Experiments With Ethyne
Chapter 7
The chemicals required for C2H2 production as listed in the Master Table (back cover)
are:


0.20-g calcium carbide, CaC2
5 mL distilled water
This quantity of calcium carbide will produce approximately 60 mL of C2H2. The
production of C2H2 is relatively fast, and it typically takes 15 seconds to fill a syringe.
The reaction is:
CaC2(s) + 2 H2O(l) → C2H2(g) + Ca(OH)2(s,aq)
The C2H2 gas samples used in these experiments are generated by the General Method
described in Chapter 1. Care must be taken to stop the gas generation after the syringe is
full. This is done by removing the latex syringe cap while it is directed upwards. Rotate
the syringe 180o in order to discharge the reaction mixture and then recap the syringe.
Washing the Gases. The gas-filled syringe must be “washed” in order to remove traces
of unwanted chemicals from the inside surfaces of the syringe before the gases can be
used in experiments. To do this, suction 5 mL distilled water into the syringe without
discharging any gas, cap the syringe, and shake the water to dissolve the contaminants on
the inside of the syringe. Remove the cap, and discharge the water but not any of the gas.
Repeat once or twice.
Disposal. Unwanted C2H2(g) samples can be safely discharged into the room.
Suitability. All of these experiments are suited for use as either classroom
demonstrations or as laboratory experiments conducted by groups of students.
General Safety Precautions.
Always wear safety glasses. Gases in syringes may be under pressure and could spray
liquid chemicals. Follow the instructions and only use the quantities suggested.
Caution:
Remember that you are working with a flammable gas!
Experiment 44. Byproducts of Ethyne Generation. A byproduct in the production of
ethyne is calcium hydroxide, which is only partially soluble.
CaC2(s) + 2 H2O(l) → C2H2(g) + Ca(OH)2(s,aq)
Discharge the aqueous portion of the spent reaction mixture into a test tube and stopper
tightly. Allow the suspension of Ca(OH)2(s) to settle for about 10 minutes. Remove the
stopper, and add a drop of phenolphthalein solution. The reaction mixture is alkaline
because calcium hydroxide is slightly soluble:
Ca(OH)2(aq) ↔ Ca+2(aq) + 2 OH-(aq)
Next, add a few drops of aqueous sodium carbonate or bicarbonate solution. A white
CaCO3 precipitate confirms the presence of Ca+2(aq):
Ca+2(aq) + CO32-(aq) → CaCO3(s)
Experiment 45. Ethyne Reacts with Permanganate. Prepare a very dilute aqueous
solution of potassium permanganate. Prepare a syringeful of ethyne as described above.
Wash the gas several times. Suction 5-mL of the permanganate solution into the C2H2filled syringe, and cap with the syringe cap. Shake the solution vigorously. The pink
color of permanganate will disappear, and tiny bubbles will be observed in the solution.
Ethyne and permanganate react with cleavage on the carbon-carbon triple bond and
formation of the formate ion and the carbonate ion. The stoichiometry for the oxidation
by permanganate under neutral or slightly acidic conditions is:
3 C2H2(g) + 8 MnO4-(aq)+ 8 H+(aq) → 3 HCOOH(aq) + 3 CO2(g) + 8 MnO2(s) + 4 H2O(l)
If the conditions are neutral or acidic, the formate ion is converted to formic acid and the
carbonate ion forms carbon dioxide. Indeed, as the reaction proceeds, tiny bubbles are
observed in the aqueous phase.
As a ‘control,’ the reaction between methane or propane and MnO4-(aq) can be tested.
Unlike alkenes and alkynes, alkanes do not react with MnO4-(aq).
Experiment 46. Ethyne Reacts with Aqueous Bromine. Prepare a very dilute aqueous
solution of bromine by shaking 100 mL water in a 250 mL flask into which some vapors
of bromine have been decanted. Prepare a syringeful of ethyne and, wash the gas several
times. Suction 5-mL of the bromine solution into the syringe and, install the syringe cap.
As a ‘control,’ the reaction between methane or propane and Br2(aq) can be tested.
Shake the two solutions simultaneously and vigorously. The red color of bromine will
disappear in the ethyne-filled syringe but not in the alkane-filled syringe.
Ethyne and bromine react to form first 1, 2-dibromoethene and then 1, 1, 2, 2tetrabromoethane as follows:
C2H2(g) + Br2(aq) → CHBrCHBr
CHBrCHBr + Br2(aq) → CHBr2CHBr2
This reaction is quite specific to alkenes and alkynes and is used as a test method to
confirm the presence of an unsaturated hydrocarbon.
