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7th Grade PSI
Chemical Reactions
and Definitions of Energy
www.njctl.org
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Table of Contents: Chemical Reactions
and Definitions of Energy
Click on the topic to go to that section
· What is a change and what are the signals?
· Physical change versus chemical reaction
· Conservation of Mass
· Types of Energy
· Changes in Energy During a Reaction
· Temperature versus Thermal Energy
· Energy Flow
· Summary
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Changes and Signals
Return to Table
of Contents
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Chemistry
In the last chapter, we defined Chemistry as the study of the
properties of matter and how matter changes.
What does it mean to say that matter changes?
How do you know when matter changes?
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Chemical Change
A change is when the properties of a substance are different before
and after something happens.
But how do you know when the
properties are different?
Write your ideas down here.
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Observations
The key is observations.
Unless you can observe something,
you don't have proof that it
happened.
The easiest way to make observations is with your five senses.
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Brainstorm: the five senses
List the senses on the lines above.
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Sense of sight
Our eyes let us see properties such as
· color (Example: red turns green)
· shape (Example: rolling chewed gum into a ball)
· size (Example: inflating a balloon)
· phase - solid, liquid, or gas (Example: ice melting)
· and things like light and smoke (Example: logs burning)
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Sense of hearing
Our ears do one thing really well,
they hear sounds.
Something must have happened for a sound to be produced.
EXAMPLE: a pair of cymbals being crashed together
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Sense of touch
Our sense of touch can let us know things such as
· texture change : like solid to slimy
(Example: solid hamburger grease melts)
· temperature change: warmer or colder
(Example: a glass of water with ice in it)
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IMPORTANT NOTE ABOUT
Sense of touch
BE CAREFUL! Unless your teacher tells
you it is okay, never directly touch
substances in the laboratory. Some
substances are dangerous and can cause
serious injury. Bringing your fingers near a
container like a beaker is close enough to
let you know if it is getting hotter or colder
while staying safe.
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Sense of taste
Our taste buds can let us know how something tastes.
(Example: strawbery ice cream - yum!)
BUT...
NEVER taste anything in the laboratory unless your teacher tells you
to do so, especially if there is another way to get information safely.
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Sense of smell
Our noses let us smell things as they happen in the laboratory.
When a new smell is present, something
must have happened to cause the smell.
(example: rotten banana - yuk!)
REMEMBER: Always waft chemicals instead of deeply breathing them.
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Laboratory Tools
Sometimes we use tools in the laboratory to help us make
observations, because it is safer or because we get more accurate
information than what our senses alone can provide.
(example: a ruler can measure how long a line is
accurately and a thermometer can measure the
temperature of boiling water both accurately and safely)
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1 What is always different when a change occurs?
A the substances involved
B at least one property of a substance
C the phases of the substances
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2 What do you make during an experiment to show
whether something is happening or not?
A Observations
B Explanations
C Predictions
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3 Which of the following are NOT possible to observe
with your eyes?
A Color
B Formula
C Size
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4 Which one of your senses should you never use in
the laboratory unless your teacher tells you to do so?
A Hearing
B Touch
C Taste
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5 Which tool is best for helping determine the
temperature of a substance?
A Ruler
B Graduated Cylinder
C Thermometer
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6 Which tool is best for helping determine the volume of
an irregularly shaped object?
A Ruler
B Graduated Cylinder
C Thermometer
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Physical Change vs
Chemical Reaction
Return to Table
of Contents
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Physical Change vs. Chemical Reaction
What is the difference between a physical
change and a chemical reaction?
Brainstorm with someone close by right now.
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Review:
Phase Changes are Physical Changes
drag and drop the
terms next to the
correct arrow
GAS
LIQUID
SOLID
condensation
vaporization
freezing
melting
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Chemical Reaction
A chemical reaction is when a substance changes its properties by
changing what substance it is.
The key idea is that the formula does change.
