Stoichiometry in Combustion of Acetylene

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Chemical Quantities and Calculations

Alternative titles: Acetylene Combustion and Limiting Reagents

Summary

Calcium carbide reacts with water to produce acetylene gas, which can be ignited to demonstrate combustion. By varying the ratio of acetylene to oxygen in test tubes, the experiment illustrates limiting reagents, incomplete vs. complete combustion, and stoichiometric balance.

Procedure

Fill a Pyrex® dish with water.
Invert several borosilicate test tubes filled with varying amounts of water into the dish.
Add one or two pellets of calcium carbide beneath each inverted tube to generate acetylene gas.
Allow at least one tube to fill completely with acetylene and another to fill only partially.
Remove each test tube individually and invert it over a burning splint to ignite the gas.
Compare the intensity of combustion “pops” and the cleanliness of each test tube.

Links

Stoichiometry in Combustion of Acetylene - Flinn Scientific:

📄 Stoichiometry in Combustion of Acetylene - Flinn Scientific: https://www.flinnsci.com/api/library/Download/948a21160ad648c2a3c2cd9c4be36953?srsltid=AfmBOorYgCDl7DPu4wxulWAi7vhwz31ZNYwIJ1a4DHDVN6gbhsc8ozmY

Variations

Use a gas syringe to measure exact gas volumes for more quantitative stoichiometry studies.
Test the combustion with different amounts of trapped air to refine the stoichiometric 5:2 oxygen-to-acetylene ratio.

Safety Precautions

Wear safety goggles, gloves, and a chemical-resistant apron.
Perform the experiment in a well-ventilated area or fume hood.
Keep all flames away from calcium carbide until gas collection is complete.
Use only borosilicate test tubes free of cracks or chips to prevent breakage.
Handle calcium carbide carefully; it is corrosive and reacts violently with moisture to release flammable gas.

Questions to Consider

What is the balanced chemical equation for the production of acetylene from calcium carbide? (CaC2 + 2H2O → C2H2 + Ca(OH)2)
What is the balanced chemical equation for complete combustion of acetylene? (2C2H2 + 5O2 → 4CO2 + 2H2O)
Why does a test tube filled entirely with acetylene burn weakly compared to one with both acetylene and oxygen? (Oxygen is the limiting reagent; without it, only incomplete combustion occurs at the tube’s opening.)
What evidence indicates incomplete combustion? (Soot deposits and a weak, quiet flame.)
Why does the correct oxygen-to-acetylene ratio produce a loud “pop”? (It allows complete combustion, releasing maximum energy rapidly.)