|
-111 -111  -1111 -
| E
111--------11111.
Set-up |
---112.
7 ml water + 29
mg sample |
113.11
Volumetric gas generation |
11...........14.
bBurning
ethyne1 |
 1
Weighing
samples, measuring the generated gas volumes, combustion
The calcium grain
in a previous experiment is a pure metallic
element that was weighed, beaten and cut into pieces of equal size and
weight. The calcium carbide used for the following experiment is a brittle
technical product. Its maximal purity is 80 %. The grains very much differ
in their concentrations. It starts to decrease as soon as the can (Photo
1) is first opened [Obendrauf, V., Grubenlampe u. Wühlmausgas Chemie &
Schule | Salzburg | 23 | 2008 Nr. 1, 5 - 13].
Material
Tray,
container for waste, goggles, pincers,
digital pocket scale (VOLTCRAFT
High-Precision Pocket Scale PS-20), gloves, ampoule 5 ml (high), syringe
(5ml), black permanent marker, sticky
tape, aluminium beaker of a tea light, candle, toilette paper, grains
of calcium carbide in a film canister.
Procedure
1. Transform the ampoule
into a measuring container: Add 1-ml-samples of water from a syringe.
Mark each of the water surfaces. Protect the graduation by sticky tape.
2. Photo 1:
Weigh a piece of calcium carbide in the size of a rice grain. (Attention:
Keep the calcium carbide dry!).
3. Photo 2:
Transfer the sample into a beaker. Totally fill the ampoule with water.
Carefully turn it upside down.
4. Photo 3:
Place the ampoule full of water with its mouth on
the grain of calcium carbide. Wait until the reaction is over. Measure
the gas volume.
5.
Photo 4 :
Turn
the ampulle upwards into a diagonal position. Lit the gas.
Observations
a)
The
calcium carbide pieces react with water releasing gas bubbles that replace a certain volume of water
in the ampoule. Photo 1 - 3:
A sample of 29 mg releases 3.3
ml gas. b) A white precipitate is formed (Photo 3).
c) The gas burns with a blue flame for a few seconds
(Photo 4).
Explanation
a)
During
the reaction CaC2
+ 2 H2O
--> C2H2
+ Ca(OH)2 / 1 mmol (64 mg) of CaC2
release 1 mmol (24 ml C2H2)
at room temperature. In sample 1 the expected volume
of C2H2 / 29 mg of pure CaC2
is 10.9 ml C2H2.
This means that sample 1 is not pure at all: Its concentration
of CaC2
is only 30 %. The following table shows the results of more measurements.
| Sample |
Mass
of sample
|
Volume
of C2H2
released
|
Volume
of
C2H2
expected |
Concentration
of CaC2
/ sample |
| .......11.. |
..............29
mg |
..............3.3
ml |
................10.9
ml |
.......................30
%.%%...... |
| .......2 |
..............22
mg |
..............3.4
ml |
.................
8.3 ml |
.......................41
% |
| .......3 |
..............23
mg |
..............4.1
ml |
.................
8.6 ml |
.......................48
% |
| .......4 |
|
|
|
|
This means that the calcium carbide concentrations
differ from one sample to the other one.
b) The blue flame of the burning ethyne shows: 1. Ethyne has a smaller
density than air. 2. The blue flame indicates that carbon monoxide - built
in a first step of the ethyne combustion- is burning.
________________________________________________________________________________________________
first publication:
10.10.2008 l..l...last modification:
18.01.2012
l..l..l. back
|