H
H
O
C
H
H
C
H
ethyl alcohol
H
O
C
O
carbon dioxide
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Signals of a Chemical Reaction
fire
bubbles
color changes
rust
precipitate
When two solutions mix and a solid forms, the solid
is called the precipitate.
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Good Science Reminder!
Observations do not prove what happened.
They only record what you observed.
It is up to you to explain your observations.
Someone else may explain things differently using your
observations. Sometimes further testing is required to get
more information.
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7 Is evaporation a physical change or a
chemical reaction?
A Physical Change
B Chemical Reaction
C May be either
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8 Is dissolving in water a physical change or a chemical
reaction?
A Physical Change
B Chemical Reaction
C May be either
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9 Is burning paper a physical change or a chemical
reaction?
A Physical Change
B Chemical Reaction
C May be either
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10 Is baking a cake a physical change or a chemical
reaction?
A Physical Change
B Chemical Reaction
C May be either
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11 Are smoke and flame signs of a physical
change or a chemical reaction?
A Physical Change
B Chemical Reaction
C May be either
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12 Are bubbles a sign of a physical change
or a chemical reaction?
A Physical Change
B Chemical Reaction
C May be either
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Lab: Classifying Reactions
What properties of substances are most helpful in
determining if a physical change or a chemical reaction has
occurred?
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Conservation of Mass
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of Contents
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Conservation of Mass
When a chemical reaction happens, atoms are not allowed to be
created or destroyed.
If we start with 4.2g of
substances, we must end
with 4.2g of substances.
This is known as
conservation of mass.
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Conservation of Mass
Example #1
+
150.0g
5.0g
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Conservation of Matter
The atoms are also not allowed to change type. If we start with three
carbon atoms and six oxygen atoms, we must end with three carbon
atoms and six oxygen atoms.
This is known as conservation of matter.
O
C
O
O
C
O
O
O
O
C
O
O
C
O
C
O
C
O
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Balanced Equation
To make sure that all of the mass and the matter is conserved,
equations need to be balanced. A balanced equation is one that
has the same numbers and types of atoms on both the reactant side
and the product side.
O
C
O
O
C
O
O
O
O
C
O
O
C
O
C
O
C
O
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C
O
C
C
O
Element
C
One of the tools that can help to
O
O
O
Reactant
C
O
O
O
O
O
C
O
O
Product
C
O
balance an equation is called
a REP Table.
REP = Reactant Element Product
To use a REP Table, each element
must have a row of its own.
Generally, the element symbol is placed in the middle column to
make them easier to track.
In the Reactant and Product columns, the number of atoms of each
element are written. When each element has matching numbers in
both Reactant and Product columns, the equation is balanced.
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O
C
O
O
C
O
O
O
O
C
C
O
O
Reactant
O
Element
C
O
C
C
O
O
Product
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O
C
O
O
C
O
O
O
O
C
C
O
O
Reactant
3
6
O
Element
C
O
C
C
O
O
Product
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O
C
O
O
C
O
O
O
O
C
C
O
O
Reactant
3
6
O
Element
C
O
C
C
O
O
Product
3
6
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O
C
O
O
C
O
O
O
O
O
C
C
O
O
3
6
O
3CO2
3C + 2O3
Reactant
C
Element
C
O
Product
3
6
C
O
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O
O
Fe
Fe
O
O
Fe
O
Reactant
Element
Fe
O
Product
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O
O
Fe
Fe
O
O
Fe
O
Reactant
1
2
Element
Fe
O
Product
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O
O
Fe
Fe
O
O
Fe
O
Reactant
Element
1
2
Fe
O
Product
2
3
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O
O
Fe
Fe
O
O
Fe
Fe
O
Reactant
Element
2
2
Fe
O
Product
2
3
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O
O
Fe
O
O
Fe
Fe
O
O
O
O
O
O
Fe
Fe
O
O
Reactant
Element
2
6
Fe
Fe
O
Product
4
6
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O
Fe
O
O
Fe
O
Fe
Fe
O
Fe
O
O
Fe
O
O
Fe
O
O
O
Fe
Reactant
Element
4
6
Fe
O
Product
4
6
