Stoichiometry Balloon Races

Introduction

Most stoichiometry calculations in the classroom are performed using exact (stoichiometric) mole ratios of reactants and products. In real life, however, many commercial processes for preparing compounds are carried out using an excess amount of one reactant (and thus a limiting amount of another reactant). This demonstration uses the well-known reaction of sodium bicarbonate and acetic acid to illustrate the concepts of limiting and excess reactants. By comparing the amount of carbon dioxide generated when varying amounts of sodium bicarbonate react with a given amount of acetic acid, students will be able to immediately identify the limiting and excess reactant in each case.

Concepts

• Stoichiometry
• Limiting reactant
• Mole ratio
• Excess reactant

Materials

Safety Precautions

Acetic acid is a skin and eye irritant. Avoid contact with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations, that may apply, before proceeding. All of the waste solutions may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Label six Erlenmeyer flasks 1–6. Using a graduated cylinder, add 10 mL of 2 M acetic acid to each flask.
2. Obtain six weighing dishes and label them 1–6.
3. Measure the appropriate amount of sodium bicarbonate into each weighing dish, according to Table 1.
   {13892_Preparation_Table_1}
4. Obtain six balloons. Stretch the balloons and blow them up at least once. Then let out as much air as possible.
5. Use a powder funnel to add the first sodium bicarbonate sample (1) to one of the balloons.
6. Flatten out the balloon to remove any extra air and then carefully stretch the neck of the balloon over the mouth of Erlenmeyer flask 1. Do not allow the solid to drop into the flask at this time.
7. Repeat steps 5 and 6 with the other sodium bicarbonate samples 2–6.

Procedure

1. Introduce the concepts of limiting and excess reagents by asking students how many complete automobiles can be assembled if the following parts are available: 140 car bodies, 520 tires, 200 engines, and 270 headlights.
2. Which automobile part is present in a quantity that “limits” the total number of complete automobiles that may be assembled? Is the limiting part the same as the part that is present in the least number? Explain. Note: The tires are the limiting parts in this example, even though there are more tires than anything else. There are also not enough headlights for all the car bodies.
3. Show students the balloon/flask assemblies and ask them to predict what will happen when the sodium bicarbonate is added to the acetic acid in the flask. Write the reaction equation on the board.
4. Line up flasks 1–6 from right to left on the lecture desk. Lift each balloon in turn and shake it to allow the solid to fall into the solution. Make sure the neck of each balloon stays firmly attached to the flask. 
5. The reactions will be immediate and vigorous. The white solids will dissolve, the solutions will start to bubble and fizz, and the balloons will become inflated.
6. Allow the reactions to proceed until the bubbling stops. Swirl the flasks, if necessary. Compare the size of the inflated balloons and whether all the solid has dissolved in each case. (The balloon size should increase fairly uniformly for flasks 1–4, and then stay constant. It may be hard to tell the difference between flasks 3, 4, 5 and 6.)
7. Discuss the observations and carry out the necessary calculations to explain the results (see Table 2). Identify the limiting reactant and the excess reactant in each case.
   {13892_Procedure_Table_2}

Student Worksheet PDF

13892_Student1.pdf

Teacher Tips

• Enough materials have been included in this kit to perform the demonstration at least seven times.
• Use new balloons each time the demonstration is performed. Once used, the balloons will become stretched, and their inflated sizes may vary considerably from the expected size.
• Unreacted (undissolved) sodium bicarbonate will be visible in flasks 4, 5 and 6. The solid, although normally water soluble, does not dissolve in the final solution.
• Add a few drops of universal indicator to each flask and observe the color changes as the pH of the solution changes over the course of the reaction.
• Excess reactants are used commercially in cases where reactions are reversible and thermodynamically unfavorable. An example of a gas-phase inorganic reaction is the synthesis of ammonia from nitrogen and hydrogen. Excess nitrogen is used to drive the reaction to completion. The synthesis of organic esters from organic alcohols and acids is another example of commercial processes that are normally carried out in the presence of excess reactants.
• Carry out the demonstration using sodium carbonate instead of sodium bicarbonate. The maximum amount of CO₂ evolution will be observed at a lower mass of sodium carbonate, due to the 2:1 mole ratio for reaction of acetic acid with sodium carbonate.
• This reaction also works well using 20–25 mL of 1 M acetic acid.

Sample Data

Answers to Questions

1. Calculate the number of moles of sodium bicarbonate that were present in each flask. Use the space below to work out the answer. Record your answer in the data table.

   Student work will vary. Answers are in the data table.

2. Write a balanced chemical equation for the reaction between sodium bicarbonate and acetic acid. Use the equation to determine the ideal mole ration for the reaction.

   NaHCO₃(s) + CH₃CO₂H(aq) → NaCH₃CO₂(aq) + CO₂(g) + H₂O(l)

3. Decide which chemical was the limiting reagent, and therefore how many moles of carbon dioxide were produced, in each flask. Record your answers in the data rable.

   Answers are in the data rable.

Discussion

Equation 1 summarizes the reaction between sodium bicarbonate and acetic acid.

References

This activity is from Flinn ChemTopic™ Labs, Volume 7, Molar Relationships and Stoichiometry. Cesa, I., Ed., Flinn Scientific; Batavia, IL, 2002.