Lesson

Introduction

Airbags have been a required safety feature for cars sold in the U. S. since 1998. They have undoubtedly saved lives, especially in accidents when the driver or passengers neglected to use seatbelts. Airbags have saved many lives, but only when they deploy properly. Recently, however, millions of cars have been recalled due to defective airbags that were not properly designed or manufactured.  

Objective

Working in a group you will investigate the underlying chemistry of airbags and design, build and test your own “airbag” to see if it can protect a “passenger.” 

Part 1

This lab begins with an internet exploration of how car air bags work. Your report should answer the following questions and indicate references used. To get started, here are a few references:

• HowStuffWorks: How Air Bags Work
• National Geographic Video: Air Bags—I Didn’t Know That

Additional Suggested reading

• The New York Times: Takata Airbag Recall Faces Rising Scrutiny
• The New York Times: Air Bag Flaw Long Known Led to Recalls
• Bloomberg News: Airbag Makes in Global Crisis Used Unusual Explosive

Pre-Lab Questions

1. What is the intended purpose of an air bag?
2. How does an air bag deploy? Describe the process.
3. What is the chemistry behind an air bag? Write the balanced main chemical reaction and secondary reactions. Note: Not all air bags have the same secondary reaction.
4. What gas fills the air bag? Why was this gas chosen to use in an air bag?
5. What is the function of the secondary chemical reactions in an air bag?
6. What is the approximate volume of an air bag when it is fully inflated?
7. What happens if an air bag is under inflated? What would be the cause?
8. What happens if an air bag is over inflated? What would be the cause?
9. How long does it take for an air bag to inflate? Is this timing important? Explain.
10. Do you want to have any chemicals left over in an air bag after it inflates?  Explain.
11. To make a functional and safe air bag give at least 5 factors you would consider if you are the one designing the airbag. Which factor you think is most important? Explain.
12. Research the cause of the recent massive recalls of airbags made by Takata.
    1. What went wrong?
    2. What chemicals did they use (instead of sodium azide) to inflate the air bag?
    3. What was the chemical reaction involved?
    4. Currently automakers only recall the airbags in the high humility area. Explain why. 
13. Reflect on the importance of stoichiometry, rate of chemical reaction.

Part 2

Background

In the second part of the project you are to design and build your own airbag utilizing sodium bicarbonate (baking soda) and acetic acid (vinegar).

NaHCO_3(s) + CH₃COOH_(aq) → CO_2(g) + CH₃COONa_(aq) + H₂O_(l)

Your task is to find the correct amounts of sodium bicarbonate and acetic acid to use to create the right amount of gas (carbon dioxide) to fill the bag. If done correctly, your bag (sandwich bag 0.73 quart zip lock) should fill up, but not pop open. There also should not be any sodium bicarbonate or acetic acid left in the bag.

Before starting, look at the attached data table in which you will record the amount of acid, the amount of sodium bicarbonate, a description of how the bag inflated, and whether there was acetic acid or sodium bicarbonate left over.

Hypothesis

Which bag do you predict will produce the greatest amount of CO₂? Circle your prediction.

bag #1 - 25 mL of vinegar + 0.5 g of sodium bicarbonate
bag #2 - 25 mL of vinegar + 1.0 g of sodium bicarbonate
bag #3 - 25 mL of vinegar + 1.5 g of sodium bicarbonate
bag #4 - 25 mL of vinegar + 2.0 g of sodium bicarbonate
bag #5 - 25 mL of vinegar + 2.5 g of sodium bicarbonate
bag #6 - 25 mL of vinegar + 3.0 g of sodium bicarbonate

Procedure

1. Mass 0.5 grams of sodium bicarbonate (NaHCO₃) and record the exact mass in the data table. Carefully pour it from the weighing boat into a bag. Flatten the bag to remove any air.
2. Add 25 mL of acetic acid (vinegar) to the bag and seal the bag as quickly as possible. Start the timer. The bag should begin to inflate. The chemicals will begin to react and bubble; the gas that is produced is carbon dioxide (CO₂). The bag should begin to inflate.
3. When the bubbling stops, further mix the acetic acid and sodium bicarbonate by squishing and/or shaking the bag to make sure the reaction proceeds as far as possible. When no more bubbles are produced, stop the timer. Record how long it took for the chemicals to react.
4. The test how inflated the bag is by pinching it. Write a description and rank the fullness of the bag in the data table. Also, does the bag feel warm or cold? Make a note of that.
5. Record in the data table whether there is any sodium bicarbonate left in the bag. 
6. If all of the sodium bicarbonate seems to be gone, open the bag and add a small amount of sodium bicarbonate to see if more bubbles form. If they do, then there is still some acetic acid left in the bag. If not, then all of the acetic acid reacted. Make a note in the data table if there is acetic acid left in the bag.
7. Repeat this process by increasing the amount of sodium bicarbonate by 0.5 g until you use 3.0 g of sodium bicarbonate. When you observe the presence of sodium bicarbonate in the bag that means all of the acetic acid was used up (because there was no acetic acid left to react with it).

