Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
2.4: Energy changes in chemical reactions
Goal Facet 02: The student can describe and predict the energy exchanged during chemical
reactions based on the energy of the reactants and products.
Goal Facet 10 The student can interpret energy diagrams, including labeling energy of reactants
and products, activation energy, and identifying endothermic and exothermic reactions.
Overview:
This lesson contains 2 activities to help students understand the concepts of energy changes
involved in chemical reactions. In Activity 1 students will observe two chemical reactions that
show the different ways energy can be exchanged. Exothermic and endothermic processes are
illustrated through the mixing of bleach and sodium sulfite and through the mixing of calcium
chloride and baking soda. Safety precautions must be observed in all parts of this lesson. In
Activity 2 students will role-play chemical reactions, using banking as an analogy to learn about
energy changes as reactants break apart and products are formed.
Background:
Every chemical reaction or physical change involves an exchange of energy between the system and the
environment. For example, when an ice cube melts, the ice cube, which is the system, absorbs energy
from the surroundings, or air in this case. For a chemical reaction, the actual chemicals involved are the
system, while everything else makes up the surroundings. If a reaction involves two aqueous solutions
the chemicals are the system, and the water makes up the most “significant” surroundings. An
endothermic reaction absorbs energy from the surroundings while an exothermic reaction releases
energy to the surroundings.
There are many ways to represent particles within a chemical system. Below are two ways to
represent the particles involved in the reaction between hydrogen gas (H2) and chlorine gas
(Cl2).
H2 (g) + Cl2 (g)  2 HCl (g)
H H
Cl
Cl
Cl
Cl
H
Cl
Cl
H
H H
H
Cl
H H
Cl
H
Cl
H
Cl
H H
Cl
Cl
H
Cl
Cl
H
H
Cl
Cl
1
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Materials:
Student handout
Molecular model kit
For Reaction #1
Liquid bleach (10 mL per group)
0.5 M sodium sulfite (Na2SO3); (25 mL per
group)
100 or 250 mL beaker
graduated cylinders
Thermometers
For Reaction #2
Calcium chloride
Sodium bicarbonate (NaHCO3)
Ziploc baggies
Spoons
Water
Activity 1: Observing Energy Changes in Chemical Reactions
Procedure:
Reaction #1: bleach and sodium sulfite (quantitative)
Safety Notes: Use caution when handling bleach as it will remove color from textiles.
1. Put 10ml of liquid bleach into a beaker.
2. Record the temperature or the bleach. ________
3. Add 25 ml of a 0.50M solution of sodium sulfite.
4. Record the temperature. ________
Reaction #2:calcium chloride and sodium bicarbonate (qualitative)
1. Put a tablespoon of calcium chloride into a zip lock baggie.
2. Add a tablespoon of sodium bicarbonate to the bag also.
3. Add a tablespoon of water to the baggie and QUICKLY SEAL IT.
4. Shake and Record your observations.
5. Extension—do in a beaker and measure temp change
Observations: _________________________________________________________________
_____________________________________________________________________________
2
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Activity 2: Energy Banking in Chemical Reactions
How to play the Energy Banking Game:
Students work in groups of 4. Two students will work together in representing the chemical
“system”, the actual chemical reaction taking place. They can be thought of as the bank’s
Customers. The other two students represent the “surroundings” and can be thought of as the
Bankers.
Before beginning, cut out the “Energy dollars” at the end of the handout. For future use, copy
onto card stock and laminate
Calculating Energy Changes
1. For each chemical reaction, begin with $4000 Energy dollars.
2. Balance the chemical equation and draw out the Lewis structures for all the reactants.
3. Build all the reactant molecules in the balanced equation with the model kit. (For the
more complicated structures, the atomic arrangement is shown to the right of the
reactions.) If the balanced equation included two H2O reactant molecules, build two of
these, etc…
4. In order to break the reactant bonds, the system must gain energy from the surroundings.
So for each bond you break you must withdraw/receive energy from the bank. Break all
the reactant bonds in your model and as you do this look at the Bond Energies Table to
determine the amount of energy absorbed by the system from the surroundings to break
all the bonds in the reaction.
5. Each time a bond breaks, the Bankers must give the Customers Energy dollars. The
Bankers give enough Energy dollars (representing kilojoules of energy) to the
Customers to break all the bonds. Keep track of the total amount the Customers are
receiving from the bank.
