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Introducing Limiting Reactants (3 Favorites)

DEMONSTRATION in Conservation of Mass, Limiting Reactant, Chemical Change. Last updated May 30, 2017.


Summary

In this demonstration, the teacher will perform a series of reactions between acetic acid (vinegar) and varying amounts of sodium bicarbonate (baking soda) in order to inflate several Ziploc bags. Students will observe the reactions and analyze the quantities of reactants used as well as the results in order to understand the concept of limiting reactants. Students will also determine if the reaction is an endothermic or exothermic process based on their observations.

Grade Level

High school

Objectives

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

  • Define limiting reactant.
  • Understand the implications of a limiting reactant in a chemical reaction.
  • Identify an excess reactant in a chemical reaction.
  • Identify a reaction as endothermic or exothermic based on lab observations.

Chemistry Topics

This lesson supports students’ understanding of

  • Chemical reactions
  • Stoichiometry
  • Limiting Reactant
  • Observations
  • Endothermic and Exothermic reactions

Time

Teacher Preparation: 20 minutes

Lesson: 30 minutes

MaterialsDemo introducinglimitingreactants materials

  • 1.0g, 2.0g, 3.0g, 4.0g, 5.0g baking soda (sodium bicarbonate, NaHCO3)
  • 210.ml store bought vinegar (5% by mass acetic acid solution, HC2H3O2) divided into 5 portions of 42.0ml
  • 5 empty quart-size or sandwich Ziploc bags
  • Electronic scale
  • 50ml Graduated cylinder
  • Scoopula

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
  • Students should wash their hands thoroughly before leaving the lab.
  • Sodium Bicarbonate (Baking Soda) SDS
  • Vinegar SDS

Teacher Notes

  • The reaction of sodium bicarbonate (baking soda) and acetic acid (vinegar) 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. The amount of carbon dioxide gas produced will inflate the Ziploc bag.
  • Chemical Equation:
    1 NaHCO3 + 1 HC2H3O2 → 1 NaC2H3O2 + 1 H2O + 1 CO2
  • The quantities of reactants used for the balloons and bottles are shown below:

 

Amount of Vinegar Amount of Baking Soda
Ziploc Bag 1 42.0 ml ≈1.0 g
Ziploc Bag 2 42.0 ml ≈2.0 g
Ziploc Bag 3 42.0 ml ≈3.0 g
Ziploc Bag 4 42.0 ml ≈4.0 g
Ziploc Bag 5 42.0 ml ≈5.0 g
  • Note: You can use 1.0M Acetic acid instead of vinegar. Change the volume to 35.0 ml for each bag.Demo introducinglimitingreactants teachernotes1
  • Ziploc 1 will produce a reaction that has excess vinegar, and all of the baking soda will react completely. This will produce the smallest amount of carbon dioxide, thus the smallest bag of the five. The solution in the bag will be clear; indicating all of the baking soda reacted. The bag will be cool to the touch indicating an endothermic reaction.
  • Ziploc 2 will also produce a reaction that has excess vinegar, and all of the baking soda will react completely. Demo introducinglimitingreactants teachernotes2This will produce the slightly more carbon dioxide than the first bag, thus the second smallest bag of the five. The solution in the bag will be clear; indicating all of the baking soda reacted. The bag will be cool to the touch indicating an endothermic reaction.


  • Ziploc 3 has an equal molar ratio of vinegar and baking soda. The bag formed will be larger than the first two, Demo introducinglimitingreactants teachernotes3and the solution in the bag will be clear, indicating all of the baking soda reacted. The bag will be cool to the touch indicating an endothermic reaction.



  • Ziploc 4 has excess baking soda. The bag formed will be the same size as Ziploc 3 and the resulting solution in Demo introducinglimitingreactants teachernotes4the bag will be cloudy, indicating an excess of baking soda that did not react. The bag will be cool to the touch indicating an endothermic reaction.



  • Ziploc 5 also has excess baking soda. The bag formed will be the same size as Ziploc 3 and 4 and the solution Demo introducinglimitingreactants teachernotes5in the bag will be cloudy, indicating an excess of baking soda that did not react. The bag will be cool to the touch indicating an endothermic reaction.
  • Teachers should engage students in a discussion about each of the reactions observed in this demonstration and focus on:
    • Limiting Reactants
    • Excess Reactants
    • Indicators of a Chemical Reaction (gas produced, heat absorbed)

Instructions

  1. Label 5 bags with a sharpie as: 1, 2, 3, 4, 5. Demo introducinglimitingreactants teachernotes6
  2. Measure the baking soda into the Ziploc bags, placing it in the corner of each bag, as shown on the right.
    1. ≈1.0g in Ziploc 1
    2. ≈2.0g in Ziploc 2
    3. ≈3.0g in Ziploc 3
    4. ≈4.0g in Ziploc 4
    5. ≈5.0g in Ziploc 5
  1. Measure 42.0ml of vinegar and add the vinegar to the other corner of Ziploc bag 1. Remove excess air from Demo introducinglimitingreactants teachernotes7bag and securely zip it shut. You may want to have a student assist with this step. Hold the bag as shown on the right and have the student pour the vinegar into the other corner and zip the bag shut.
  2. Shake the bag to mix the baking soda and vinegar. When bubbling stops make observations about the contents and temperature of the bag.
  3. Repeat this measurement with each of the remaining bags, making observations and comparisons to previous bags after each.
  4. Do not open or empty bags until you have made observations and comparisons of all of them.

Opportunities for extension

This demonstration can also be used as a student lab activity. You can also have students do supporting calculations if they measure and record the exact mass of baking soda and volume of vinegar used for each bag. See sample data table below. Some information they will need:

  • The density of household vinegar is 1.01 g/ml.
  • There are 5.00 grams of acetic acid in 100. grams of vinegar.
Bag Exact Mass Baking Soda  (g) Volume of Vinegar (mL) Observations:  Relative size? Temperature change? What does the final mixture look like?

1

 

42.0 mL

 

2

 

42.0 mL

 

3

 

42.0 mL

 

4

 

42.0 mL

 

5

 

42.0 mL