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Acid/Base Stoichiometry (3 Favorites)

LAB in Acid Base Reactions, Limiting Reactant. Last updated April 25, 2019.


Summary

In this lab, students experience a limiting reactant and can physically see the difference in amounts of product generated. They also see which reactant is in excess.

Grade Level

High school

AP Chemistry Curriculum Framework

This lab supports the following learning objectives:

  • Big Idea 3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons.
    • 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
    • 3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
  • Big Idea 4: Rates of chemical reactions are determined by details of the molecular collisions.
    • 4.1 The student is able to design and/or interpret the results of an experiment regarding the factors (i.e., temperature, concentration, surface area) that may influence the rate of a reaction.
  • Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
    • 5.10 The student can support the claim about whether a process is a chemical or physical change (or may be classified as both) based on whether the process involves changes in intramolecular versus intermolecular interactions.
    • 5.16 The student can use Le Chatelier’s principle to make qualitative predictions for systems in which coupled reactions that share a common intermediate drive formation of a product.
  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.3 The student can connect kinetics to equilibrium by using reasoning about equilibrium, such as Le Chatelier’s principle, to infer the relative rates of the forward and reverse reactions.
    • 6.13 The student can interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base.

Objectives

By the end of this lesson, students should be able to

  • Understand the concept of limiting reactant.

Chemistry Topics

This lesson supports students’ understanding of

  • Limiting reactant
  • Acids and bases
  • Stoichiometry

Time

Teacher Preparation: 30 minutes

Lesson: 1–2 hours

Materials

  • 250-mL Erlenmeyer flask (7)
  • Balloon (7)
  • Balance
  • Vinegar with universal indicator added
  • Sodium bicarbonate

Safety

  • Always wear safety goggles when working in a lab setting.
  • Wearing an apron is recommended.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

  • If you don’t have enough glassware for each group to have seven Erlenmeyer flasks, the flasks can be reused.
  • The experiment could be done as a class and each group can be responsible for collecting one or two pieces of data.

For the Student

Lesson

Background

Chemistry is like baking. Correct proportions are essential to obtain the desired product. Imagine a cake baked with too much flour, it turns out dry. And if you omit baking powder, the cake becomes hard or rubbery. Knowing how much of one material will react with a given amount of another is extremely important. This activity will give you the opportunity to both visually and quantitatively observe the phenomenon of a limiting reactant. From the quantitative data obtained, you will be able to determine the amount of the active ingredient in a sample of vinegar.

Acid/Base Reactions

If an acid and base react, hydronium and hydroxide react to create water:

H3O+ (aq) + OH- (aq) ⇾ 2 H2O (l)

This is called a neutralization reaction. This is a favorable reaction that releases a lot of energy, so if a strong acid and strong base react, the reaction can be quite dangerous.

You will explore the results of a neutralization reaction between a weak acid and base, using vinegar (acetic acid, HC2H3O2) and baking soda (sodium bicarbonate, NaHCO3, which dissociates into Na+ and HCO3- in water). When these two compounds react, they produce acetate and carbonic acid. The production of carbonic acid leads to the formation of carbon dioxide gas in a second reaction:

HCO3-(aq) + HC2H3O2(aq) ⇾ C2H3O2-+ H2CO3 (aq)

… and then H2CO3 (aq) ⇾ CO2 (g) + H2O (l)

You will explore how the pH and the products of this reaction are affected if you react different amounts of vinegar and baking soda by trapping the final product carbon dioxide in a balloon over the flask. You will also use a universal indicator, which has already been dissolved in your solutions of vinegar, to monitor the pH of the solutions before and after the reaction

Universal indicator key

red orange-yellow yellow-green blue purple

0–3 3–6 6–7 8–11 11–14

Safety

  • Always wear safety goggles when working with chemicals in a lab setting.
  • Barium iodate is a serious eye irritant and a mild skin irritant so take suitable precautions.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Procedure

PART I

  1. Measure 10 mL, 20 mL, 30 mL, 40 mL, 50 mL, 60 mL, and 70 mL of the vinegar + indicator solution and pour it into seven 250-mL flasks.
  2. Record the color and approximate pH of each solution based on the universal indicator key you created.
  3. Measure 3.0 g of baking soda into seven different balloons.
  4. Carefully fasten the balloon on the top of the flask without inverting it to avoid emptying the baking soda into the flask. Take the mass of this unreacted system.
  5. Once secure, empty the contents of the balloons into the flasks. Hold the balloon to prevent it from coming off. Make sure all of the baking soda drops into the flask.
  6. After the reactions are complete, record the color and approximate pH of each solution, based on the universal indicator key.
  7. Remeasure the mass of each reacted system.
  8. Carefully allow the gas to escape from the balloon. Reattach the balloon to the top of each flask. Remeasure the mass for each reacted system with the now-empty balloon.

PART II

  1. Pour 50 mL of the vinegar + indicator solution into seven 250-mL flasks.
  2. Record the color and approximate pH of each solution based on the universal indicator key.
  3. Measure 1.0 g 2.0 g, 3.0 g, 4.0 g, 5.0 g, 6.0 g, and 7.0 g of baking soda into seven balloons.
  4. Carefully fasten the balloon on the top of the flask without inverting it to avoid emptying the baking soda into the flask. Take the mass of this unreacted system.
  5. Once secure, empty the contents of the balloons into the flasks. Hold the balloon to prevent it from coming off. Make sure all of the baking soda drops into the flask.
  6. After the reactions are complete, record the color and approximate pH of each solution, based on the universal indicator key.
  7. Remeasure the mass of each reacted system.
  8. Carefully allow the gas to escape from the balloon. Reattach the balloon to the top of each flask. Remeasure the mass for each reacted system with the now-empty balloon.

Results/Observations

  1. For each part, create a table for the seven trials to record color and pH before and after mixing.
  2. For each part, create a table to record your before and after mass measurements.

Calculations

Show all work for at least one set of data for each part.

  1. What was the mass difference of the reaction?
  2. What was the mass difference when you released the gas in the balloon?
  3. How many moles of baking soda were added to the reaction?
  4. The vinegar solution has a concentration of 0.83 mol/L. How many moles of acetic acid were in each reaction?
  5. How many moles of carbon dioxide were evolved in each reaction?
  6. The limiting reagent is the one that runs out first and thus prevents the reaction from going any further. What is the limiting reagent in each reaction?
  7. Describe the amount of carbon dioxide evolved as you increased the volume of vinegar from 10 to 70 mL and the mass of baking soda from 1.0 g to 7.0 g. How does your answer support your answer to question six?
  8. Construct a graph of moles of vinegar vs. moles of carbon dioxide produced.
  9. Compare the moles of baking soda used and the moles of carbon dioxide evolved. For each reaction, how do they compare? Does this comparison match with the limiting reagent answers you found in question six?
  10. How do the moles of vinegar used compare to the number of moles of carbon dioxide evolved for each reaction? Does this comparison match with the limiting reagent answers you found in question six?

Analysis

  1. What did you see when the baking soda was dropped into the vinegar? Why did the balloon inflate?
  2. What must be leftover (in excess) when the pH is <7? What must be leftover (in excess) when pH is >7? Why is the pH different in each flask?
  3. If you have 1.0 g of baking soda and 10 mL of vinegar, how many moles of each do you have? Which would be the limiting reagent?

Conclusion

Answer these in paragraph form.

  • What did you learn about stoichiometry from this lab? What did you learn about limiting reagents?
  • Did you have any problems or difficulties making measurements? If so, how were these resolved? What were some sources of error and how did you minimize the error in your measurements?
  • How does this lab apply to the real world?