Bond Energy Activity
Name_______
Learning Target: Students will calculate ΔHf° for reactions and determine if a reaction is
exothermic or endothermic
There is energy involved in chemical changes. It is this energy that you may have referred to
as “chemical energy” in previous courses. It can be used to determine the enthalpy of a reaction. This activity will teach you how to use ΔHf° to determine if reactions are exothermic or endothermic.
First take a look at the following chart:
Bond
Energy (kJ/mol)
Bond
Energy (kJ/mol)
H–H
436
C–C
347
O–H
464
C=C
607
C–H
413
CΞC
839
C–O
360
C=O
805
N–H
393
O=O
498
O – Cl
205
C-F
488
C – Cl
330
I–I
151
O–I
201
Cl – Cl
243
OΞC
351
F–F
153
NΞN
945
Notice that the energy has the unit of “kJ/mol.” The higher the number the more energy is involved.
Using the chart:
1. Would it take more energy to break H – H or C – H ? Explain.
2. Would you get more energy by forming C – O or O – H? Explain
3. Compare C-C and C=C and CΞC.
4. Using the chart, what is the relationship between energy and single vs double vs triple
bonds?
Part 1: Modeling the Reactions
In this activity there are 3 roles:
· Reaction Coordinators: These students will create the reactants out of models. They
will go to the Bond Breakers to have atoms pulled apart and to the Bond Formers so
they can have them put together to make the products.
· Bond Breakers: These students pull apart the models. Every time they pull apart a pair
of atoms they will need to give a blue card to the reaction coordinator that has the appropriate amount of energy printed on it (use the chart above.)
· Bond Formers: These students put models together for the reactants. Every time they
put a pair of atoms together they will need to give a green card to the reaction coordinator that has the appropriate amount of energy printed on it (use the chart above.)
The reaction coordinator will then arrange the cards to calculate the ΔHf° for the reaction.
Indicate if the reaction is endothermic or exothermic.
****REMINDER: ALWAYS BALANCE THE EQUATION FIRST!
Reaction 1:
2 H2 + O2  2 H2O
Reaction 2:
C2H4 + 3 O2  2 CO2 + 2 H2O
Reaction 3:
CH3OH + O2  CO2 + H2O
Reaction 4:
C + O2  CO2
Reaction 5:
CH4 + O2  CO2 + H2O
Reaction 6:
CCl4 + F2  CF4 + Cl2
Reaction 7:
OCl2 + I2  OI2 + Cl2
Part 2: Calculating Overall Energy Changes
1. Add up all the Bond Energy cards that are blue – this is the total energy involved in
breaking the bonds: _________
2. Add up all the Bond Energy cards that are green – this is the total energy involved in
breaking the bonds: _________
3. Are the amounts the same? Which is greater?
4. All of these reactions are exothermic, which process (breaking or forming bonds) must
give off energy? How do you know?
5. What would happen if a reaction was endothermic?
6. The amount of energy involved in Reaction 1 should have been 486 kJ/mol. Since it is
exothermic (gives off energy), would it be more appropriate to put this amount of energy
on the left side of the reaction or the right side? Explain.
486 kJ/mol (?) + 2 H2 + O2  2 H2O + 486 kJ/mol (?)
7. If a reaction is endothermic, should the energy be written on the left or right side of the
reaction? Why?
ON YOUR OWN
For the following reactions, use the bond energy chart to calculate the energy required to
break all the bonds and the energy that will be release in forming the new bonds. Then decide
if the reaction will be endo or exothermic. Add the energy to the appropriate side.
Reaction A:
H2 + CO  H2O + C
Reaction B:
N2 + H2  NH3
Reaction C:
C + H2  C2H4
Reaction D:
C3H8 + O2  CO2 + H2O
Reaction E:
Inside of you the following reactions is taking place:
C6H12O6 + O2  CO2 + H2O
This reaction describes what your body does with glucose to get energy. Why does it make
sense that it is exothermic?
Besides glucose, what is the other reactant & where do you get it?
What are the products and how does your body release them?
Reaction F:
The following reaction describes the photosynthesis reaction happening inside a plant:
CO2 + H2O  C6H12O6 + O2
This describes a reaction that happens inside a plant – it is making a larger compound (synthesis) and the process is endothermic.
Look at the reactants, where does the plant get these from?
Where does the plant get the energy to do this reaction?
Look at the Reactions E & F (what happens inside of you and the reaction inside a plant.)
What do you notice about them?