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PHOTOSYNTHESIS
CO2 + H2O
Reactant
C6H12O6 + O2
Element
C
H
O
Product
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PHOTOSYNTHESIS
CO2 + H2O
Reactant
1
2
2+1=3
C6H12O6 + O2
Element
C
H
O
Product
6
12
6+2=8
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PHOTOSYNTHESIS
CO2 + 6H2O
Reactant
1
12
2+6=8
C6H12O6 + O2
Element
C
H
O
Product
6
12
6+2=8
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PHOTOSYNTHESIS
6CO2 + 6H2O
Reactant
C6H12O6 + O2
Element
Product
6
C
6
12
H
12
12 + 6 = 18
O
6+2=8
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PHOTOSYNTHESIS
6CO2 + 6H2O
Reactant
C6H12O6 + 6O2
Element
Product
6
C
6
12
H
12
12 + 6 = 18
O
6 + 12 = 18
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Iron Oxide
O
Fe2O3
Element
Fe
O
Product
Teacher Notes
O
O
Fe + O2
Reactant
Fe
O
O
Fe
Fe
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13 Which of the following must be followed when
balancing chemical equations?
A Conservation of Mass
B Conservation of Matter
C neither
D both
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14 Does the following equation follow Conservation of
Mass?
Yes
No
Ca
Br
C
Br
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15 Does the following equation follow Conservation of
Mass?
Yes
No
Br
Br
Br
Ca
Ca
Br
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16 Does the following equation follow Conservation of
Mass?
Yes
No
2Na + Cl2
2NaCl
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17 Does the following equation follow Conservation of
Mass?
Yes
No
CH3OH + O2
CO2 + H2O
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18 What number should be in front of the H2O to
make the equation balanced?
A 1
B 2
C 3
D 4
H2SO4 + 2NaOH
Na2SO4 + ??H2O
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Lab 2: Atomic Rearrangement
How do the atoms of reactants form the products?
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Types of Energy
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of Contents
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Kinetic energy
Kinetic energy is energy something has because it is
moving.
The bigger the object
or the faster it is moving
the more kinetic energy
it has.
REMEMBER: Molecules and atoms are constantly moving
even if you can't see them.
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Potential energy
Potential energy is energy that is stored in an object.
When you hang something from a spring
and the spring stretches, you are storing
elastic potential energy. To get the
energy back, you would release the object
and let the spring return to normal length.
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Potential energy
H
O
O
When energy is stored in chemical
substances, it is called chemical
potential energy. To release this
energy, a chemical reaction must occur.
O
O
H
Electromagnetic energy is the energy of
electromagnetic radiation such as sunlight,
radio waves, microwaves that is stored in the
electric and magnetic fields. This energy can
be absorbed by an object.
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Thermal Energy
Thermal Energy is the portion of an object's
average potential and kinetic energies per atom
or molecule, depending on what substance it is.
Thermal Energy is responsible for the object
having a measurable temperature.
Heat is the energy that is
transferred between two
objects that are at different
initial temperatures.
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Conversion of Energy
A wind turbine uses the
wind as its source of
energy. The wind actually
has kinetic energy since
it is moving that the
turbine converts to
electromagnetic energy.
The faster the wind is
moving the more energy
the turbine can convert.
REMEMBER: Types of energy are different
from sources of energy.
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19 Which type of energy is best illustrated by a bee
moving very quickly?
A kinetic energy
B chemical potential energy
C electromagnetic energy
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20 Which type of energy is best illustrated by calories in
food?
A kinetic energy
B chemical potential energy
C electromagnetic energy
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21 Which type of energy is best illustrated by a a
waterfall being used to turn a turbine?
A kinetic energy
B chemical potential energy
C electromagnetic energy
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22 Which type of energy is best illustrated by burning
fossil fuels to release energy?
A kinetic energy
B chemical potential energy
C electromagnetic energy
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23 Which type of energy is transferred between objects
that are different temperatures?
