.

Demonstration 1: 2NO2(g) N2O4(g) and Temperature

Purpose

To demonstrate the effect of temperature on a system at equilibrium.

Materials

Hot water bath

Ice water bath

3 NO₂ (g)-filled tubes

Dry ice/acetone bath (optional)

Overhead projector

Safety

No special precautions are necessary other than those given in the directions.

Procedure

Obtain NO₂ /N₂ O₄ (nitrogen dioxide/dinitrogen tetroxide) gas tubes from an educational scientific supply house (e.g. ,Temperature Equilibrium Tubes, Sargent-Welch #4426). Place one tube into a beaker of hot water 70 to 80 °C, one tube into a beaker of ice water (alternative: a dry ice/acetone bath; it will turn the gas colorless), and leave one tube at room temperature. Place all three tubes on an overhead projector and ask students to make observations. Write the equilibrium equation on the board:

Ask students to use LeChatelier's principle to account for their observations.

Demonstration 2: 2NO2(g)N2O4(g) and Pressure

Purpose

To demonstrate the effect of pressure on a system at equilibrium.

Materials

Glass syringes, 30-mL

Plastic syringe cap

Nitrogen dioxide, NO₂ (g)

Safety

Care must be taken to contain the NO₂ gas, preventing it from escaping into the room.

Procedure

In a hood collect some NO₂ in a syringe (Cu + HNO₃ (conc.) produces NO₂) by air displacement. Seal the syringe with a plastic syringe cap so no gas can escape. Firmly and rapidly press on the syringe to compress the gas as much as possible. At first the NO₂ gas appears darker (increased concentration) and then should fade (equilibrium shift). Demonstrate this reaction for individual students or small groups, since it may be hard to see. Write the following equilibrium equation:

Demonstration 3: Temperature Effects on Cobalt Complexes

Purpose

To demonstrate the effect of temperature on a system at equilibrium.

Materials

• Cobalt(II) chloride hexahydrate, CoCl ₂ ¥ 6H₂ O, (9.5 g per 100 mL solution)
• Concentrated HCl (hydrochloric acid), 100 mL
• Hot plate
• Ice water bath
• 3 Beakers, 250-mL

Safety

Since the solution is highly acidic it should be neutralized with sodium bicarbonate before disposal and flushed down the drain.

Procedure

Pour 100 mL CoCl₂ solution into a 250-mL beaker; add concentrated HCl until the solution changes color (pink to blue). This may take considerable HCl (about 100 mL). Divide the solution into three beakers. Place one on a hot plate, one in an ice bath, and the third at room temperature. Have students explain the colors in terms of the following equation:

Demonstration 4: Rubber Band Equilibrium

Purpose

To demonstrate the effect of temperature on a physical equilibrium system.

Materials

• Heavy rubber band
• Hooked weights (100-500 grams depending on rubber band)
• Ringstand and ring
• Hair dryer
• Meter stick

Safety

No special precautions are necessary.

Procedure

Suspend the weight from a rubber band attached to a ring stand. Put a meter stick next to the weight so that its distance from the table top can be measured. Discuss this system as an example of a physical system in static equilibrium. Ask students to predict what will happen if the rubber band is heated. Heat the rubber band with a hair dryer and measure its distance from the table. Allow it to cool and measure again.

Heated rubber band will be shorter than cooled rubber band due to entropy. Heating the system causes the ordered orientation of the molecules to become disordered. In the three dimensional system the rubber band becomes shorter.

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