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Where natural gas and petrol are
not available, hydrocarbons can be made from
lime, coal and water. In a first step quicklime and
coal react to form calcium carbide, a brittle, very hard salt.
In the following experiment calcium carbide will react with water resulting
in a combustible gas.
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| E 111--------1111Set-up, weighing | ---17 mL water + 29 mg Ca carbide | 1111 Volumetric gas generation | 11...........1Burning ethyne1 |
Material
Tray,
container for waste, goggles, pincers, digital
pocket scale, 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.
Experiment
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. Weigh a piece of calcium carbide in the size of a rice grain. (Attention:
Keep the calcium carbide dry!) (Fig. 1).
3.
Transfer the sample into a beaker. Totally fill the ampoule with water. Carefully
turn it upside down (Fig. 2).
4. Place
the ampoule full of water with its mouth on the grain of calcium carbide (Fig. 3).
Wait until the reaction is over. Measure the gas volume
(Fig. 4).
5.
Turn
the ampulle upwards into a diagonal position. Lit the gas (Fig. 5).
Observations
a)
A
sample of 29 mg
calcium carbide reacts with water releases 3.3 mL of gas
(Fig. 4).
b)
A white precipitate is formed
(Fig. 4).
c) The gas burns with a blue flame for a few seconds
(Fig. 5).
Explanation
a)
During
the reaction of
calcium carbide and water ethine and calcium hydroxide are formed: CaC2
+ 2 H2O
--> C2H2
+ Ca(OH)2
/ . 1 mmol (64 mg) of CaC2
displace 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 |
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 an intermediate during combustion.
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