Purpose
Illustrate limiting reagent by comparing incomplete combustion of hydrogen versus a stoichiometric mix of hydrogen and oxygen.
Materials
- Hydrogen balloon
- Oxygen balloon
- Hydrogen and oxygen balloon (2:1 ratio; fill with oxygen first)
- Match on a meter stick
- Ringstand with clamps
Procedure
- Setup:
- Attach one balloon at a time to the ringstand using a pinch clamp with floral wire.
- Start with the hydrogen balloon.
- Hydrogen Balloon Test:
- Announce to the class: “This is a hydrogen balloon. What are some of the properties of hydrogen?”
- Encourage students to suggest “flammable.”
- Apply the match to the hydrogen balloon.
- Expect a bang and a large flame.
- Discuss the reaction: “What is going on when something burns? It is reacting with oxygen and releasing a lot of energy.”
- Oxygen Balloon Test:
- Explain that our atmosphere contains about 20% oxygen, which isn’t enough to use up all the hydrogen.
- Introduce the second balloon (oxygen).
- Have students predict what will happen when the oxygen balloon is tested (bigger, smaller, about the same).
- Test it out.
- Discuss the result: “Well, that wasn’t very exciting, was it? Why was this disappointing?”
- Explain: There wasn’t anything for the oxygen to react with because there is only a tiny trace amount of hydrogen in the atmosphere.
- Stoichiometric Mixture:
- Discuss the need for an appropriate mixture of fuel and oxygen.
- Explain that hydrogen means “water-making” because it combines with oxygen to form water vapor (H₂O).
- Identify the correct ratio: “So what is the ratio? We’re making H₂O, so it’s 2:1. Let’s try that.”
- Note safety precautions: “PLUG YOUR EARS WITH SOMETHING; this can be very loud!”
- Final Test:
- Set off the last balloon.
- Emphasize that with the proper mixture, all of the hydrogen is consumed, resulting in a much bigger bang.
- Relate the concept to practical applications: “Ratios like this are very important for things like car engines. You need the right mix of fuel and oxygen to maximize power.”