A heat
B chemical potential energy
C electromagnetic energy
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24 Energy can be created from nothing as part of a
chemical reaction.
True
False
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Energy Changes
During Reactions
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of Contents
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Chemical Reactions
Conservation of Mass and
Conservation of Matter mean that
atoms are not allowed to be
created or destroyed during a
chemical reaction. They are only
allowed to change the way they
are attached to each other.
What about energy?
Is it allowed to change during a chemical reaction?
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Conservation of Energy
Conservation of Energy explains that energy may not be
created or destroyed during a chemical reaction. It may be
transferred between substances or change its type.
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Endothermic reactions
Endothermic
reactions absorb
energy from their
surroundings. This
makes the area
around the reaction
feel cold.
Making scrambled eggs requires adding energy by heating the
pan on the stovetop. That energy transfers into the eggs until
they cook.
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Exothermic reactions
Exothermic reactions
release energy to their
surroundings. This
makes the area around
the reaction feel warm or
hot.
When using a gas
stovetop, the heat
energy is released by the
natural gas as it burns.
Flames are a good indication that an exothermic reaction is
taking place.
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Good Science Reminder!
The system is chosen by the scientist and
typically involves the reacting substances.
The surroundings are everything
else that isn't part of the system.
The system and surroundings combine to form the universe.
Matter, mass, and energy must be constant in the universe
during chemical reactions.
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Energy diagrams
Energy diagrams are used to visually show if a reaction is
endothermic or exothermic. It also can give hints about if
the reaction is likely to happen or not.
E
N
E
R
G
Y
REACTION COMPLETION
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Energy diagrams
For an endothermic reaction, the energy of the products is higher
than the energy of the reactants. Energy was absorbed.
E
N
E
R
G
Y
products
reactants
REACTION COMPLETION
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Energy diagrams
For an exothermic reaction, the energy of the products is lower than
the energy of the reactants. Energy was released.
E
N
E
R
G
Y
reactants
products
REACTION COMPLETION
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25 An instant cold pack is an example of what kind of
reaction?
A Endothermic Reaction
B Exothermic Reaction
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26 An instant hand warmer is an example of
what kind of reaction?
A Endothermic Reaction
B Exothermic Reaction
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27 Burning oil in a camping lantern is an
example of what kind of reaction?
A Endothermic Reaction
B Exothermic Reaction
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28 Which reaction pictured is endothermic?
E
N
E
R
G
Y
B
A
C
REACTION COMPLETION
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29 Which reaction pictured is the most
exothermic?
E
N
E
R
G
Y
B
A
C
REACTION COMPLETION
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30 Which reaction has the products with the most
energy?
E
N
E
R
G
Y
B
A
C
REACTION COMPLETION
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Temperature and
Thermal Energy
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of Contents
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Brainstorm
What is the relationship between temperature and
thermal energy?
Why not use temperature instead of energy for the
reaction diagrams?
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Reminder
FROM BEFORE:
Thermal Energy is the
portion of an object's
average potential and
kinetic energies per atom or
molecule, depending on
what substance it is.
Thermal Energy is
responsible for the object
having a measurable
temperature.
NEW:
Temperature is not energy.
It is related to energy but
also is influenced by what
the substance is and what
phase of matter it is.
Temperature is something
we can directly measure.
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Similar Science
Size is not the only thing that
influences how much mass an
object has. It depends on what
the object is made of as well. A
soccer ball filled with air will
have a different mass than a
soccer ball filled with water or a
soccer ball filled with cement,
even though they are the same
size.
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Similar Science
When making a pizza, sometimes the
recipe calls for preheating a pizza
stone in the oven. This allows the
pizza stone to be the same
temperature as the oven.
The air inside the oven
also heats up to the
temperature of the oven.
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Similar Science
When you open the
oven, if you touch the
pizza stone or the oven
itself, you will probably
get burned. The air
inside the oven doesn't
burn you, though.
