Heat in Reactions
Learning Objectives:
Students will be able to define exothermic and endothermic reactions and identify
them through calculations, observations, and data readings.
Students will use enthalpy to determine heat flow in reactions.
Students will use data gathered in the lab to support conclusions about unique
reactions.
Assessment Criteria:
Students can identify exothermic and endothermic reactions from observations such
as changing thermometer readings.
Students will be able to correctly determine and label enthalpy change for given
chemical reactions.
Students will use proper lab techniques to gather the necessary data and report this
data for analysis upon completion of the lab.
Benchmarks and Standards:
11.A.4b Conduct controlled experiments or simulations to test hypotheses.
12.C.4a Use kinetic theory, wave theory, quantum theory and the laws of thermodynamics to explain energy transformations.
13.A.4d Explain how peer review helps to assure the accurate use of data and
improves the scientific process.
Prior Knowledge/Conceptions:
Students should be able to balance any given reaction equation.
Students should be able to properly use lab equipment and take appropriate data
readings.
Students should be able to perform unit conversions, especially in terms of energy,
as well as stoichiometry calculations.
Instructional Strategies:
The lesson will first focus on demonstrations of exothermic and endothermic
reactions. Students will then gather in small and discuss from where the heat in
these reactions comes. Brief lectures will be given to define key terms and ideas.
The second half of the lesson will focus on interactive lab work, with each group
investigating a unique reaction. The groups will then present their findings to the
class in brief presentations and results will be discussed. The unit will end with a
quiz and conversation of practical uses for exothermic and endothermic reactions as
well as natural examples of them.
Instructional Resources used:
Adapted from Endothermic and Exothermic Reactions by the University of
Minnesota Chemistry Department
(http://www.chem.umn.edu/outreach/EndoExo.html)
Adapted from What would make a good hot pack or cold pack - Introduction to
Thermochemistry by Victor Chen
(http://teacherknowledge.wikispaces.com/Demonstration+Lesson++Acid+in+water+puzzle+(by+Victor+Chen))
Materials and set-up needed:
For Day One:
For initial demonstrations:
-Safety gear
-2 400 mL beakers filled with 250 mL of water
-20 g of NaOH (s)
-20 g of Urea (s)
-Stir rod
For discussion/lecture:
-Projector and screen for videos
-White board
For Day Two:
-Worksheets
-Common Enthalpy Values Table
-Exit Slips
For Day Three:
-Lab sheets
- Appropriate safety gear for students (aprons, goggles, gloves)
-Balance at every table
-16 200 mL beakers
-4 large beakers filled with water
-4 stir rods
-4 thermometers
-4 Lab Scoops
-50 g of NaOH
-50 g of KCl
-50 g of CaCl and C2H3NaO2
-50 g of NH4NO3
For Day Four:
Copies of quiz
Time required: 3.5 class periods
Cautions:
The reactants can be irritating to skin and fumes produced can be harmful if
breathed in. Do not handle beakers when they are holding reactions, the
temperatures can cause minor burns to hands. Be careful when using the
thermometers and beakers; be ready to clean up glass and mild acids or bases if
beakers are dropped or spilled. During lab, all participants should be wearing
proper safety equipment, such as goggles and aprons.
Instructional Sequence:
Day One:
Introduction: Wear lab coat and safety goggles as students enter classroom so they
anticipate interesting demonstrations will be taking place. Have the necessary
components of the reactions already assembled and ready to go. Draw student
attention to the learning goal for the day: describing heat flow in chemical reactions.
This will be written on the board for all students to see and starts the class. This
will transition into the introduction to heat. Inform students that the new unit we’re
covering focuses on heat and that we will be demonstrating how is involved in
chemical reactions. Pull out cart containing necessary components for reactions.
Ask students what temperature the reactants are at (we can assume room
temperature ~25 C).
Perform exothermic reaction by mixing NaOH into one of the beakers of water.
Allow nearby students to feel beaker as the reaction occurs.
Perform endothermic reaction by mixing the urea into the other beaker of water.
Allow nearby students to feel beaker after the reaction occurs.
Ask the students where the heat or cold came from in the reaction. Remind students
that we observed both reactants started at room temperature when mixed and there
was no Bunsen burner or heat source involved.
Possible Responses: It came from the air. There was a hotplate hidden under the
table. Molecules hit each other when mixed and the friction heats them up, like
rubbing your hands together. The reaction makes ice, so it gets cold. The reactions
release heat in the molecules.
Transition into brief lecture on exothermic and endothermic reactions by telling
students: There are actually two answers, the hot beaker is releasing energy
because bonds are being broken, and the cold beaker is absorbing heat from nearby
sources to continue the reaction. Let’s look at this in a bit more detail.