Experiments 47 –49. Ethyne/Oxygen Stoichiometry. Caution! Never mix ethyne and
oxygen in the same syringe. The mixture has been known to spontaneously explode.
The law of combining volumes states that the theoretically ideal ratio of ethyne and
oxygen for use in Experiments 47 – 49 is 2 parts C2H2(g) for every 5 parts O2(g).
2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g)
Experiment 47. Sooty Combustible of Ethyne. Fill a medium sized test tube with water.
Invert the test tube into a beaker of water. Prepare a syringeful of C2H2(g). Equip the
syringe with a 15-cm piece of latex tubing. Use water displacement to fill the test tube
with ethyne as shown in Fig. 29. Completely fill the test tube with C2H2(g).
Remove the test tube from the beaker of water. Clamp in a ring stand, and immediately
ignite with a lighted candle as shown in Fig. 30. Soot will be produced as the C2H2(g)
burns.
Space for figures
Experiment 48. Very Loud Bang! Prepare a syringe with O2(g) as described in Chapter
4 and clamp it cap-up in a ring stand as shown in Fig. 31. (Do not remove the plunger
yet.)
Experiment 48-A. Next prepare a syringeful of C2H2(g). Replace the latex cap with the
latex tubing. Remove the plunger from the O2-filled syringe and transfer about 20 mL of
the C2H2 into the O2-filled syringe as shown in Fig. 31. Caution! Do NOT stopper the
mixture or reinstall the plunger! Bring a lit candle up close to the mouth of the open
syringe as shown in Fig. 32. A very loud BANG! will be heard. Alert others nearby
before you do this experiment. The reaction is very clean and will not damage the
syringe.
Space for figures.
Experiment 48-B. Clamp both the O2(g) and C2H2(g) syringes to a
ringstand as shown in Fig. 33. Bubble various quantities of O2(g)
and C2H2(g) through a 3% dish soap solution forming a mound of
bubbles as shown in Fig. 33. Remove the tubes from the bubble
solution and set it aside. Ignite the bubbles with a piezoelectric
lighter, candle, or match. As with Experiment 48-A, the pop and
flame produced will depend on the mixture of C2H2 and O2. Pure
ethyne will also produce soot. Caution! The combustion of C2H2/O2
mixtures can be extremely loud. Warn viewers that ear protection
may be appropriate.
Experiment 49. Ethyne/Oxygen Rockets. This experiment uses the
piezoelectric lighter and the plastic ‘pipet rockets’ first described in
Chapter 4, Experiment 22. Instructions for the assembly of this
device from an empty piezoelectric lighter are provided in the
Appendix.
Fill the pipet rockets completely with water, and store them
open-end down in test tubes filled with water. Remove the cap
from the O2-filled syringe, and slip the water-filled pipet rocket
over the LuerLOK fitting as shown in Fig. 34. Bubble theO2(g)
into the pipet rocket until it is about half full. Next, complete
the water-displacement with C2H2(g) until the rocket is nearly
filled. Leave some water in the pipet stem.
Position the rocket over the wire of the igniter. Water must remain in the stem because
this serves as the propellant, however the ends of the wire leads must be above the water
in the gas-filled region of the rocket. Trigger the igniter, and the rocket will fly 5 m or
more. We have found that ethyne works particularly well and that a small fireball is
noticed as the gas mixture ignites.
Experiment 50. Spectacular Underwater Fireworks!
Ethyne reacts with chlorine accompanied by spectacular
flashes of fire. Prepare two 15-cm lengths of latex
tubing. Tape the pieces together near one end as shown
in Fig. 35. The design will allow bubbles of ethyne and
chlorine to come in contact under water. Generate a
syringeful of ethyne and set it aside. Generate a syringeful
of Cl2(g) using the General Procedure in the original
article: Place 1 mL 6 M HCl(aq) in a vial cap. (A
fairly large vial cap is necessary.) Suction 3 mL household bleach into the syringe.
Commence Cl2(g) generation as described. Cl2(g) is rapidly produced. Vent unwanted
Cl2(g) into a fume hood or into water. Connect the two syringes to their respective pieces
of tubing as shown with the arrows in Fig. 35. Immerse the taped end of the tubing into a
beaker of water. Simultaneously bubble C2H2(g) and Cl2(g) into the water using small
bursts of 5 mL at a time. The gases react to produce HCl(aq) and sooty carbon:
C2H2(g) + Cl2(g) → 2 C(s) + HCl(aq)
Last updated 8-01