Example:
Balanced chemical equation for the combustion of methane:
CH4 + 2 O2  CO2 + 2 H2O
Draw the Lewis structures, determine the number of bonds broken and calculate the energy
required to break all the bonds:
H
|
H―C―H
|
H
O=O
Bond
O=O
C-H
O=O
REACTANTS
Bond energy # of bonds
(kJ/mol)
broken
413
4
498
2
Total:
Energy required
(kJ)
1652
996
2648
The chemicals absorb 2648 kJ from the surroundings. Thus, the Customers must “receive”
2648 Energy dollars from the bank to break these bonds.
3
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
6. Now do the same thing for the product molecules – draw Lewis structures, determine the
total number of bonds formed and how much energy this involves. When bonds are
formed, energy is released to the surroundings.
7. Build the product molecules from the reactant atoms and for each bond formed the
Customers now give the Bankers back the amount of Energy dollars released for those
bonds.
H
H
O
O
Bond
H
O=C=O
H
H-O
C=O
Products
Bond energy # of bonds
(kJ/mol)
broken
459
4
799
2
Total:
Energy required
(kJ)
1836
1598
3434
8. Once all the Energy dollars have been exchanged, the Bankers and Customers now
count up all their Energy dollars and see how much they gained or lost. Record these
values in the Data Table. (Remember that each group started with $4000.)
9. To calculate the heat of the reaction from the system’s point of view, the customers
subtract the amount of money they had to return to the bank from the amount of money
they received from the bank. If this number is positive, the reaction is endothermic (has
more energy at the end) and if this number is negative, the reaction is exothermic (lost
energy to the surroundings).
10. After finishing the calculations for each reaction, redistribute the money so that each
pair has $4000 and then do the next reaction.
Reactions:
1. CH4 + I2  CH3I + HI
O=O
(O2)
2. 2 C2H6 + 7 O2  4 CO2 + 6 H2O
3. H2 + Cl2  2 HCl
4. CH4 + 2 F2  CF4 + 2 H2
O=C=O
(CO2)
H H
| |
H―C―C―H
| |
H H
( C2H6)
4
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Bond Type
C-Br
C-C
C=C
C≡C
C-Cl
C-F
C-H
C-I
C-N
C-O
C=O
C-S
Energy
(kJ/mol)
288
348
389
839
330
488
413
216
308
360
799
272
Table of Bond Energies
Energy
Bond Type
(kJ/mol)
H-Br
366
H-Cl
432
H-F
568
H-H
436
H-I
298
H-N
391
H-O
459
Bond
Type
Br-Br
Cl-Cl
F-F
I-I
N-N
N≡N
O-O
O=O
Energy
(kJ/mol)
193
243
158
151
170
945
145
498
Data Table
System (Customers)
Energy
Energy lost
gained
Surroundings (Bankers)
Energy
Energy lost
gained
Heat of Reaction
Exothermic or
Endothermic
Reaction 1
Reaction 2
Reaction 3
Reaction 4
5
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Questions:
Interpreting your findings:
1. For reaction #1, draw a representation at the molecular level of the methane gas (CH4) and
solid iodine (I2) in the container before any reaction occurred.
2. Draw a representation of the products after the reaction took place.
Before reaction
After reaction
3. Did the chemical system gain or lose energy?
4. If this reaction was carried out in the flask shown above, would the glass get warmer or
colder? If you had a thermometer in the flask would the temperature go up or down?
Explain.
5. Describe for each of the reactions #2-#4 whether the surroundings would have heated up or
cooled down.
6. Explain how a chemical reaction can be considered to have both endothermic and
exothermic aspects during the reacting process.
7. Can you determine the heat of reaction from total bonds broken and formed?
8. Does it matter how many dollars each side starts with?
6
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Teacher Notes
Activity 1: Observing Energy Changes in Chemical Reactions
The chemical equation for reaction #1 is:
NaClO(aq) + Na2SO3(aq)  NaCl(aq) + Na2SO4(aq) + heat (exothermic)
The chemical equations for reaction #2 are:
CaCl2(aq) + NaHCO3(aq) + Heat  NaCl(aq) + CaCO3(aq) + H2CO3(aq) (endothermic)
H2CO3(aq)  H2O(l) + CO2(g)
7
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
Energy dollars
500
500
500
500
100
100
100
100
8
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
100
100
100
100
100
100
50
50
9
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
50
50
20
20
20
20
20
20
10
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
10
10
10
10
10
10
5
5
11
Energy of Chemical Reactions Developmental Lesson
“Energy dollars” and Energy of Reactions
Teacher Notes and Materials
5
5
1
1
1
1
1
1
12