SNC 2DComplete and Incomplete Combustion

Introduction

Acetylene (C2H2) can be produced by the reaction of calcium carbide (CaC2) with water. In this experiment you will be collecting different volumes of acetylene in separate test tubes and mixing these samples with air. By examining how each mixture burns, you will be able to observe the effect that the ratio of fuel to air has on combustion.

Background

Complete combustion of a hydrocarbon requires excess oxygen and produces only carbon dioxide and water. Incomplete combustion produces carbon monoxide or carbon along with water.

Problem

How does the ratio of fuel to air affect the completeness of the combustion of acetylene?

Hypothesis

Balance the following reactions

production of acetylene:

CaC2(s) + H2O(l)  Ca(OH)2(s) + C2H2(g)

by-product acetylene gas

complete combustion of acetylene:

C2H2(g) + O2(g)  CO2(g) + H2O(g)

Which of the ratios (full, ½-full, ¼-full, or -full of acetylene) will achieve complete combustion? Write your predictions in your table of observations.

Equipment and Materials

\SNC2D\chem\incomp comp combustion.doc.doc

  • 600 mL (large) beaker
  • 4 medium-sized test tubes
  • Calcium carbide (CaC2) rocks
  • Wooden splints
  • Candle and match

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Procedure(see reverse for diagram)

  1. Half-fill a large beaker with water. Fill four medium test tubes with water and invert them in the water in the beaker.
  1. Drop a small lump of calcium carbide into the beaker and observe the reaction.
  1. Place one of the inverted test tubes over the lump of calcium carbide and fill the test tube with acetylene. Slowly lift the test tube out of the beaker and, keeping it mouth downward, place it on the top of the lab bench (see Fig. 1).
  1. Half-filla second test tube with acetylene. When the desired volume of gas has been collected, lift the test tube straight up and hold it mouth downward over the beaker until the remaining water has drained out. Keeping it inverted, place the test tube on the bench. In the same manner, collect a third test tube ¼-full and a fourth test tube -full of acetylene.
  1. Placing your thumb tightly over the mouth of the first test tube, hold it horizontally at arm’s length. Remove your thumb and bring a burning splint to the mouth of the test tube. Observe the flame produced, the duration of the burning, and any visible combustion products.
  1. Repeat the previous step, but SHAKE the test tube before removing your thumb.
  1. Clean the test tubes using a test tube brush.

Questions

  1. Write the balanced chemical equations for the:
  2. production of acetylene from the reaction of calcium carbide with water;
  3. complete combustion of acetylene;
  4. incomplete combustion of acetylene in which only carbon and water vapour are produced.
  1. Compare the way in which the acetylene burned in each of the four test tubes.
  1. What evidence from your reaction is there that the combustion in the test tube containing the smallest quantity of acetylene is complete?
  1. Why does the acetylene burn so poorly in the test tube completely filled with acetylene? What evidence is there to indicate that the combustion is incomplete?
  1. The test tube that was half-full of acetylene did not combust perfectly either, even though half the test tube was full of air. Use the balanced chemical equation from the hypothesis to explain this observation.

Extend

  1. Only about 20% of air is oxygen gas. How does this affect the air to acetylene gas ratio needed for complete combustion? (HINT: note that the balanced chemical equation for combustion involves O2 and NOT air.)
  1. Why do you think pure oxygen rather than air is used in an oxyacetylene torch?

SNC 2DComplete and Incomplete Combustion

OBSERVATIONS

Amount of Acetylene in test tube / Prediction
Rating of Combustion
1 – least
4 - most / Experimental
Rating of Combustion
1 – least
4 - most / Observations of Combustion
Full
½ full
¼ full
-full

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