Data

Calculations

Assume the concentration of acetic acid is 5% and its density is 1 g/mL.

Analysis

1. Write the balanced chemical equation. What is the mole ratio of acetic acid and sodium bicarbonate? What is the molar mass of acetic acid? What is the molar mass of sodium bicarbonate?
2. Assuming acetic acid is 5% and has a density of 1g/ml, calculate how many moles of acetic acid in bags 1-6. Enter the values in the table.
   Mass of acetic acid: 25ml x (1g/1ml) x (5%) = 1.25g
3. Calculate how many moles of sodium bicarbonate there are in bags 1-6. Enter the values in the table.
4. Calculate how many moles of CO₂ formed in bags 1-6. Enter the values in the table.
5. Determine how many moles of acetic acid or sodium bicarbonate left. Which reactant is the limiting reagent? Which one is the excess reagent?
6. Calculate how many grams of sodium bicarbonate are needed to completely react with 25ml vinegar (5% acetic acid) by the stoichiometry.
7. Which bags have less sodium bicarbonate compared with the result of question #6? Do you observe any leftover of sodium bicarbonate or vinegar in these bags? Which bag has the closest amount of sodium bicarbonate compared with the result of question #6? Do you observe any leftover of sodium bicarbonate or vinegar in this bag? Which bags have more sodium bicarbonate compared with the result of question #6? Do you observe any leftover sodium bicarbonate or vinegar in these bags?
8. Which bag is the most inflated with no leftovers? Is this what you predicted in your hypotheses? How would you revise your hypotheses?
9. How are your observations of fullness of the bag compared with how much CO2 formed according to the calculations?
10. Is there any bag fully inflated? If not, explain how you would do to fully inflate the bag? Can you propose a recipe for a full bag?
11. From your observations, how long does it take to inflate the bag fully?
12. From your observations, can you determine if this reaction is endothermic or exothermic?
13. Why is this reaction a good candidate for the real car airbag? Why is this reaction not a good candidate for the real car airbag? Explain your answer. (You may want to revisit Part 1 of this project.)

Conclusion (include the following in your summary)

Part 3:

Now you will put what you found in part 2 to the test—a crash test. You will design and build a vehicle for your crash test dummy (a raw egg). The goal is to build a vehicle that will protect an egg from breaking, even when dropped from a height of three stories.

The only materials you can use are:

• an egg in a plastic bag
• two 1-quart zip lock bags
• 100 mL 5% acetic acid
• sodium bicarbonate
• tape
• a graduated cylinder
• a balance

Procedure

1. Review the difference of car air bags and your air bag and the assessment rubric before you start your work.
2. Complete your group planning steps one, two, and three. Show your work to your teacher.
3. Once your procedure is approved, carry out the reaction and inflate both bags according to your procedure.
4. Show your bags to your teacher for an examination of any leftover of reactants.
5. Assemble the inflated bag(s) and the egg (in a plastic bag) together.
6. Design your vehicle (step four) and get approval from your teacher.
7. When it’s your turn, drop your vehicle.
8. Check your passenger. Did it survive the crash? Show your vehicle to your teacher after the crash test.
9. Complete and submit your worksheet.

Planning your work

1. Look at the data from your experiment (part 2). How many grams of sodium bicarbonate and how many mL of acetic acid did you add without any excess after the reaction?
2. Did this amount of sodium bicarbonate inflate the bag fully? If not, how could you change the amounts of the reactants so the bag is full but with no reactants left over? Assume 1.0 L of CO₂ at STP will fully inflate a 1-quart bag.
3. On a separate piece of paper, write a procedure for your air bag and show it to your teacher before you proceed. 
4. Remember, your air bag is going to protect an egg as it’s dropped from three stories. Design your vehicle and draw a picture of it next to your procedure.
5. If your passenger did not survive the “crash test,” explain what you could do to improve your design. If your passenger did survive, explain why it did.
6. What is another real-life example where stoichiometry is important?

Conclusion

From part 1, you know the following reaction occurs in a real air bag:

NaN₃(s)  → Na(s) +  N₂(g)

If 65.1 L at STP of N₂ gas are needed to inflate a real air bag to the proper size, how many grams of NaN₃ must be included in the real air bag to generate this amount of N₂?