Limiting Reagent

Description

Equal numbers of moles of acid are placed into each of three vessels. Different amounts of sodium bicarbonate are reacted with the acid. In the middle vessel, equal numbers of moles of acid and bicarbonate are present. In the first vessel there is excess acid; in the third vessel there is excess bicarbonate. By capturing the gas released in balloons, students can see differences in the number of moles. Mixing unreacted materials confirms the presence of excess chemicals.

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Eye protection is required.

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• Acetic acid reacts with sodium bicarbonate according to the reaction:

  CH₃COOH + NaHCO₃ --> NaCH₃COO + CO₂ + H₂O

• According to the balanced equation, one mole of acid reacts with one mole of sodium bicarbonate.

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1. Weigh three portions of sodium bicarbonate: A-2.0 g; B-3.5 g; and C-5.0 g.
2. Transfer each portion to a balloon using a powder funnel.
3. Use a marking pen to mark the balloon with the mass of sodium bicarbonate added.
4.  Place 50 mL of vinegar into each of 3 250-mL Erlenmeyer flasks.
5. Attach a balloon contained a preweighed portion of sodium bicarbonate to each of the three flasks.
6. Lift and jostle each balloon so as to add the sodium bicarbonate to each flask. Observe the result.
7. Note the apparent amount of gas produced.
8. Note the appearance of the material inside each flask.
9. Set the middle flask aside. Remove the balloons from the remaining flasks.
10. Mix their contents. Note any evidence for reaction.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 024 Limiting Reagent

1. Acetic acid reacts with sodium bicarbonate according to the reaction:

   CH₃COOH + NaHCO₃ --> NaCH₃COO + CO₂ + H₂O

   According to the balanced equation, one mole of acid reacts with one mole of sodium bicarbonate.

2. Which balloon will inflate the most? See the movies in the procedure section of this experiment.
3. Which substance is the limiting reagent in each balloon?

   A (left side)

   B (middle)

   C (right side)

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Purpose

To demonstrate some consequences of a limiting reagent.

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(for 10 students working in pairs)

• 50-mL graduated cylinder
• 3 250-mL Erlenmeyer flask
• 3 large balloon
• 150 mL vinegar
• triple beam balance
• weighing paper
• powder funnel
• 15 g sodium bicarbonate
• scoopula

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Prepare the balloons filled with sodium bicarbonate in advance. Write the mass of sodium bicarbonate on the balloon.

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Teacher preparation: 40 minutes

Presentation: 15 minutes

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Balloons are filled with gas under pressure. They may detach from the flasks and fly around.

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Eye protection is required. Check that the eye wash fountains are in working order.

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The materials used in this experiment may be disposed of safely at the sink.

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Presentation Question:

• Which balloon will inflate the most? (Ask this before the demonstration is performed. Then ask students to explain the outcome.)
  • The sample of vinegar is just enough to react with 3.5 g of sodium bicarbonate. In the third flask, there is 1.5 g of sodium bicarbonate in excess. There is excess vinegar in the first flask. Therefore, second and third balloons should be about the same size.

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• mole
• mole relationship
• formula
• equation
• balanced equation
• limiting reagent
• excess

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