An Afternoon of Apple Pie
Introduction: For this topic, you’ll need to read through and make note of new vocabulary
words that you will need to use. Take notes throughout if you need! Your first terms are:
SYSTEM- the point of interest
SURROUNDINGS- everything else outside of the system
ENTHALPY- the heat energy of a system, notated with an ‘H’
EXOTHERMIC- heat exits the system, a NEGATIVE change in enthalpy (-ΔH)
ENDOTHERMIC- heat enters the system, a POSITIVE change in enthalpy (+ΔH)
Part I.
Consider a chef hard at work in a kitchen. Let’s call her Chef Ms. C. She is the best in the state
and is most famous for her desserts, especially her world renowned apple pie. Today you and
your partner are assisting her in the kitchen as she cooks for her harshest critic… Mama
Cordeiro. Everything must be perfect!
To start with, Ms. C takes out a cast iron pan to make her crust in. She turns on the stove top and
puts the pan on it. The fire starts to heat it and the air above it.
1. Do the molecules of the air above the pan move faster when they are at a lower temperature
or a higher temperature? Why?
2. Using what you know about temperature and your answer from #1, would the air molecules
have more energy or less energy when they are at a higher temperature?
3. Draw one picture showing what the molecules look like in cool air and another showing what
molecules look like in hot air based on your answers to #1 and #2.
As the pie crust is baking, Ms. C asks you to take out a jar of her special pie filling from the
refrigerator and set it on the counter, which you gladly do. When you take it out of the
refrigerator, you notice that the glass jar feels very cold. You set the jar on the counter and go
back to helping Ms. C. After a while, she tells you that she is ready for the filling and sends you
to go get it. When you pick up the jar, you notice it is much warmer than it was when you first
took it out of the refrigerator. As you take it over to Ms. C, she sees your puzzled expression and
asks what’s wrong. You explain to her your observations and she responds by asking you why
you think the jar got warmer.
4. Explain to Ms. C how the jar got warmer by just sitting on the counter. Make sure to relate
your answer back to the energy of molecules.
5. Using the space below, draw a picture showing how the change happened.
Part II.
Ms. C nods her head as you finish your explanation. She tells you that what you are talking
about, the dispersal of the initial motional energy of molecules so that the energy is spread out
over a bigger volume is called a change in entropy and is represented in chemistry by the
symbol ∆S. She tells you that an object’s entropy stands for the number of possible states or
configurations it can be in and is usually referred to as a degree of disorder or randomness in a
system. She begins to draw a picture to help you understand:
6.
Ms. C draws a picture of a box with 6 molecules in it. Would you say that the system is
ordered or disordered? Explain.
7.
Ms. C then draws a second picture below. Would you saw the second picture is ordered or
disordered compared to Ms. C’s first picture?
8. Using your new-found knowledge of entropy, explain which picture has higher entropy?
9. Would you say the first picture better represents the atomic arrangement of a solid or gas?
The second picture? Why?
10. Explain whether a solid or a gas has more entropy.
Ms. C tells you that systems with more molecules or the same number of molecules in a smaller
volume have more entropy because there are more collisions and more possible states. She also
tells you molecules that have higher temperatures have higher entropies because they have
higher kinetic energy and thus more collisions and a higher frequency of changes.
11. Based on what Ms. C just told you, predict whether the following changes would have an
increase in entropy (∆S>0) or a decrease in entropy (∆S<0):
a. a cube of ice melting into water
b. a container with 2 moles of helium expanding from 1 L to 2 L
c. cooling a glass of water from 20 degrees C to 5 degrees C
Part III.
Ms. C congratulates you on your grasp of entropy so far. She asks if you have any questions and
you say that you’ve learned about enthalpy in class before and you were wondering which was
more important, enthalpy or entropy, in determining whether or not a reaction will occur? She
smiles and tells you what a great question that is and asks you one back.
12. Consider a glass of water evaporating into steam. Is the change in enthalpy of the process
positive or negative? Is it an exothermic or endothermic process?
13. For that same process, is the change in entropy positive or negative?
14. Based on what you know about water changing into steam, does the process happen at high
temperatures or low temperatures? Is it possible to go from steam back to water?
15. Would you say then that the entropy change, the enthalpy change, both, or neither are
responsible for determining whether or not a reaction will happen?
After hearing your answer, Ms. C tells you about another chemistry term: Gibbs Free Energy
(G). She reminds you that energy is the ability to do work and the change of Gibbs free energy of
a system (∆G) represents the energy absorbed or released by a reaction that is available, or free,
to do work. Ms. C tells you that if a reaction releases energy that can do work (∆G<0), then we
call that reaction spontaneous or favorable, meaning it will happen naturally without any
external input of energy as long as the activation energy can be overcome. The change in Gibbs
free energy of a system is related to the change in entropy, the change in enthalpy, and the
temperature of the system by the equation:
∆G = ∆H – T*∆S
16. Based on Ms. C’s explanation, complete the chart below, indicating the sign of the change in
Gibbs free energy and whether or not the reaction is always spontaneous, never spontaneous,
or spontaneous at certain temperatures. Remember that a reaction is spontaneous only if ∆G
< 0!
Sign of ∆H Sign of ∆S Sign of ∆G
+
+
+
+
Spontaneity
17. Going back to the water into steam example, is the reaction always spontaneous, never
spontaneous, or spontaneous only at certain temperatures? Tell the signs of ∆H and ∆S. If
you say only at certain temperatures, at what temperature would it become spontaneous?
18. Can you think of an example of a reaction that would be always spontaneous? What about
one that would never be spontaneous?
Ms. C applauds you as you finish your explanation. Suddenly both of you smell something
burning and turn frantically towards the stove. You run over and notice that the apple pie is
smoking! Oh no! And Mama C is due in only a few minutes! There will be time for more
chemistry later, but for now it’s time to bake!