Sharing chemistry with the community: Limiting and excess reagents

The determination of the limiting and excess reagents in a chemical reaction tends to be a challenge for many chemistry students. Teachers have used the making of cookies and hamburgers, or combinations of nuts and bolts, among other common items to help students understand these concepts. While students understand that if you have 10 hamburger patties and only 6 pickles one can only make 3 hamburgers, if each requires 2 pickles. They have difficulty transferring this idea to chemical reactions. Their difficulties, in part, can be attributed to the fact that these concepts are expressed in symbolic and mathematical expressions, and require an understanding from the molecular viewpoint.

This demonstration uses the simple reaction between vinegar and baking soda to provide observational evidence of limiting and excess reagents. Students can see that the limiting and excess reagents depend upon the amounts of each reactant.

The reaction:

CH₃COOH   +   NaHCO₃   →   CO₂   +  H₂O  +  CH₃COONa

Five flasks, each containing 70 mL of vinegar, have balloons attached which contain 2.0 g, 4.0 g, 6.0 g, 8.0 g and 10.0 g of baking soda consecutively (Fig. 1).

The balloons are raised to allow the baking soda to fall in and react with the vinegar. The students note the size of each balloon (see Fig. 2) and the bottoms of each flask to see whether baking soda remains or not. The addition of bromothymol blue allows one (Fig. 3) to see if the solution is acidic or basic, and confirm predictions by adding more baking soda or vinegar to the flasks. The students are guided through a series of calculations to confirm their observations for the flask with 2.0 g of baking soda added — see Table 1 below. Then the students confirm their observations for the other four flasks by completing similar calculations. Lastly, they determine the stoichiometric amount of each reactant necessary for neither to remain when the reaction reaches completion.

This demonstration has several advantages. Except for the bromothymol blue*, only common, inexpensive and readily available household items are used. Other acid-base indicators found in most chemistry labs could be substituted. The experiment is easy to perform, providing a variety of observations to support the concepts of limiting and excess reagents. If enough equipment is available, each student can perform this safe, hands-on activity. There are two disadvantages. First, you will need to find balloons that have equivalent stretch to yield appropriate volumes depending upon the amount of gas generated. Second, the reactants in this demonstration have a one to one ratio. Are students able to apply their understanding gained to other reactant ratios? As a demonstrator, I have not investigated this.

Limiting and excess reagents

Concepts

• Limiting reagent and excess reagent(s)
• Stoichiometric quantities of reagents
• Stoichiometric calculations (assuming 100% reaction), such as determination of limiting reagent, amount of excess reagent (amount reacted, amount remaining), amounts of products

Materials

• Five 125 mL Erlenmeyer flasks
• Five 12” helium quality latex rubber balloons (same color)
• 100 mL graduated cylinder
• ~400 mL white vinegar (~5% acetic acid; this demo, the concentration was calculated to be 0.83 mol/L)
• Balance, 150 mL beaker and plastic funnel for weighing and transferring solids
• ~50 g pure baking soda (sodium hydrogen carbonate)
• Permanent marker
• Crystallization dish or large beaker
• Two 100 mL beakers
• ~50 mL of water
• Bromothymol blue indicator solution and dropper

Table 1: Completed table for 2.0 g NaHCO3 with 70 mL of vinegar

Fig. 1. Left to right: NaHCO₃ (2.0 g, 4.0 g, 6.0 g, 8.0 g, 10.0 g) in the balloons with all flasks with 70 mL of vinegar.

Safety

• Although the chemicals used are not harmful one should model good practices. Wear protective gloves and eyewear. Avoid ingestion, inhalation or contact with bromothymol blue.

Advance preparation

• Fill each 125 mL Erlenmeyer flask with 70 mL of vinegar. Label each flask respectively, 2 g, 4 g, 6 g, 8 g and 10 g.
• Measure about 2.0, 4.0, 6.0, 8.0, 10.0 g of baking soda into separate balloons. I do this by attaching a balloon to the plastic funnel and standing in the 150 mL beaker. Place the entire setup on the balance, tare and weigh baking soda.
• Carefully attach each balloon to the designated flask being careful not to spill any baking soda into the vinegar. Be sure to squeeze out any air in the balloon before fully attaching. Arrange the flasks in consecutive order.

