Download Unit 1 Cycle 2: Interactions and Energy

CYCLE 6
Applying Ideas
ACTIVITY 8: Explanations Involving Chemical
Changes KEY
Comparing the Class Consensus and Scientists' Ideas
During the activities in this cycle the class developed ideas about different
characteristic chemical properties and changes in chemical properties as a
result of chemical changes.
B
Your instructor will give you a copy of the handout Scientists' Ideas:
Chemical Properties and Changes. Take a few minutes to review the Scientists'
Ideas and make sure they correspond to the ideas the class has developed. In
the space below each of the scientists’ ideas you should make a note of any
evidence (or examples) you have seen in this cycle that supports each idea.
In this activity, we will be trying to explain everyday events or phenomena in
terms of ideas that we developed in class.
An explanation of a chemical change of a material should say something
about how changes in the small particles cause changes we can observe
directly (e.g. production of new materials, changes in color, production of a
gas and changes in temperature) of the material.
The balanced chemical equation represents the Conservation of Mass for both
the macroscopic and particle levels and is a useful way to represent a system
undergoing chemical change in energy diagrams also.
An energy perspective is also useful is explaining most chemical changes.
This perspective allows us to explain phenomena in terms of the Law of
Conservation of Energy using Macro/Micro energy diagrams to describe
energy changes in the system due to other system energy changes, energy
inputs and energy outputs.
As always, your written narrative should be accurate, complete and should
be clear and logically consistent.
© 2007 PSET
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Cycle 6
Experiment #1: Physical or chemical change?
You will need:
1 gallon-size zippered plastic bag
dry ice (one pea-sized pellet)
1 sandwich-size zippered plastic bag
4 oz DixieTM cup
50 mL vinegar (a solution of acetic acid in water)
2 antacid tablets (calcium carbonate)
Electronic balance
STEP 1. Place a one or two pellets of dry ice in the gallon-sized zippered bag,
then seal the bag.
STEP 2. Measure the mass and carefully observe the system (e.g. appearance,
relative temperature, rigidity of the bag)
Record the initial mass and your observations in Table 1.
Table 1. Dry Ice
Mass
Observations
Initial
g
Final
g
STEP 3. Handle the bag until most of the dry ice has disappeared, or until the
bag appears “full”.
STEP 4. Measure the mass and carefully observe the system (e.g. appearance,
relative temperature, rigidity of the bag)
Record the final mass and your observations in Table 1.
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Activity 8: Explanations Involving Chemical Changes KEY
Did the dry ice (solid carbon dioxide) undergo a physical change or a
chemical change? Explain your reasoning.
A physical change. It sublimes (from solid state to gas state) but is still carbon dioxide.
During this change is heat energy transferred into or out of the system.
Explain your reasoning.
Heat energy is transferred to the system, the potential energy of the system increases.
STEP 5. Use a hammer to crush the antacid tablets into powder, and then
add the powder to the quart-sized zippered bag. Observe the powder (e.g.
physical state, color, texture, relative temperature, etc.)
Record your initial observations for the antacid in Table 2.
STEP 6. Measure 50 mL of vinegar and pour it into the DixieTM cup (a little
less than half of a 4-oz DixieTM cup). Observe the vinegar (e.g. physical state,
color, texture, relative temperature, etc.)
Record your initial observations for vinegar in Table 2.
Table 2. Antacid and Vinegar
Mass
Observations
Initial
g
Final
g
STEP 7. Place the zippered bag on the electronic balance. Open the bag and
stand the DixieTM cup containing vinegar right-side up inside the bag, being
careful not to spill any vinegar. Seal the bag and measure the mass of the
system.
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Cycle 6
Record the initial mass in Table 2.
STEP 8. With the bag sealed, tip the cup so that all of the vinegar pours into
the bag containing antacid powder.
STEP 9. Measure the final mass of the system.
Record the final mass in Table 2.
STEP 10. Observe the system (e.g. physical state, color, texture, relative
temperature, rigidity of bag, etc.)
Record your final observations for the system in Table 2.
Did the antacid and vinegar undergo a physical change or a chemical
change? Explain your reasoning.
Chemical change. Gas forms from a solid and a liquid when mixed together. They have
different characteristic properties.
During this change is heat energy transferred into or out of the system?
Explain your reasoning.
Heat energy is transferred into the system, we felt the baggie get cold.
Is this change endothermic or exothermic?
Endothermic.
Simulator Exploration #2: Does the Small Particle Model confirm your
prediction—physical or chemical change—for antacid and vinegar?
STEP 1. Open the Cycle 6 Activity 8 Setup.
When the setup opens you should see the Macro region which contains the
Chemical Analyzer, a balance, a thermal container, and a solubility container.
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Activity 8: Explanations Involving Chemical Changes KEY
STEP 2. Use any methods that you have used in previous activities to
determine characteristic properties for both calcium carbonate (a reactant)
and carbon dioxide (a product).
calcium carbonate
Density (g/cm3)
Melting point (ºC)
Boiling point (ºC)
Solubility (g solute/100 g water)
Molar mass (g / mole)
Chemical formula
2.71
1339
decomposes
0.0014
100.09
CaCO3
carbon dioxide
0.00198
-78.5
-78.5
0.14
44.01
CO2
Does the simulator confirm that the antacid and vinegar undergo a
physical change or a chemical change? Explain your reasoning.
