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Chemical Reactionsand Definitions of Energy
www.njctl.org
7th Grade PSI
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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
· Types of Energy· Conservation of Mass
· Changes in Energy During a Reaction· Temperature versus Thermal Energy· Energy Flow· Summary
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In the last chapter, we defined Chemistry as the study of the
properties of matter and how matter changes.
How do you know when matter changes?
Chemistry
What does it mean to say that matter changes?
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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.
Chemical Change
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The easiest way to make observations is with your five senses.
The key is observations.
Observations
Unless you can observe something, you don't have proof that it happened.
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Brainstorm: the five senses
List the senses on the lines above.
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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)
Sense of sight
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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
Sense of hearing
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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)
Sense of touch
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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.
IMPORTANT NOTE ABOUT Sense of touch
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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.
Sense of taste
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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!)
Sense of smell
REMEMBER: Always waft chemicals instead of deeply breathing them.
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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.
Laboratory Tools
(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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What is the difference between a physical change and a chemical reaction?
Brainstorm with someone close by right now.
Physical Change vs. Chemical Reaction
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LIQUID
SOLID
GAS
Review: Phase Changes are Physical Changes
vaporization
freezing
condensation
melting
drag and drop the terms next to the correct arrow
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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.
HO
H
H H
H
H
C CC OO
ethyl alcohol
carbon dioxide
Chemical Reaction
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fire
bubbles
color changes
rust
precipitate
When two solutions mix and a solid forms, the solid is called the precipitate.
Signals of a Chemical Reaction
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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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When a chemical reaction happens, atoms are not allowed to be
created or destroyed.
Conservation of Mass
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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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.
O
O
O
O
O
O
C OO
C OO
C OO
C
CC
Conservation of Matter
This is known as conservation of matter.
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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
O
O
O
O
O
C OO
C OO
C OO
C
CC
Balanced Equation
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OO
O
OO
O
C OO
C OO
C OO
C
C C
Reactant Element Product
C
O
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.
One of the tools that can help to
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.
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O
O
O
O
O
O
C OO
C OO
C OO
C
CC
Reactant Element Product
C
O
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O
O
O
O
O
O
C OO
C OO
C OO
C
CC
Reactant Element Product
3 C
6O
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O
O
O
O
O
O
C OO
C OO
C OO
C
CC
Reactant Element Product
3 C 3
6O
6
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O
O
O
O
O
O
C OO
C OO
C OO
C
CC
Reactant Element Product
3C
3
6O
6
3C + 2O3 3CO2
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Reactant Element Product
Fe
O
Fe
O
O
Fe
Fe
O
O
O
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Reactant Element Product
1 Fe
2 O
Fe
O
O
Fe
Fe
O
O
O
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Reactant Element Product
1 Fe 2
2O
3
Fe
O
O
Fe
Fe
O
O
O
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Reactant Element Product
2 Fe 2
2O
3
Fe
O
O
Fe
Fe
O
O
O
Fe
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Reactant Element Product
2 Fe 4
6O
6
Fe
O
O
Fe
Fe
O
O
O
Fe
Fe
Fe
O
O
O
O
O
O O
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Reactant Element Product
4 Fe 4
6O
6
Fe
O
O
Fe
Fe
O
O
O
Fe
Fe
Fe
O
O
OO
O
Fe
Fe
O
O
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C6H12O6 + O2CO2 + H2O
PHOTOSYNTHESIS
Reactant Element Product
C
H
O
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C6H12O6 + O2CO2 + H2O
PHOTOSYNTHESIS
Reactant Element Product
1C
6
2H
12
2 + 1 = 3O
6 + 2 = 8
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C6H12O6 + O2CO2 + 6H2O
PHOTOSYNTHESIS
Reactant Element Product
1C
6
12H
12
2 + 6 = 8O
6 + 2 = 8
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C6H12O6 + O26CO2 + 6H2O
PHOTOSYNTHESIS
Reactant Element Product
6C
6
12H
12
12 + 6 = 18O
6 + 2 = 8
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C6H12O6 + 6O26CO2 + 6H2O
PHOTOSYNTHESIS
Reactant Element Product
6 C 6
12H
12
12 + 6 = 18O
6 + 12 = 18
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Fe2O3Fe + O2
Reactant Element Product
Fe
O
Fe
O
O
Fe
O O
Fe
O
Teac
her N
otes
Iron Oxide
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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
CaBr
C Br
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Ca
Br
Br
BrBr
15 Does the following equation follow Conservation of Mass?
Yes
No
Ca
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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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Kinetic energy is energy something has because it is moving.
The bigger the objector 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.
Kinetic energy
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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.
Potential energy
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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.
When energy is stored in chemical substances, it is called chemical potential energy. To release this energy, a chemical reaction must occur.
H
O
OHO
O
Potential energy
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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.
Thermal Energy
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REMEMBER: Types of energy are different from sources 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.
Conversion 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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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?
Chemical Reactions
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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.
Conservation of Energy
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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.
Endothermic reactions
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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.
Exothermic reactions
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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 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.
ENERGY
REACTION COMPLETION
Energy diagrams
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For an endothermic reaction, the energy of the products is higher than the energy of the reactants. Energy was absorbed.
ENERGY
REACTION COMPLETION
reactants
products
Energy diagrams
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For an exothermic reaction, the energy of the products is lower than the energy of the reactants. Energy was released.
ENERGY
REACTION COMPLETION
reactants
products
Energy diagrams
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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?
ENERGY
REACTION COMPLETION
AB
C
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29 Which reaction pictured is the most exothermic?
ENERGY
REACTION COMPLETION
AB
C
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30 Which reaction has the products with the most energy?
ENERGY
REACTION COMPLETION
AB
C
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What is the relationship between temperature and thermal energy?
Why not use temperature instead of energy for the reaction diagrams?
Brainstorm
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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.
Reminder
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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.
Similar Science
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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.
Similar Science
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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.
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.
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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.
Phase Changes
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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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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?
Brainstorm
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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.
Energy Flow
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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.
Energy Flow
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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 TransferBuild 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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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, _________ and __________ must be conserved and may not change.
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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.
