Download Chapter 2: Chemical Reactions Section 1

Chemical Reactions
Section 1: Observing Chemical Changes
How can 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
 Substances that undergo
chemical changes – reactants
 The new substance formed product

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
– More energy is
required to break the
bond of the reactants
than is released by
the formation of the
new products
– Examples: baking
soda and vinegar
gets cooler when
combined

Exothermic – A
reaction that
releases energy in
the form of heat
– The energy released
as the products form
is greater than the
energy required to
break the bonds of
the reactants
– Examples: burning
of airplane fuel
endothermic
exothermic
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)