Body of the Lesson: Display the term exothermic on the board. Define it as a
reaction that releases heat. Break down exothermic into exo, meaning outward, and
therm, meaning heat. Heat is given off as a product. Ask the students which beaker
contained an exothermic reaction. Most students should answer that the hot beaker
is exothermic, reasoning that the heat produced warmed up the beaker. Write the
chemical reaction that took place on the board and show “heat” alongside the
product.
Write the term endothermic on the board and define it as a reaction that uses heat
to proceed. Break down endothermic into endo, defined as inward, and therm,
again meaning heat. Heat is a reactant. Confirm with the class that the other beaker
contained an endothermic reaction. Ask the class: If an endothermic reaction needs
all this heat, why does it feel cold to the touch?
Possible Answers: It used up all the heat inside the beaker. If it’s using up heat, it’s
giving off cold.
Explain that the heat it needs is taken from anything nearby, so when you touch it,
heat from your hands is being drawn out and into the beaker. So that lack of heat in
your hands now makes them feel cold. It may be beneficial to define “cold” as a lack
of heat and heat is just a form of energy, so something cold has less energy. Show
short videos demonstrating some interesting examples of exothermic and
endothermic reactions. Exothermic Reaction Endothermic Reaction
Wrapping up the Lesson: Review chemical bonds between atoms in molecules. Ask
students what keeps those bonds together between atoms (energy) and what
happens to energy when those bonds are broken (it is released). In exothermic
reactions, these bonds are being broken, so that energy is being released into the
system. In endothermic reactions, bonds are typically forming, so they need to take
in energy from outside in order to accomplish this. Address any questions or
discrepancies students have with this, and tell them they will be investigating a
reaction in lab in the near future. Based on student responses, if further explanation
is needed, the material can be reviewed the next day to start class.
Evaluating Student Learning: Because this is the first day of the unit, there will be
no explicit evaluation. Student participation will be a major indicator of their
learning. If most of the class is involved in answering questions and can explain the
difference between the two reaction types, then I believe it’s safe to assume they
have achieved the objective for the day.
Day Two:
Introduction: Display the term enthalpy on the board. Once the students have
entered the classroom, direct their attention to the learning goal: we will be defining
enthalpy and discussing what it means. Review briefly yesterday’s topic of heat as
either a product or reactant in chemical reactions. Move into short lecture on
enthalpy.
Possible student question: I can’t remember the difference between exothermic and
endothermic. Possible Response: Engage the rest of the class by trying to have
another student explain the difference. Correct Response: Exothermic reactions
produce heat, think of the prefix exo-, it means outward or going out, so heat moves
out of the system. It’s being released. Endo- means inward, so it’s the opposite and
heat is moving into the system.
Body: Begin short lecture with the focus describing the difference between heat and
temperature. Temperature is a property of a system, while heat is an expression for
energy transferred between objects. Students will be taking notes.
Define enthalpy as the total energy for a system, H. Pose the questions: So if
enthalpy is the total energy, what does a change in enthalpy mean?
Possible Student Responses: It means a temperature change happens. It means the
heat in the system changes. The total energy changes.
Teacher Response: Yes, enthalpy change describes a change in energy and we call
that ∆H. Now think back to how heat moves in exothermic and endothermic
reactions. In exothermic reactions, heat is a product, right? So do you think the
change in energy will be positive or negative? Student responses will probably be
split evenly between positive and negative. Explanation: well, in exothermic
reactions, we know energy is released and the system loses that energy. So the
energy change will be negative. So for an endothermic process, where energy is
gained, ∆H will be positive.
Possible student question: So it’s negative for exothermic reactions even though it
gets hotter? Response: Yes, it seems counterintuitive at first, but remember that
when heat is produced, it’s being lost by the system, so we show that loss as a
negative number.
Describe enthalpy for different substances: These values can be found in tables for
common substances. (Hand out a table to each student.) Ask students what they
think these values mean. Possible Responses: If we burned it, that’s how hot it
would be. That’s how hot it needs to be for the reaction to take place. It’s how much
energy it needs.
Explanation: Each molecule has a unique ∆Hf value, the “heat of formation,” which is
essentially how much energy it takes to make a mole of it. We can apply these
values to reactions so we can actually calculate the total ∆H values for many
chemical reactions. This will tell us how much energy goes into or is released
during a chemical reaction.
Pass out enthalpy worksheet to the students and work through the first example on
the board. If the students understand how to perform the calculation, give them
most of the remaining time to complete the worksheet. Circulate around classroom
and assist students with any questions. Use this time to also make sure students are
properly performing the calculations.
Possible Questions: Do I need to balance the reactions first? Absolutely, that could
change your results entirely.
For ∆H do I subtract the products from the reactants or the other way around?
Whenever you have a ∆ value, you need to subtract total reactants from the total
products.
Wrapping up the Lesson: Bring student attention back to the front of the board and
tell them the worksheet is homework if it is incomplete. Write these exit ticket
questions on the board for students to complete:
-Calculate ∆H for: C4H8 + O2  CO2 + H2O
-Endothermic reactions have a ______ ∆H while exothermic reactions have a ______ ∆H.
