Transcript Chemical Reactions

Chemical Reactions

What’s the MATTER?
• Matter is anything that
has MASS and takes up
SPACE
Physical Changes
• Changes in the
physical properties
of a substance:
including changes in
size, shape, or its
state of matter
(whether it’s solid,
liquid, or gas).
• How are these
pictures physical
changes??
Physical Changes
• Examples:
• Tearing paper
• Freezing water
• Water boiling and
becoming water vapor
• Cutting your hair
• Cutting food
• Flipping a light switch
• Mixing Kool-Aid and
water
Chemical Changes
• Changes that produce new substances
with chemical properties different from
the original substances.
• Can you name
some chemical
changes?
Chemical Changes
• Examples:
• Process of rusting
• Burning paper
• Silver tarnishing
• Light bulb coming on
• Milk spoiling
• Baking soda and
vinegar reacting
Chemical Reactions
• A process that produces a chemical change is a
chemical reaction.
• Signs that a chemical reaction has occurred
include:
 Color change
 Bubbles released
 Heat released
 Light given off
 Temperature change
 New substance formed
 Substances CANNOT be changed back to
original substances by physical means
NEXT 4 SLIDES
are for Pre-AP
CLASSES
**Bonus for General
Ed Classes
Energy in Chemical Reactions
• Exothermic reactions RELEASE heat energy
resulting in the raising of temperature.
• Endothermic reactions ABSORB heat energy
resulting in the lowering of temperature
Exothermic Reactions
• The extra energy is released
in various forms—light,
sound, and heat.
• Example: burning is an
exothermic chemical
reaction in which a
substance combines with
oxygen to produce heat
along with light, carbon
dioxide, and water.
Endothermic Reactions
• Example: instant cold pack
• The heavy plastic cold pack holds
ammonium nitrate and water.
• The two substances are separated by a
plastic divider.
• When you squeeze the bag, you break
the divider so that the ammonium
nitrate dissolves in the water.
• The dissolving process absorbs heat energy.
• The reaction requires energy which is
absorbed, used in the reaction, and thus the
temperature DROPS and the pack gets cold!
Endothermic OR Exothermic?
Chemical Formulas
• Chemical formulas are combinations of
chemical symbols and numbers that indicates
how many atoms of each element are in a
compound
• Subscripts, located to the right of a symbol, tell
you the number of atoms of that element only
(O3)
• If an atom has no subscript, it means that only
one atom of that element is in the compound.
NaHCO3 (baking soda)
Chemical Equations
• To describe a chemical reaction, you must
know which substances react and which
substances are formed in the reaction.
• A chemical equation tells chemists at a glance
the reactants, products, and the amounts of
each substance present in the reaction.
Reactants & Products
• Reactants are the
substances that exist
before the reaction begins
(LEFT side of arrow)
• Products are the
substances that form as a
result of the reaction
(RIGHT side of arrow)
• The arrow in the
equations is known as the
yield arrow and it means:
“produces”
The Law of Conservation of Mass
• States that
mass/matter is
neither created nor
destroyed, but
only changes from
one form to
another.
The Law of Conservation of Mass
• Chemical equations
are much like math
equations: the left and
right sides of the
equation are
numerically EQUAL
• The number and type
of atoms must be the
same on each side of
the equation (the mass • This means the equation
is BALANCED and the
of the reactants must
law of conservation of
be the same as the
mass/matter is observed.
mass of the products)
Practice #1
Na + Cl
• Reactants:
– # of atoms of Na = 1
– # of atoms of Cl = 1
NaCl
• Products:
– # of atoms of Na = 1
– # of atoms of Cl = 1
Does this example follow the law of
conservation of mass?
YES!
Practice #2
H2 + O2
• Reactants:
– # of atoms of H =
– # of atoms of O =
H2O
• Products:
2
2
– # of atoms of H =
– # of atoms of O =
Does this example follow the law of
conservation of mass?
2
1
NO!
Adjusting Coefficients
• When balancing chemical equations, the
numbers placed BEFORE the formulas
are coefficients.
• They are the ONLY numbers changed
when balancing equations (unlike when
building compounds)
• Example:
2H2O H = 4 O = 2
• The coefficient numbers are distributed
across entire compound
Practice with Coefficients
1. 2KF
K=2 F=2
2. 4CO2
C=4 O=8
3. 5NaCl Na = 5 Cl = 5
4. 3AlCl3 Al = 3
Cl = 9
5. 6CaCO2 Ca = 6 C = 6 O = 12
Practice #2, Continued…
2 H2O
2 H2 + O2
• Reactants:
– # of atoms of H =
– # of atoms of O =
• Products:
24
2
– # of atoms of H =
– # of atoms of O =
24
12
Is it now BALANCED and follow the law of
conservation of mass?
YES!