That is because, even though everything in the oven is the same
temperature, the air has a lot less thermal energy than the oven or
the pizza stone. The interactions of the atoms in each substance
cause them to require different amounts of energy.
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Phase Changes
Thermal energy also changes when a substance
changes phase. If you measure the temperature of ice
cream when it melts or of water when it boils, the
temperature stays constant until the phase change
finishes. Extra energy must be added to make those
phase change happen.
Sometimes, objects need to lose energy for a phase change to
happen. When water freezes energy must be released before the ice
can form.
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31 Which has more thermal energy, 5.0g of solid
candle wax or 5.0g of liquid candle wax if they
have the same temperature?
A the solid wax
B the liquid wax
C they have the same
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32 Which has more thermal energy, 5.0g of solid
candle wax or 15.0g of solid candle wax if they
have the same temperature?
A the 5.0g sample
B the 15.0g sample
C they have the same
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33 What energy change must happen for a gas to
condense to a liquid?
A decrease energy
B increase energy
C more information is needed
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34 What energy change must happen for sublimation
to occur?
A decrease energy
B increase energy
C more information is needed
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35 If substance one and substance two are at the
same temperature, which one has more thermal
energy?
A substance one
B substance two
C more information is needed
D they have the same energy
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36 If 150g of iron skillet and 150g of water are both
at 100 degrees Celsius, which has more thermal
energy, the iron or the water?
A the iron skillet
B the water
C more information is needed
D they have the same energy
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Energy Flow
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Brainstorm
If two objects can have the same amount of
thermal energy but different temperatures,
or different thermal energies and the same
temperature, when does energy transfer
between them?
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Energy Flow
When two substances touch, if they have
different temperatures, energy will flow from
the hotter substance to the colder substance
until their temperatures are the same.
Once the temperatures are the same,
the energy transfer process stops.
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Energy Flow
In science, unless you are talking about a disease, "cold" is an
adjective, not a noun. Heat is the energy that transfers
between objects.
Ice doesn't transfer cold to the juice in
the glass. The juice actually transfers
energy to the ice.
So what physically happens? Explain it
in your own words.
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37 What quantity tries to balance out when energy is
transferred via heat?
A thermal energy
B temperature
C they both must be the same
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38 Which direction does heat flow?
A higher temperature to lower temperature
B lower temperature to higher temperature
C higher thermal energy to lower thermal energy
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39 Which of the following best describes how energy
transfers when you cook an egg in a skillet on a
stovetop?
A the stovetop transfers energy to the egg
B the egg absorbs energy from the stovetop
C the egg absorbs energy from the skillet
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40 Which of the following best describes why energy
transfers when you cook an egg in a skillet on a
stovetop?
A the stovetop has a higher temperature
than the egg
B the skillet has a lower temperature
than the egg
C the egg has a lower temperature
than the skillet
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41 If object one is the same temperature as object
two but has twice as much thermal energy, what
happens when the objects touch?
A object one warms object two
B no energy is transferred
C object one gives energy to object two
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42 If object one is the same temperature as object
two but has twice as much mass, what happens
when the objects touch?
A object one warms object two
B no energy is transferred
C object one gives energy to object two
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Lab 3: Temperature & Thermal Energy
How can the difference between temperature and thermal
energy be observed?
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Lab 4: Energy Transfer
Build a device that takes the thermal energy change from a
chemical reaction and uses it to do something different than it
would normally be used for.
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Summary:
Fill in the blanks!
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of Contents
Slide 119 / 120
When a substance has different properties before and after
something happens, a ___________ has taken place.
Observable changes can be either ___________ changes where
the substance does not change its formula or _________
______________ where the substance changes into a new
substance with a new formula.
When changes occur, _________
conserved and may not change.
and __________ must be
Slide 120 / 120
A ___________ equation shows the correct ratios of reactants and
products that allow mass and matter to be conserved.
There are several types of ________ such as kinetic, chemical
potential, and thermal.
Energy may be absorbed or released during a __________
reaction.
___________ energy is different from temperature.
Energy flows from__________ temperature to _________
temperature.