Fig. 2. Left to right: NaHCO₃ (2.0 g, 4.0 g, 6.0 g, 8.0 g, 10.0 g) added to all flasks with 70 mL of vinegar. Students can see the first balloon is the smallest. The second balloon is bigger. All of the three balloons on right have a larger and similar volume.  

Procedure

• Demonstrate the reaction between baking soda and vinegar by placing a small amount of baking soda in the crystallization dish and adding a few milliliters of vinegar.
• Show and discuss the chemical reaction.
• Explain the setup of the five flasks, pointing out that all contains 70 mL of vinegar, and that there are increasing amounts of baking soda in the balloons.
• Holding onto the balloon where it is attached to the flask, raise the balloon containing 2.0 g of baking soda so that all of the baking soda will fall into the vinegar. Allow it to completely react.
• Repeat the above process in order for each flask.
• Compare the volumes of each balloon and examine the bottoms of each flask for excess baking soda.
• Show the color of bromothymol blue in vinegar and in a water solution of baking soda — yellow and blue, respectively.
• Remove the balloons from each flask and add bromothymol blue (about 15 drops) to each. Add baking soda to the first flask that originally had 2.0 of baking soda. That a reaction occurs shows that the baking soda was the limiting reactant.
• Add vinegar to the last flask that originally had 10.0 g of baking soda. That a reaction occurs shows that the vinegar was the limiting reactant.
• Show the mathematic calculations to determine the limiting and excess reactants, and the amount of each product formed — assuming 100% reaction — for the 2.0 g of baking soda reacting with the 70 mL of vinegar. Use the results to explain the physical observations.
• Have students repeat the mathematical calculations for
  4.0 g, 6.0 g, 8.0 g and 10.0 g, and use results to explain the physical observations.
• Have students determine the stoichiometric amounts of reactants so that neither reactant remains at the end of the reaction.

Fig. 3. Left to right: bromothymol blue added to NaHCO₃ (2.0 g, 4.0 g, 6.0 g, 8.0 g, 10.0 g) added to all flasks with 70 mL of vinegar. The solutions in the first two flasks (left) are yellow; the third is light green; and the last two are bluish-green. The beakers on each end show bromothymol blue in vinegar (far left, yellow), and in water plus baking soda (far right, blue).

Observations

As shown in Fig. 2, the second balloon on right (4.0 g NaHCO₃) is about double the volume of the balloon on the first flask (2.0 g NaHCO₃). The balloon on the third flask (6.0 g NaHCO₃) is larger but not quite triple the size of first balloon. The balloons on the last two flasks (8.0 g, 10.0 g NaHCO₃) are about the same size as the one on the third flask (6.0 g NaHCO₃). No left over baking soda in the first two flasks was observed whereas there was baking soda observed in the last three flasks in consecutively increasing amounts. When there was no evidence of baking soda; this means it was used up — limiting reagent. When there was evidence of leftover baking soda, students could clearly see it was the reagent in excess.

As shown in Fig. 3, a yellow color is produced when bromothymol blue is added to the first two flasks. This indicates the vinegar is in excess as the solution is acidic. The third flask is a lighter green (neutral, pH 6.0 -7.6). The last two have a darker bluish-green (pH greater than 7.6); this would indicate the baking soda is in excess.

Clean up

• All mixtures can be disposed of in the sink. Check local regulations before doing so.

Notes

[* Bromothymol blue can be purchased at pet stores. The indicator is used to check pH of an aquarium.]

Dr. Kenneth Lyle is a lecturing-fellow in the Department of Chemistry at Duke University NC. The Powell Family Trust, the Duke-Durham Neighborhood Partnership, and Biogen Idec – Research Triangle Park fund the Duke Chemistry Outreach Program.