Co-Authored by Dean J. Campbell*, Carley Steres*, and Kaitlyn Walls*
*Bradley University, Peoria, Illinois
Self-inflating balloons based on the chemical reaction between citric acid and baking soda can be used to illustrate multiple chemical principles.1,2,3 One variety, marketed as Wack-A-Pack balloons, have been sold in local Dollar Tree stores as St. Valentine’s Day novelties. As of this writing, they are currently $1.25 for a four-pack of balloons. We have purchased many of these balloons over the years for use in classroom demonstrations and chemistry-related outreach events. Each Wack-A-Pack consists of an outer sealed pouch containing a folded and sealed balloon. The balloon contains loose sodium hydrogen carbonate powder and a small inner pouch containing citric acid solution. When the entire Wack-A-Pack is struck by a hand or a foot (“whacked”), the innermost pouch breaks and releases the citric acid solution onto the sodium hydrogen carbonate, Figure 1.
Figure 1.Inflated Wack-A-Pack balloon and some of its components CLOCKWISE FROM UPPER LEFT: outer packaging, inner pouch, inflated balloon, balloon sliced in half.
The reaction of the citric acid and the sodium hydrogen carbonate produces carbon dioxide gas which inflates the balloon.
H3C8H5O7 + NaHCO3 à 3 CO2 + 3 H2O + Na3C8H5O7
Usually, enough pressure from the carbon dioxide causes the outer pouch to pop open and release the balloon, although sometimes the outer pouch needs a little help to open. With a little information about the quantities of reactants and other aspects of this process, chemical principles can be covered in a bit more depth.
There is an opportunity to use these balloons to discuss limiting reactants. Three unopened Wack-A-Pack balloons were cut open and the sodium hydrogen carbonate powder was weighed. The measured masses varied, but averaged 1.439 g, or 0.01713 moles, of sodium hydrogen carbonate. If this compound was the limiting reactant and completely reacted with the citric acid, it would produce 0.01713 moles of carbon dioxide. A couple of approaches were to measure the amount of carbon dioxide actually produced in five balloons. We first utilized a 2 L graduated cylinder and water to determine balloon volume through displacement. Beakers and even a kitchen measuring cup could also possibly be used for this type of volume measurement. Each balloon was submerged in the water with tongs, and volume change was measured. Neglecting the volume of the balloon walls, the displacement technique gave an average volume of 135 mL with a standard deviation of 8 mL. At the room temperature and pressure conditions, this volume corresponded to 0.00564 moles of carbon dioxide, much less than could be produced by complete reaction of the sodium hydrogen carbonate. However, the balloons must have a pressure at least equal to that of ambient pressure. Sometimes the balloons are very firmly inflated. To get measurements of the carbon dioxide gas volumes outside of their balloons, a 250 mL graduated cylinder filled with water was inverted in a plastic tub of water. A beaker or kitchen measuring cup might also work for this measurement. Each inflated balloon with its volume previously measured was held under the submerged opening of the graduated cylinder and was punctured. The carbon dioxide gas that bubbled out of the balloon was captured as a gas pocket at the top of the inverted cylinder. The average volume of the gas pocket was 147 mL with a standard deviation of 27 mL. At the room temperature and pressure conditions, this volume corresponded to 0.00614 moles of carbon dioxide, again much less than could be produced by complete reaction of the sodium hydrogen carbonate, but a little more than that obtained by measuring the inflated balloons themselves. This small increase in volume supports the notion that the pressure inside the inflated balloon is not much greater than the pressure outside the balloon. If all of the sodium hydrogen carbonate were to react in the balloon, a much greater pressure or a much greater volume of carbon dioxide could be produced. In Video 1, this idea is discussed and reinforced with a demonstration where the sodium hydrogen carbonate from a Wack-A-Pack balloon was added to sulfuric acid solution with dish soap added to produce roughly 350 mL of foam, much more than what was needed to inflate a 150 mL balloon.