Collect the slips as students leave the classroom and mention that they will be doing
a lab tomorrow.
Evaluating Student Learning: The exits slips will be the main source of evaluation
for this lesson. It covers the two main topics of the day: relating exothermic and
endothermic to energy changes and calculating energy changes for given reactions.
Having the students complete the worksheet also allows for addressing students
one-on-one to discuss any problems. Going over the problems with them will also
provides the opportunity to observe the students work through the problems
correctly.
Day Three:
Introduction: The lab stations should already be set up for each group. Each table
will have several 200 mL beakers, a larger beaker filled with water, a thermometer,
stir rod, balance, and 50 g of the table’s unique reactant (either NaOH, KCl,
ammonium nitrate, or calcium chloride and sodium acetate). Be dressed and ready
for lab and make students aware of lab work scheduled for today. At the start of
class, bring attention to the learning goal: we will use the lab to apply our
knowledge on heat and use data gathered to make conclusions about reactions.
Explain that each group will have one reaction to perform at each table and they
need to work quickly and productively. Students will mix about 10 g of the reactant
in a beaker of water and record the temperature change. This data will be used later
to come to a conclusion about reaction type (endothermic or exothermic) and
enthalpy change (positive or negative).
Body: Pass out copies of the lab sheet to each student and have him or her gather in
the pre-assigned groups. Have them begin working at their current station.
Circulate through the lab to make sure students are on task and correctly
performing the reactions. Students will most likely need about 5 minutes to
perform the reaction and several more minutes to record data and observations.
The groups should rotate to different stations roughly every 10 minutes.
Possible Questions during this lab: Does it have to be exactly 10 g of the salt? No,
but make sure you record exactly the amount you mixed in. Do we need exactly 150
mL of water? No, just record the amount you have. Does it matter if I put the water
or salt in first? Yes, fill the beaker with the water first, and then add the salt. What
should I measure the temperature to? These thermometers are really only accurate
to .5 degrees.
Wrapping up the Lesson: When all the reactions have been performed by each
group, call the class back to their normal seats. A blank chart will be displayed on
the board, with a section for each reaction (divided further into four sections, one
for each lab group). Each lab group will report their initial amounts and conditions
as well as the temperature change they observed. As a class, come to a consensus as
to the type of reaction this is and whether the enthalpy change is positive or
negative, based on the results of each group’s tests. Pose the question: What
practical uses would a reaction such as this, when a salt is mixed with water and
creates either a cold or hot solution, have? Tell students to think about this as they
prepare for a quiz tomorrow.
Evaluating Student Learning: The primary evaluation tool in this lesson involves
student use of the lab. If they follow the procedure correctly and make proper
observations and data collections, the objective will be successfully achieved. By
gathering their results after completing the experiment and discussing them as a
class, it allows another chance to verify results or deal with any inconsistencies
among the groups.
Day Four:
Conclusion of the lesson: The lesson will conclude with a quiz on the previous three
days. This quiz should only take about 15 minutes and will be collected
immediately. The rest of the class time can then be devoted to whatever unit
follows. The quiz provides a useful evaluation tool over the entire unit. It will also
focus on examples of exothermic and endothermic reactions to create relevant and
practical uses that students can relate to other areas.
Design Rationale
The entirety of the lesson plan is based around providing demonstrations for
the students, describing why those demonstrations occur, and then having the
students investigate the phenomena on their own. Leading the lesson with two
curious demonstrations of reactions is meant to get the students thinking
immediately. Having a beaker suddenly dramatically heat up or cool down seems
completely unexplainable and nonsensical. This should get students trying to
explain it themselves and make them think.
Then, by defining exothermic and endothermic reactions, reason and
explanation are given to the student. The reaction is no longer a mystery. The
theory has been created, and now students can begin to apply it elsewhere. By
following this up with an explanation on enthalpy, a more quantifiable, meaningful
term is given to the reactions. We can get a calculated theoretical result rather than
a general observable conclusion.
Following up these explanations with a worksheet and exit ticket provide
assessment tools at roughly the midpoint of the unit. Student progress can be
readily gauged and there is still enough time left in the unit to go over topics another
time if needed. Modifications can still be made to lesson plan as necessary.
The lab work near the end of the lesson gives students the opportunity to put
what they’ve learned into practice. They can now demonstrate that they understand
the material covered in the unit. Working in groups and going over the results
together at the end of lab offers more experience with collaboration. The students
can also get differing opinions or results from each other, which can lead to more
varied approaches to future lab projects.
The quiz at the end of the lesson provides a fitting conclusion and summation
of all the concepts learned over the past three days. It reviews the important points
of the lessons and synthesizes them all. The quiz will also take the knowledge a step
further and apply it to the real world and encourage students to think about how
heat flow in reactions drive many natural cycles. Finishing the lesson with a quiz
also provides a final evaluation tool for the instructor.