Unit 15: Lesson 2
Name ________________________
Temperature and Le Chatelier's Principle
Introduction: If you’re into magic shows, this is a good one to perform for an audience, because the solution
goes from purple to pink to green to blue and back again!
Le Chatelier’s principle states that when the temperature is raised, an equilibrium will shift away from the side
that contains energy. When temperature is lowered, the reaction shifts toward the side that contains the energy. That’s a little hard to understand, so that’s why there’s a really cool experiment that will show you exactly what we see happening with this principle.
Remember that exothermic reactions are chemical reactions that give off energy. In this experiment, this reaction is exothermic, which is going to be an important key in predicting which way the system will balance itself
as it gets subjected to temperature changes.
Materials:
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three test tubes
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two medicine droppers
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two beakers or glass jars
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two small disposable cups
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clear ammonia (not yellow) (IMPORTANT: Adult Supervision Required!)
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distilled white vinegar (clear)
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stove or alcohol burner
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saucepans OR stand for the burner (with the wire mesh) and an extra beaker
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water
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crushed ice
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red cabbage (not green)
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safety goggles
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gloves
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adult help
Procedure:
1.
Perform this experiment in a well ventilated area and next to a sink or water hose.
2.
Keep away all small children and pets!
3.
Place 200 mL (a little more than 3/4 cup) in the pan and add a handful of diced leaves of red cabbage and boil for 3
minutes. Turn off the heat and let cool, discarding the solids. The liquid should be purple or dark blue.
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Unit 15: Lesson 2
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4.
Label one of the cups with “NH3” and pour a small amount of clear ammonia in one of the disposable cups.
5.
Label the other cup with “CH3COOH” and pour a small amount of vinegar inside.
6.
Fill a glass jar or beaker with mostly ice and a little water.
7.
Place a glass jar mostly full of water in a pan and also fill the pan with water. Turn on the stove and heat the water
and glass jar. When it’s hot (but not boiling), set it in the sink so you don’t accidentally bump or splash it. (You can
also heat a jar of water using a microwave if you have one.)
8.
Place 20 drops of hot cabbage juice (be careful here!) into each test tube using your medicine dropper.
9.
Rinse out the medicine dropper. (Don’t skip this step!)
10. Place 15 drops vinegar to each of two of the test tubes. Put a stopper on the top and swirl it around. Don’t put anything in the third one except cabbage juice, because this one is your “control” so you can compare the color
changes.
11. What color is the test tube now compared with plain cabbage juice?
12. Add ammonia one drop at a time to only one of the test tubes with vinegar in it until it turns green (any shade). Swirl
the test tube after each drop. Record the number of drops required to turn the liquid green below.
13. Add the same amount of ammonia to the other test tube with vinegar until it also turns green. Swirl the test tube
after each drop.
14. Place both test tubes in the ice water bath to cool them down for at least a minute. When you pull them out, they
should be the same color, right?
15. Now put only one of the ammonia test tubes back in the ice bath and put the other in the jar that has the hot water
(be careful!).
16. After 30-45 seconds, pull out the test tubes from the ice bath and the hot water and compare. What happened? (If
the one from the hot water is not bluer than the ice bath, try again with hotter water. Don’t let it sit for longer than a
minute or you’ll drive the ammonia out of the solution.) Record your observations about what happened to the color
below.
17. Now put both test tubes in the ice bath for a couple of minutes. What color are they now? Are they the same or different?
Number of ammonia drops
required to turn liquid green
Color of liquid in ice bath
Color of liquid in hot bath
What you should have noticed is that the temperature of the liquid affected the color. Why is that? Well, the
solution that you created was an exothermic reaction which was in the following equilibrium:
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Unit 15: Lesson 2
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NH3 + C2H4O2 ↔ NH4+ + C2H3O2-
The cabbage juice is just there to act as a color indicator. What we are really seeing change is the pH of the
solutions as they change in temperature. Le Chatleier's principle says that when we subject an equilibrium
solution to colder temperatures, the reaction shifts toward the products. This caused the cabbage to turn
green (more basic). When you subjected the solution to hot water, the reaction shifted towards the reactants.
This caused the solution to turn blue (more acidic).
What is pH? It stands for “power of hydrogen” and describes the concentration of hydrogen ions in a solution.
Water has a pH of around 7. Anything with a pH of less than 7 is considered acidic. Anything with a pH of more
than 7 is considered basic.
Human blood has a normal pH of 7.4. This is a very important value. If the pH of your blood gets outside the
range of 7.35 – 7.45 it can be fatal. Water and blood have a very similar pH, which makes sense because we
are mostly made of water!
Problems:
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Unit 15: Lesson 2
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1. Which is a more basic liquid: ammonia or vinegar?
2. What does it mean to be in equilibrium?
3. What does it mean to be a neutral solution?
Answers:
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Unit 15: Lesson 2
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1. Ammonia is more basic with a pH of about 11
2. Equilibrium is when both the reactants and products are both present in the solution, and have no
tendency to change
3. A neutral solution has the same concentration H+ and OH-, meaning it's neither acidic or basic
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