Video 1. Discussion of limiting reactants with gas volume experiments for Wack A Pack balloons. ChemDemos YouTube Channel (accessed 2/2/2022)
Knowing the volume of the inflated balloon and that it contains mostly carbon dioxide gas enables the calculation of the difference between the molar mass of carbon dioxide and the average molar mass of the ambient air in the room. To do this, the volume of three inflated balloons was measured by water displacement and the balloons were dried. Each dry balloon was placed on a laboratory balance, which was then tared. The balloon was removed from the balance and gently punctured with a needle to release extra carbon dioxide pressure and enable the pressure inside and outside the balloon to equilibrate. Since the walls of the balloons were not comprised of stretchy latex, and were more likely the polymers described below, the balloon mostly held its shape. The still-inflated balloon was then placed back on the balance and the mass loss was noted. In the three trials, the average mass loss was 0.0993 g, corresponding to 0.00226 moles of carbon dioxide. Given that the average volume of the inflated balloons was 150 mL, this corresponded to a pressure loss of 0.0650 atm at ambient room conditions. Again, the initial pressure inside the inflated balloons was slightly greater than outside of the balloons.
With the equilibrated but still inflated balloon on the balance pan, the balance was again tared. The balloon was again removed from the balance and this time was gently pressed flat to expel all the air, but not any liquid or solid, from the balloon. The balloon was again placed back on the balance and the mass loss was noted. This mass loss in grams was divided by the inflated balloon volume to give the density difference (in g/L) between the carbon dioxide in the balloon and the air surrounding the balloon.
Incorporating that density (d), and the ambient pressure (P) and temperature (T), into the equation P × Mm = d × R × T, gives the molar mass difference (Mm) between the gases.4 Using the data from the three trials, a difference of 16.13 g/mol was measured. For comparison, the average molar mass of the air in the lower atmosphere is 28.96 g/mol,5 which is 15.05 g/mol lower than the carbon dioxide molar mass of 44.01 g/mol.
If the excess gas pressure in a freshly inflated balloon can be released quickly through a narrow hole, the balloon can be propelled in the opposite direction of the hole and behave a bit like a rocket. The challenge is to make a small hole in the balloon without actually pushing on the balloon and moving it. However, heat from a flame placed near the balloon can weaken the plastic until it fails. The carbon dioxide that quickly vents from the balloon can provide thrust to move the balloon. Video 2 shows a Wack-A-Pack balloon hanging from a paper clip tied to a piece of dental floss. The heat from a butane lighter placed near the balloon caused a small hole to form, releasing carbon dioxide and causing the balloon to spin. This demonstration can be connected to other rocket or space-related demonstrations.6 Another space-related connection is the fact these balloons are inflated with carbon dioxide, which is the major component of the atmospheres of Venus and Mars,7 and a minor yet environmentally significant component of Earth’s atmosphere.
Video 2. Wack-A-Pack balloon spinning from pressure release through heat-weakened hole. ChemDemos YouTube Channel (accessed 2/2/2022)
In addition to the gas pressure aspects of the Wack-A-Pack balloons, there are thermochemical aspects to consider. As others have already noted, the chemical reaction itself is clearly spontaneous, with a negative change in Gibbs free energy.2 The reaction proceeds after initiation without outside intervention. The initiation of the process, the “whack” could be a sort of analogy for the activation energy required to get the inflation process started. However, details like transition states might be beyond the scope of the analogy. A readily observable thermochemical phenomenon with the Wack-A-Pack balloons is that they develop a cold spot as they inflate that can be felt with fingers as the balloon is held. This appears to be due to the dissolution of the sodium hydrogen carbonate in water and its reaction with the citric acid both being endothermic. Video 3 shows a Wack-A-Pack balloon inflating while being viewed with an infrared camera.8 The bottom corner of the balloon was notably cooler where the citric acid solution and sodium hydrogen carbonate were actually combining.
