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A Comparison of Two Chemical Reactions (2 Favorites)

LAB in Chemical Change, Reaction Rate, Chemical Change, Heat, Temperature, Exothermic & Endothermic. Last updated June 24, 2020.


Summary

In this lab, students will perform two chemical reactions, one between acetic acid and sodium bicarbonate and the other between the citric acid and the sodium bicarbonate in an Alka-Seltzer tablet when dissolved in water. Both reactions will produce gas while reacting in a closed plastic sandwich bag, causing it to inflate. Students will observe the reactions and analyze the results in order to understand indicators of chemical changes, heat flow, and factors that affect reaction rates.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
  • HS-PS1-4: Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
  • HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
  • HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Engaging in Argument from Evidence

Objectives

By the end of this laboratory experiment, students should be able to:

  • Define chemical reaction.
  • Identify simple acids and bases.
  • Observe different indicators of chemical changes.
  • Identify a reaction as endothermic or exothermic based on lab observations.
  • Explain the difference between heat energy and temperature.
  • Understand how changes in temperature, surface area, and concentration can affect reaction rates.

Chemistry Topics

This lab supports students’ understanding of:

  • Chemical reactions, specifically acid-base reactions
  • Indicators and observations of chemical changes
  • Endothermic and exothermic reactions
  • Heat energy
  • Temperature
  • Reaction rates

Time

Teacher Preparation: 25 minutes
Lesson: 60 minutes

Materials

  • 13 grams baking soda (sodium bicarbonate, NaHCO3)
  • 200 mL store bought vinegar (5% by mass acetic acid solution, HC2H3O2)
  • 200 mL water
  • 6 Alka-Seltzer tablets
  • 6 plastic sandwich bags
  • 6 small rubber bands
  • 2 weigh boats or disposable cups
  • 1- 100 mL graduated cylinder
  • 1- electronic balance
  • 1- plastic spoon or scoopula
  • 1- hot plate
  • 1- thermometer
  • 1- permanent marker
  • 1- stopwatch or cell phone timer

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • 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.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water. Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • An operational fire extinguisher should be in the classroom.

Teacher Notes

  • This lab was originally designed as a demonstration for elementary school students, Comparing Chemical Reactions. This lab is a modification, inspired by that resource, for use in the high school classroom as part of the AACT Strategic Plan and the work of the AACT Grade-Level Ambassadors.
  • The reaction of acetic acid (vinegar) and sodium bicarbonate (baking soda) produces carbon dioxide gas, water, and sodium acetate (soluble in water). The carbon dioxide gas can originally be seen as bubbles in the solution but will quickly be released from the solution as gas. The carbon dioxide gas produced will inflate the plastic sandwich bag.
  • Chemical equation: HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2
  • An Alka-Seltzer tablet contains aspirin, citric acid, and sodium bicarbonate. When it is mixed with water, citric acid reacts with sodium bicarbonate to produce sodium citrate, water, and carbon dioxide gas. The carbon dioxide gas can originally be seen as bubbles in the solution but it, too, will quickly be released from the solution. The carbon dioxide gas produced will inflate the plastic sandwich bag.
  • Chemical equation: HC6H7O7 + NaHCO3 → NaC6H7O7 + H2O + CO2
  • Depending on the academic level of students, teachers can engage students in a discussion about each of the reactions observed in this lab to help students decide what observations to report and how to answer the post-lab questions. Teachers can focus on the following topics, if they choose:
    • Indicators of a chemical reaction (gas produced, temperature changes, precipitation, and color changes)
    • Amount of reactants used
    • Amount of products formed
    • Endothermic vs. exothermic reactions
    • Factors that affect reaction rates like temperature, surface area, and concentration
  • A short video clip of this experiment can be found here.

For the Student

Lesson

Background

Acid-Base reactions are a type of chemical reaction that involve the transfer of a hydrogen ion, H+, from the acid to the base. In this lab, two acid-base reactions will be performed. First, vinegar (acetic acid) will be added to baking soda (sodium bicarbonate). Three trials will be executed for this reaction: one control, one that has an increased temperature, and one that has an increased concentration. Second, citric acid and sodium bicarbonate (both found in Alka-Seltzer tablets) will be added to water. Three trials will also be carried out for this reaction: one control, one that has an increased temperature, and one that has an increased surface area.

Some acid-base reactions are endothermic and some are exothermic. This can be determined by measuring the temperature change of the surroundings either qualitatively by using touch or quantitatively by using a thermometer.

Prelab Questions

  1. Define heat energy.
  2. Define temperature.
  3. How are heat energy and temperature related? Explain.
  4. What is an endothermic reaction? How does the container of an endothermic reaction feel?
  5. What is an exothermic reaction? How does the container of an endothermic reaction feel?
  6. What is reaction rate?
  7. What four factors can affect reaction rate, and how do they help to speed up the rate of the reaction? Explain on the particle/molecular level.

