038 Henry's Law

Expt 038 -- Henry's Law

The pressure of a gas over a solution determines the solubility of the gas; more gas dissolves at high pressure. When soda water is placed in a syringe with a suitable pH indicator, decreasing the pressure of the CO₂ decreases the amount dissolved, and the change can be detected by noting its effect on the carbonic acid equilibrium.

Background

The solubility of a gas depends upon pressure. Nitrogen is not very soluble in aqueous solution. When considerable pressure is applied, the solubility increases. Divers who work at great depths breathe air that is highly pressurized and, as such, they have more nitrogen dissolved than under normal conditions. If the divers are brought to the surface too quickly, the pressure goes down, the solubility decreases, and bubbles of nitrogen form in their blood. The result is a dangerous diver's disease -- the bends.

Safety

Wear goggles and apron. The indicators can stain clothing. The syringes under pressure can snap back into place causing some damage. Wash hands after the experiment.

Procedure

Work with a partner. You will need a stopper, a syringe with a small hole to hold it open, and a nail.

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Practice placing to stopper on the syringe. This is more difficult when the syringe is filled with solution. Pull the plunger on the syringe to see if it is air tight.

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Practice placing the nail in the hole of the syringe with the syringe held open.

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Draw 1.0 mL of syringe filling solution into the syringe.

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Expel air bubbles in the syringe (it will still work with some bubbles).

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Carefully, place the stopper on the syringe. Note and record the color of the liquid in the syringe.

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Pull the plunger of the syringe out, and hold it with a nail in place.

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Shake the syringe to accelerate this process. Note and record any change in the color of the solution.
Compare the color of the chlorophenol red before and after expansion.

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Methyl Red gives a different color change.

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Carefully remove the nail and allow the plunger to return to its initial position, pressing as necessary. After a few moments, tap the syringe a few times to mix the liquid.

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Note and record any change in the color of the solution.

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Questions

Account for the formation of gas bubbles.

CO₂ has an equilibrium in water:
CO₂ + H₂O --> H⁺ + HCO₃^-
Account for any color changes upon decompression in terms of this equilibrium.
Account for observations when the syringe is recompressed.
Explain why one must pull so hard to move the plunger out to the 10 mL mark. Does the force required change?

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 038 Henry's Law

Watch the movies.

Describe the color of the soda solution before and after compression for the chlorophenol red indicator. State whether compressed or expanded is more acidic.

Answer the questions.

Curriculum-

This experiment comes late in an introductory course. To appreciate the color change, equilibrium must have been previously or be concurrently covered. Gases, solutions, and gas laws also should have been covered. This also is an excellent AP experiment. When the theory isn't stressed, the dissolved gases idea fits well into many areas of an applied chemistry course.

Safety-

Wear goggles and apron. The indicators can stain clothing. The syringes under pressure can snap back into place causing some damage. Wash hands after the experiment.

Time-

Teacher Preparation: 15 minutes

Class Time: 30 minutes

Materials-

5 mL of club soda
nail
burner
tongs
matches
scissors
# 3 to # 6solid stopper
#0 solid rubber stopper.
10-mL disposable plastic syringe without a needle
0.5 g NaHCO₃ -- use as solid
1 mL of 0.1 M HCl (Dissolve 3.3 mL of 3 M HCl in enough water to make 100 mL of solution.)
- chlorophenol red indicator -- dissolve 0.1 g of chlorophenol red in 25 mL of 0.01 M NaOH; then add 225 mL of H₂O.
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  - or
- methyl red indicator -- Dissolve 0.10 g of methyl red in 100 mL ethanol. (Alternate preparation--Dissolve 0.10 g of methyl red in 35 ml of 0.01 M NaOH; then add 215 ml of H₂O.) If you are using methyl red the colors are red in acid and yellow in base which is just the reverse of the ones described below for chlorophenol red.
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Disposal-

Dispose of them at the sink with running water.

Lab Hints-

Filling solution (prepare fresh): pour 100 mL of Soda Water into a 150-mL beaker. Add 15 drops of chlorophenol red indicator solution. The solution is usually yellow. Add a small amount of sodium bicarbonate (a quarter the size of a split pea). The solution will turn red violet. (If not, add more sodium bicarbonate).

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Titrate this solution dropwise with 0.1 M HCl until it turns yellow-orange. You must be within one drop of the end point. If you over shoot the end point by more than one drop, add a little more bicarbonate and titrate again.

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The solution will NOT keep long so you may wish to plan this titration as a demonstration for your class at the beginning of the laboratory.

Answers-

Q1. Account for the formation of gas bubbles. A1. CO₂ gas comes out of solution because of the reduced pressure.
Q2. CO₂ has an equilibrium in water:
CO₂ + H₂O --> H⁺ + HCO₃^-
Account for any color changes upon decompression in terms of this equilibrium. A2. When the CO₂ comes out of solution, it pulls the equilibrium to the left (Le Chatelier's principle). As [H⁺] is lowered, a pH indicator whose change is in the appropriate region may change color. Chlorophenol red is an excellent choice for this indicator.
Q3. Account for observations when the syringe is recompressed. A3. At first, no changes are noted. However, when the syringe cylinder is tapped a few times, the initial color reappears. The redissolved CO₂ is dissolved in a thin region at the interface, and requires mixing for a dramatic color change to be apparent.
Q4. Explain why one must pull so hard to move the plunger out to the 10 mL mark. Does the force required change? A4. In order to reduce the pressure, a force must be applied. The laboratory worker is applying a force to overcome the force of air pressure. In a sense, the plunger is being made to compress the atmosphere. As CO₂ escapes from the solution, this amount of force required diminishes. The CO₂ starts pushing to assist the lab worker.

Handout Ans.-

Clorophenol solution is red violet expanded and yellow compressed. The compressed solution is more acidic.

CoopLearn-

This experiment is best performed in pairs; one student pulls on the syringe plunger, while the other inserts the nail.

Reference-

A similar experiment was developed by Andy Edington and published in Microchemistry Lab Experiments, Daedalus, P. O. Box 307, Mequon, WI 53092, experiment 1-13. Note the reference to Zidick therein.

Key Words 1-

acid, pH indicator, equilibrium, weak acid, pressure, gas pressure, Henry's law, solution, dissolve

Elements-