Video 3. Wack A Pack balloon viewed with an infrared camera while inflating. ChemDemos YouTube Channel (accessed 2/2/2022)
Another chemical topic to consider is the “greenness” of these demonstrations from the perspective of the Twelve Principles of Green Chemistry, which yields some positive and some negative outcomes.9 The carbon dioxide-producing reaction itself, based on the relatively safe reactants sodium hydrogen carbonate (baking soda) and aqueous citric acid, seem to mesh well with principles 3. Less Hazardous Chemical Synthesis, 5. Safer Solvents & Auxiliaries, and 10. Design for Degradation. There is, however, definitely room for improvements in these demonstrations with respect to principles 1. Waste Prevention and 10. Design for Degradation. There seems to be an excess of sodium hydrogen carbonate in the Wack-A-Pack balloons, and it is unclear whether or not that excess is necessary to help ensure that the balloons inflate in a timely manner. Perhaps more significant is the use of all the plastics in the balloons and the packaging. Raman microscopy performed in our lab of the balloon and its packaging indicates that these materials are comprised of polypropylene, polyethylene terephthalate, and possibly other polymers. It would be desirable to switch to materials that are less persistent in the environment at the end their useful lifetimes.10 Despite this need for improvement, these low-cost yet eye-catching novelty balloons can be used to illustrate multiple chemical principles associated with moles, gas laws, thermochemistry, and Green Chemistry.
Safety Precautions, including proper personal protective equipment such as goggles, should be used when working with demonstrations. Avoid spilling strong acid solutions on skin or clothing. All solution containers should be clearly labeled. Do not set the plastic balloons on fire with the butane lighter. Always wash your hands after completing the demonstrations.
Acknowledgements This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium. The FLIR camera was provided through a subcontract from a grant from the National Science Foundation Division of Undergraduate Education, awards 1813313 and 1626228.
1. Modic, A. Self-Inflatable Valentine Balloons – Chemistry is Everywhere! https://www.chemedx.org/blog/self-inflatable-valentine-balloons-%E2%80%9... (accessed February, 2022).
2. The STEMAzing Project. Wack-a-Pack Science PQRST. https://stemazing.org/wp-content/uploads/2019/04/Wack-a-Pack-Science.pdf (accessed February, 2022).
3. Campbell, D. Dr. Campbell’s Favorite Demos: Whak-A-Pack Valentines. http://campbelldemo.blogspot.com/2014/02/whak-pack-valentines.html (accessed February, 2022).
4. Tro, N. J. Chemistry: A Molecular Approach, 5th Edition; Pearson Education, Inc., 2020.
5. The Engineering Toolbox. Air - Molecular Weight and Composition. https://www.engineeringtoolbox.com/amp/molecular-mass-air-d_679.html (accessed February, 2022).
6. Campbell, D. J.; Kahila, T.; Kraft, C. “Soda Fountains from Aluminum Cans.” ChemEd Exchange. September 16, 2021. https://www.chemedx.org/blog/soda-fountains-aluminum-cans (accessed February, 2022).
7. Campbell, D. J. “LEGO Brick Atmosphere Sticks.” ChemEd Exchange. June 6, 2021. https://www.chemedx.org/article/lego-brick-atmosphere-sticks (accessed February, 2022).
8. Green, T.; Gresh, R.; Cochran, D.; Crobar, K.; Blass, P.; Ostrowski, A.; Campbell, D.; Xie, C.; Torelli, A. “Invisibility Cloaks and Hot Reactions: Applying Infrared Thermography in the Chemistry Education Laboratory.” J. Chem. Educ., 2020, 97, 710-718.
9. Compound Interest. The Twelve Principles of Green Chemistry: What it is, & Why it Matters. https://www.compoundchem.com/2015/09/24/green-chemistry/ (accessed February, 2022).
10. Campbell, D. J.; Lojpur, B. “Microplastics, Liquid Nitrogen, and Iodine: Polystyrene vs. Starch Foam Packing Peanuts.” ChemEd Exchange. July 28, 2021. https://www.chemedx.org/blog/microplastics-liquid-nitrogen-and-iodine-po... (accessed February, 2022).
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies