Lesson

PART 1

Prelab Questions
This project begins with an internet exploration of how car air bag works. Answer the following question. Here are some references you may use. At the end, indicate which of these references you used in addition to any other references.

• How Airbags Work
• Airbag Chemistry
• Car, Airbag, Money: Building an Airbag
• I Didn’t Know That: Air Bags

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. What five factors would you consider if you had to design a functional and safe air bag? Which factor do you think is most important? Explain.

References:

PART 2: The Air Bag Stoichiometry Project

Background

Create your own air bag technology using sodium bicarbonate and acetic acid.

Find the right amounts of sodium bicarbonate and acetic acid needed to fill the bag. “The right amount” means your bag, a 1-quart zip lock, should fill up and not pop open, and no sodium bicarbonate or acetic acid should be left in the bag.

Before starting, look at the data table. You will record the amounts of acetic acid and sodium bicarbonate, a description of how the bag inflated, what chemical was in excess, and the amount of time it took for the reaction to take place.

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 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 vinegar to the bag and seal the bag as quickly as possible. Start the timer. 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. 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. Record in the data table the excess reactant.
7. Repeat this process by increasing the amount of sodium bicarbonate by 0.5 g until you use 3.0 g of sodium bicarbonate.

Data

Calculations

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

Analysis

1. Write the balanced chemical equation between acetic acid and sodium bicarbonate. What is the mole ratio of acetic acid and sodium bicarbonate?
2. Calculate how many grams of sodium bicarbonate are needed to completely react with 25 mL of vinegar (5% acetic acid).
3. Which bags have less sodium bicarbonate than your calculation in question two? Did you observe any leftover sodium bicarbonate or vinegar in these bags?
4. Which bag has the closest amount of sodium bicarbonate you calculated in question two? Did you observe any leftover of sodium bicarbonate or vinegar in that bag?
5. Which bags have more sodium bicarbonate than your calculation in question two? Did you observe any leftover sodium bicarbonate or vinegar in these bags?
6. Which bag is most inflated with no leftovers? Is this what you predicted in your hypotheses? How would you revise your hypotheses?
7. How are your observations of fullness of the bag compared with how much CO₂ formed according to the calculations?
8. Did any bag fully inflate? If not, explain what you would do to fully inflate the bag. Can you propose a procedure to make a full bag?
9. From your observations, how long does it take to fully inflate the bag?
10. From your observations, can you determine whether this reaction is endothermic or exothermic? Explain.
11. Why is this reaction a good candidate for the real car air bag? Why is this reaction not a good candidate for the real car air bag? Explain your answer. You can revisit Part 1.

Conclusion

PART 3: The Crash Test

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, which is 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

Your bags should fully inflate and not have any acetic acid or sodium bicarbonate left over.

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. One 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.
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?