the
reaction
How do you balance chemical equations?
Count
the # of atoms of
each element in the
reactants & products
ID element that is not equal
on both sides
Add coefficient to the front
of the formula that will
make the # = on both sides
for that element
Sample Problems
Sample Problem:
Reactants
Products
1 Zn
1 Zn
1H
2H
1 Br
2 Br
Zn + HBr H2 + ZnBr2
Multiply HBr x 2 to correct
(balance)
Reactants Products
Zn + 2HBr H2 + ZnBr2
1 Zn
1 Zn
2H
2H
2 Br
2 Br
Try these!!
2Na + Cl2 2NaCl
Ca + Cl2 CaCl2
2H2O 2H2 + O2
N2 + 3 H2 2NH3
2Al2O3 4 Al + 3 O2
P4 + 3 O2 P4O6
3Fe + 4H2O Fe3O4 + 4 H2
What are the three categories of
chemical reactions?
Synthesis
Decomposition
Replacement
What is a synthesis reaction?
When two or more
substances combine to
make a more complex
substance
–A + B AB
–2H2 + O2 2H2O
Example: hydrogen and
oxygen to make water
What is a Decomposition
reaction?
Breaking down compounds into
simpler products
–AB A + B
–2H2O2 2H2O + O2
Example: Hydrogen peroxide
decomposes into water and
oxygen gas
What is a replacement
reaction?
When one element replaces another in a
compound, or when two elements in
different compounds trade places
Example: copper metal obtained by heating
copper oxide with carbon
Single (one element replaces another)
–AB + C AC + B
–Zn + 2HCl ZnCl2 + H2
or double (elements appear to trade places
with another compound)
–AB + CD AC + BD
–NaCl + AgF NaF +AgCl
Section 3: Controlling Chemical Reactions
How is activation energy related to chemical
reactions?
The minimum
amount of
energy needed to
start a chemical
reaction
All chemical
reactions need a
certain amount
of activation
energy to get
started
What factors affect the rate of a
chemical reaction?
Surface area – the greater the surface area that
faster the reaction (ex. Chewing)
Temperature – raising the temperature causes
particles to move faster and therefore have more
energy; they also come into contact more often;
lowering temperature slows things down
Concentration – amount of substance in a given
volume; increased concentration-increased
reaction
Catalysts – increases the rate of a reaction by
decreasing the energy needed to start
– Enzymes: biological catalysts
Inhibitors – material used to decrease the rate of
reaction (ex. preservatives in food)