Since the chemical formulas, molar masses, and characteristic physical properties are
clearly different for a starting material (calcium carbonate) and a product (carbon
dioxide), this was a chemical change.
Recall from Activity 2 that some solids (such as iodine and dry ice) sublime—
that is, when they are heated, the solid state vaporizes, forming a gas. Even
though it is a physical change, a chemical equation can be written to represent
the system’s changes:
CO2 (s) Æ CO2 (g)
The antacid and vinegar undergoes an endothermic chemical change
producing a gas that clearly has properties that are distinct from either of the
two starting materials. The chemical equation that represents the system’s
changes is:
CaCO3(s) + 2 CH3CO2H(aq) Æ (CH3CO2)2Ca(aq) + CO2(g) + H2O(l)
(antacid)
(vinegar)
We will use these chemical equations throughout this activity to develop
different explanations involving these two systems.
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Cycle 6
Explanation #1: Even though carbon dioxide is produced in both cases,
why is one a physical change and the other a chemical change?
Describe the Interaction(s) using a balanced chemical equation:
CO2 (s) Æ CO2 (g)
(carbon dioxide)
CaCO3 (s) + 2 CH3CO2H (aq) Æ (CH3CO2)2Ca (aq) + CO2 (g)
(calcium carbonate) (acetic acid)
+ H2O (l)
(calcium acetate) (carbon dioxide) (water)
Write the narrative:
When dry ice sublimes, it undergoes a change of state from solid to gas.
Because the carbon dioxide particles do not change composition (chemical
identity), they only change in their motion and spatial configuration, this
is a physical change.
When antacid and vinegar are mixed, they undergo a reaction. The atoms
of the calcium carbonate and acetic acid particles rearrange into
different spatial configurations, forming calcium acetate, carbon dioxide
and water. Because the reactants and products have different
composition (chemical identities) and characteristic properties, this is a
chemical change.
Now evaluate this explanation in terms of the following criteria. If all criteria
are met, the explanation is good. If one or more of the criteria are not met, the
explanation is poor. In that case, state what is wrong and correct it.
Present
YES/NO
YES/NO
YES/NO
Criterion
Completeness: All ideas needed are included
Accuracy: All ideas included correspond to established ideas
Logical Reasoning and Clarity: Narrative connects ideas to the
phenomenon, and is well written.
The explanation is good because all criteria are met.
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Activity 8: Explanations Involving Chemical Changes KEY
Explanation #2: How does the Small Particle Model explain how mass is
conserved in both physical and chemical changes?
Describe the Interaction(s) using a balanced chemical equation and molar
masses:
CO2 (s) Æ CO2 (g)
(carbon dioxide)
MM: 44.01
CaCO3 (s) + 2 CH3CO2H (aq) Æ (CH3CO2)2Ca (aq) + CO2 (g)
(calcium carbonate) (acetic acid)
MM:100.09
MM:60.06
+ H2O (l)
(calcium acetate) (carbon dioxide) (water)
MM:158.18
MM:44.01 MM:18.02
Write the narrative:
When dry ice sublimes, it undergoes a change of state from solid to gas. The
balanced chemical equation shows that the number of carbon dioxide
molecules will be the same before and after the physical change. The
balanced chemical equation shows that the total molar mass is the same
before and after the physical change.
When calcium carbonate and acetic acid are mixed, they undergo a reaction
(chemical change). The balanced chemical equation shows that the number of
calcium, carbon, oxygen and hydrogen atoms will be the same before and
after the chemical change. The chemical equation shows that the total
molar mass is the same before and after the chemical change.
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Cycle 6
Explanation #3: How is energy involved in both changes?
Describe the Interaction(s) using energy diagrams:
heat
energy
CO2 (s) Æ CO2 (g)
CaCO3 (s) + 2 CH3CO2H (aq)Æ
(CH3CO2)2Ca (aq) + CO2 (g) + H2O (l)
Increase in
potential energy
Increase in
potential energy
Write the narrative:
When dry ice sublimes, it undergoes a change of state from solid to gas. HC/IR interactions
occur when dry ice is sitting at room temperature. Heat energy is transferred from the
surroundings into the system of carbon dioxide particles (at 20 ºC, or 293 K) in the solid state,
and all of the heat energy increases the potential energy. Because the potential energy of the
carbon dioxide particles increases and the distance between particles also changes
significantly, the strength of the attractive forces between the carbon dioxide particles becomes
so weak that the particles are no longer held together. Thus, the dry ice sublimes, changing
from carbon dioxide in the solid state to carbon dioxide in the gas state.
When calcium carbonate and acetic acid are mixed, they undergo a reaction (chemical
change). During the reaction the potential energy of the system increases as the carbon
dioxide gas is produced from the solid calcium carbonate and liquid acetic acid. The increase
in potential energy causes heat energy to be transferred from the surroundings to the system.
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heat
energy