Complete and Incomplete Combustion
SNC 2D
Introduction
Acetylene (C2H2) can be produced by the reaction of calcium carbide (CaC 2) 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)

by-product
+
C2H2(g)
acetylene gas
complete combustion of acetylene:
C2H2(g)
+
O2(g)
Which of the ratios (full, ½-full, ¼-full, or
CO2(g)

1
12
+
H2O(g)
-full of acetylene) will achieve complete combustion?
Write your predictions in your table of observations.
Equipment and Materials



600 mL (large) beaker
4 medium-sized test tubes
Calcium carbide (CaC2) rocks


Wooden splints
Candle and match
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.
2. Drop a small lump of calcium carbide into the beaker and observe the reaction.
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Acetylene Gas
3. 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).
Calcium
carbide
Fig. 1
4. Half-fill a 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
1 -full of acetylene.
test tube 12
5. 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.
6. Repeat the previous step, but SHAKE the test tube before removing your thumb.
7. Clean the test tubes using a test tube brush.
Questions
1.
Write the balanced chemical equations for the:
a. production of acetylene from the reaction of calcium carbide with water;
b. complete combustion of acetylene;
c. incomplete combustion of acetylene in which only carbon and water vapour are produced.
2. Compare the way in which the acetylene burned in each of the four test tubes.
3. What evidence from your reaction is there that the combustion in the test tube containing
the smallest quantity of acetylene is complete?
4. 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?
5. 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
6. 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.)
7. Why do you think pure oxygen rather than air is used in an oxyacetylene torch?
\SNC2D\chem\106751865.doc
Complete and Incomplete Combustion
SNC 2D
OBSERVATIONS
Amount of
Acetylene in
test tube
Full
½ full
¼ full
1
12
-full
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Prediction
Rating of Combustion
1 – least
4 - most
Experimental
Rating of Combustion
1 – least
4 - most
Observations of Combustion