SCH3U
Combustion of Acetylene
NAME:_______________
Materials Required:
 Large granules of Calcium Carbide
 Scoop or spatula to handle the calcium carbide
 3 large test tubes of similar size with stoppers (held in large empty beaker)
 Wooden splint and lit candle
 Large beaker (500mL to 1L) half full of water
Pre-lab Exercise:
1. In this lab we will produce acetylene by placing calcium carbide (CaC2) in
water. Write the chemical formula for the reaction between calcium carbide
and water knowing that the product is acetylene (C2H2).
2. The acetylene in this lab will be collected in a test tube and burned. Write
the balanced equation for the combustion of acetylene.
3. Look at the balanced equation. Note the coefficients of the reactants. What
proportion of oxygen molecules are required for the most effective burning
of acetylene.
Procedure Summary and Important INFO:
The beaker is filled with water. A calcium carbide granule is dropped in the water in
the container and bubbles of acetylene gas begin to be produced. If a test tube full
of water is inverted and placed over the evolving gas the gas displaces the water
and the entire test tube fills with acetylene. The test tube can be stoppered to
retain the gas. Evidence of any black soot produced indicates that there is not
enough oxygen present for complete combustion to occur. If the test tube is pulled
out before the water is fully displaced we can get a test tube partially filled with
acetylene as oxygen will rush in to replace the water.
In the steps that follow you will be filling 3 test tubes with different amounts
of acetylene.
1
Diagram of gas collection:
Test tube inverted in
beaker filled with tap
water
Bubbles of
acetylene gas
filling test tube
and forcing
water out
Large granule of
calcium carbide
Beaker full of tap
water
Procedure
1. Fill a test tube with water and invert into the large beaker. Try to minimize
air bubbles at the top of the test tube. Your teacher can show you a trick!
2. Drop a piece of calcium carbide into the beaker and place the mouth of the
test tube over the calcium carbide in order to trap the gas formed.
3. Completely fill the first test tube with the acetylene gas then remove from
the beaker and stopper IMMEDIATELY! LABEL the testube #1 then place in
empty beaker to store for later.
4. Repeat steps 1 and 2 with a second test tube but only fill about half way
with acetylene gas. The excess water will drain out when the test tube is
removed from beaker, stopper IMMEDIATELY. Label and store for later.
5. Repeat steps 1 and 2 with a third test tube but only fill about one quarter of
the way with acetylene gas. The excess water will drain out when the test
tube is removed from beaker, stopper IMMEDIATELY. Label and store for
later
STOP
At this point you should have 3 test tubes with varying amounts of acetylene sitting
in your test tube rack.
2
Making Some Predictions:
1. Which of the 3 test tubes do you think will burn the best/fastest when
ignited? Why do you think this?
2. Whish of the 3 test tubes do you think will burn the poorest/slowest when
ignited? Why do you think this?
3. Use the chart below to rank your predictions (1- most reactive/fastest, 3least reactive/slowest)
Burning the Acetylene:
Test tube
Acetylene
Amount
Predicted Reactivity
1 (fastest) –
3 (slowest)
Actual Reactivity
1 (fastest) –
3 (slowest)
Observations of Burning
FULL
Test tube
#1
½ Full
Test tube
#2
¼ Full
Test tube
#3
4. Once you have made your prediction ignite each of the test tubes and record
observations pertaining to their burning rate and any evidence of complete
or incomplete combustion.
3
Further considerations:
1. Which acetylene sample burned the best? Why do you suggest that is the
case? Two things are necessary for you to answer this question.
First consider the balanced chemical equation
2 C2H2(g) + 5 O2(g)  4 CO2(g) + 2 H2O(l)
Second consider that air is only about 20% oxygen.
2. Which acetylene sample burned the slowest/worst? Why do you suggest that
is the case?
4