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Alka-Seltzer Reveals Gas Solubility and Reaction Rate
Solids are generally more soluble in hot water than in cold water so it may be surprising to learn that all gases become less soluble in water as the water temperature rises. Use Alka-Seltzer to create an interesting and very visual demonstration of gas solubility along with other chemical concepts.
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Equipment and Materials:
3 Pyrex beakers, 250 mL
3 Erlenmeyer flasks, 125 mL
1 graduated cylinder, 50 mL
2 Alka-Seltzer tablets
10 mL, bromthymol blue indicator solution,
0.04% aqueous
5 mL, sodium hydroxide solution, NaOH,
around 1 M
Laboratory balance
Ice
Hot plate
Dropping bottles
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Advanced Preparation:
Aqueous bromthymol blue can be purchased in the correct concentration but if you don’t have it available then follow this procedure. Dissolve 0.04 grams of bromthymol blue indicator powder in approximately 80 mL of distilled or deionized water. Stir to dissolve the powder and dilute the solution to 100 mL with additional water.
Procedure:
- Add 200 mL of water to each of three 250-mL beakers. Place one beaker in an ice bath, heat the second on a hot plate, and keep the third at room temperature to serve as the experimental control.
- Select two smooth, unbroken Alka-Seltzer tablets.
- When the water in the ice bath has cooled to near the temperature of the bath and the water on the hot plate has reached a temperature of 70 to 80 oC, remove them from the ice bath and hot plate and place them on either side of the room temperature beaker. To each of the three beakers add 3 mL of bromthymol blue solution.
- Simultaneously drop the two Alka-Seltzer tablets into the hot and cold beakers. Carefully observe and record the reactions and whatever changes occur. When the reaction has stopped and the Alka-Seltzer has fully dissolved note the appearance of the liquids in each beaker.
- Remove 25-mL samples from each of the three beakers and place them in corresponding Erlenmeyer flasks.
- Using the samples in the flasks, titrate the hot and cold samples by the dropwise addition of NaOH solution, swirling to mix between drops. The end point is reached when the solution color matches that of the room temperature “control” tube. Wait two minutes before recording the final drop counts as the colors may revert slightly.
Tips:
If bromthymol blue is not available then phenol red may be substituted as the indicator. The strength of the NaOH solution is not critical, although it should be in the range of 0.5 M to 1.0 M for best results. After the indicator has been added to the three beakers, check the color. All three solutions should be bright blue. A drop of NaOH solution may be added to adjust the color, if needed.
An entire Alka-Seltzer tablet may cause the hot beaker to over flow. The vigorous reaction is certain to peak student attention but requires extra planning. Placing the beakers in a shallow tray or on two sections of old newsprint should be adequate. If the overflow appears to be a problem then use carefully measured one-gram masses of Alka-Seltzer.
Discussion:
Alka-Seltzer contains aspirin (acetylsalicylic acid), sodium bicarbonate and citric acid. When the Alka-Seltzer is dissolved in water, three different equilibria occur. Initially, the hydrogen ions from the two weak acids react with the bicarbonate ion to form carbonic acid (equation 1). Carbonic acid is in equilibrium with CO2(aq) and water (equation 2). Finally, the dissolved carbon dioxide is in equilibrium with gaseous CO2 based on the solubility of carbon dioxide in water.
Eq.1) H+(aq) + HCO3-(aq) -> H2CO3(aq)
Eq.2) H2CO3(aq) -> H2O(l) + CO2(aq)
Eq.3) CO2(aq) -> CO2(g)
The amount of carbonic acid in solution depends upon the solubility of carbon dioxide in the solution. The greater the solubility, the greater the amount of carbonic acid and the lower the pH of the solution. When the aqueous solution of the Alka-Seltzer is titrated with a strong base, all of the acetylsalicylic acid, citric acid, and carbonic acid are neutralized. Since both the hot and cold solutions will have the same amount of citric acid and acetyl salicylic acid, the difference in the NaOH required to titrate is due to the quantity of carbonic acid in solution. The more NaOH required to neutralize the solution, the greater the amount of carbonic acid available and the greater the concentration of dissolved carbon dioxide. The cold solution requires more sodium hydroxide for neutralization because the solubility of carbon dioxide is greater in the cold water than the hot water.
The implications of this experiment in chemistry include gas solubility, reaction rate, acid-base indicators, and acid-base neutralization reactions. In biology the discussion might focus on oxygen solubility in water and what will inevitably result when natural waters are warmed either naturally or artificially by thermal pollution. What will happen as less oxygen becomes available to aquatic organisms? The increase in atmospheric CO2 from burning fossil fuels and rain forests has been proposed as a potential cause of global warming through the greenhouse effect. The oceans are a major storehouse for CO2. As global temperatures rise, ocean temperatures will rise and some of the stored CO2 will be driven out of solution.
Safety Precautions:
Sodium hydroxide and its solution are very corrosive; skin burns are possible; very dangerous to eyes. Wear chemical splash goggles, chemical-resistant gloves, and chemical-resistant apron. Tongs are recommended whenever handling beakers of hot liquids.
Disposal:
The resulting solution and any left over chemicals can be rinsed down the drain with an excess of tap water. Consult a reputable manual for detailed instructions.
Acknowledgements:
An excellent version of this activity was presented by Adele Gomez, St. John’s School, Santurce, Puerto Rico during the 1995 Woodrow Wilson National Fellowship Foundation Chemistry Team 11 workshop at Northern Kentucky University.
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