Temperature and Le Chatelier’s Principle

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.

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Here’s how you do it:

ADULT SUPERVISION REQUIRED for this experiment because it involves ammonia and boiling water!!

Materials:

• three test tubes
• two medicine droppers
• two beakers or glass jars
• two small disposable cups
• clear ammonia (not yellow) (IMPORTANT: Adult Supervision Required!) (MSDS)
• distilled white vinegar (clear)
• stove or alcohol burner
• saucepans OR stand for the burner (with the wire mesh) and an extra beaker
• water
• crushed ice
• red cabbage (not green)
• safety goggles
• gloves
• adult help

Advanced Students: Download your Worksheet Lab here!

Experiment Steps:

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.
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.
13. Add ammonia one drop at a time to the other test tubes with vinegar in it until it turns green (any shade). 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.)
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?

What’s going on?

When ammonia and vinegar were mixed in the solution, it created this equilibrium:

NH₃ + C₂H₄O₂ <–> NH₄⁺ + C₂H₃O₂^–

This reaction produces energy, which means it’s exothermic. Placing the test tube in the ice bath lowers the temperature shifts the reaction toward the products which causes the cabbage juice to turn green, indicating a basic solution.

When you added the cabbage juice, it served as an indicator to tell whither the solution was acidic or basic. The anthocyanin from the cabbage juice turns pink with acids like vinegar and blue with bases like baking soda, and green with bases like ammonia.

When the test tube is placed in the hot water, the solution turns blue to indicate that the reaction shifted toward the reactants, making a less basic solution than it was in the ice water. The solution is still basic, but not as strongly as when placed in the ice bath.

There’s more to this principle (including how pressure or concentration affect the equilibrium), but it’s the same idea. If the temperature, pressure (volume) or concentration of a chemical system at equilibrium changes, then the equilibrium shifts to compensate for that change.  Chemists use this principle to predict how a change in pressure, volume, concentration, or temperature will affect a chemical system in equilibrium. Knowing this ahead of time allows chemists to figure out how to get the most products out of (or least out of, such as with smog) a reaction.

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