Objectives

  • To define chemical reaction.
  • To identify simple acids and bases.
  • To observe different indicators of chemical changes.
  • To identify a reaction as endothermic or exothermic based on lab observations.
  • To explain the difference between heat energy and temperature.
  • To understand how changes in temperature, surface area, and concentration can affect reaction rates.

Materials (per lab group)

  • 13 grams baking soda (sodium bicarbonate, NaHCO3)
  • 200 mL store bought vinegar (5% by mass acetic acid solution, HC2H3O2)
  • 200 mL water
  • 6 Alka-Seltzer tablets
  • 6 plastic sandwich bags
  • 6 small rubber bands
  • 2 weigh boats or disposable cups
  • 1- 100 mL graduated cylinder
  • 1- electronic balance
  • 1- plastic spoon or scoopula
  • 1- hot plate
  • 1- thermometer
  • 1- permanent marker
  • 1- stopwatch or cell phone timer

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.

Procedure

  1. With a permanent marker, label six plastic sandwich bags as #1, 2, 3, 4, 5, & 6.
  2. Measure approximately 3.00 grams of baking soda and place it into the corner of bag #1, and tie it off with a rubber band.
  3. Measure approximately 3.00 grams of baking soda and place it into the corner of bag #2, and tie it off with a rubber band.
  4. Measure approximately 5.00 grams of baking soda and place it into the corner of bag #3, and tie it off with a rubber band.
  5. Place two Alka-Seltzer tablets in the corner of bag #4, and tie it off with a rubber band.
  6. Place two Alka-Seltzer tablets in the corner of bag #5, and tie it off with a rubber band.
  7. Place two Alka-Seltzer tablets in the corner of bag #6, and tie it off with a rubber band. Break the tablets into small pieces.
  8. Measure approximately 60.0 mL of vinegar and record its temperature using a thermometer. Add the vinegar to the other corner of bag #1.
  9. Remove excess air from the bag and securely zip it shut. Use a stopwatch or cell phone timer to time the chemical reaction. Start the timer as soon as the rubber band is removed and the vinegar and baking soda mix. Shake the bag to ensure thorough mixing. Touch the bag to feel if there is a temperature change occurring. Stop the timer when the reaction is complete. Record the necessary data and observations in the data table.
  10. Measure approximately 60.0 mL of vinegar and heat it on a hot plate until its temperature reaches approximately 35-40 oC. Record its temperature and then add the warm vinegar to the other corner of bag #2. Complete the reaction using the procedure outlined in step #9.
  11. Measure approximately 60.0 mL of vinegar and record its temperature using a thermometer. Add the vinegar to the other corner of bag #3. Complete the reaction using the procedure outlined in step #9.
  12. Measure approximately 60.0 mL of water and record its temperature using a thermometer. Add the water to the other corner of bag #4. Complete the reaction using the procedure outlined in step #9.
  13. Measure approximately 60.0 mL of water and heat it on a hot plate until its temperature reaches approximately 35-40 oC. Record its temperature and then add the warm water to the other corner of bag #5. Complete the reaction using the procedure outlined in step #9.
  14. Measure approximately 60.0 mL of water and record its temperature using a thermometer. Add the water to the other corner of bag #6. Complete the reaction using the procedure outlined in step #9.Do not open or empty bags until you have made observations and comparisons of all of them.

Data and Observations

Complete the data table with observations and results collected and analyzed during the lab.

Bag # Name and Amount of Acid Name and Amount of Base Temp. of Solution Reaction Time Observations
1
2
3
4
5
6

Post-Lab and Analysis Questions

  1. Write the balanced chemical reaction that took place between the vinegar and the baking soda.
  2. Write the balanced chemical reaction that took place between the citric acid and the sodium bicarbonate when the Alka-Seltzer tablet was dissolved in water.
  3. Why was it necessary to add water to the Alka-Seltzer tablets if both reacting chemicals were already present inside the solid tablet?
  4. How did each bag feel as the reaction took place? Classify each bag reaction as endothermic or exothermic.
  5. What affect did increasing the temperature of each reaction have on the rate of the reaction? Explain what occurred on the particle/molecular level.
  6. What affect did increasing the concentration of baking soda have on the rate of the reaction? Explain what occurred on the particle/molecular level.
  7. What affect did increasing the surface area of the Alka-Seltzer tablets by breaking them up have on the rate of the reaction? Explain what occurred on the particle/molecular level.

Conclusion

An overall summary of the lab as well as concepts covered in the lab should be included. Sample questions to include in your conclusion: What did you do? What did you find? What do you think your results mean? Connect the hypothesis, procedures, data, and results together with concepts covered in the lab. Someone should be able to read your conclusion and essentially know what you did in the lab and why you did it.

Error Analysis

A minimum of three possible errors are needed. List the errors and how each one affected the outcome/results of your lab to the best of your knowledge. This is the ANALYSIS part of this section and, in grading, is the most important part. Do not include errors which are totally insignificant or that are mathematical, as these will not be accepted.