Microscale Gas Chemistry:

Experiments with Ethyne

     Link to ethyne data page including physical properties.

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: Ethyne forms explosive mixtures with air and oxygen.

Toxicity.
    Ethyne is relatively non-toxic; however, it is a simple asphyxiant if inhaled in very large quantities.

Suitability.
    All of these experiments are suited for use as either classroom demonstrations or as laboratory experiments conducted by students.

Syringe Lubrication.
    We recommend lubricating the black rubber diaphragm of the plunger with silicone spray (available from hardware stores) or medium-grade silicone oil (Educational Innovations, $5.95 Part #GAS-150; Fisher Catalog Number S159-500; $34/500 mL.)
 

Equipment. (This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), Micro Mole, and Fisher Scientific.  Part numbers and links to their websites are provided.)

Chemicals.
  • 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 Method A.  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 outdoors or in a fume hood.
     


    Experiments with Ethyne










    Experiment 1. Byproducts of Ethyne Generation.
    Equipment:

    Chemicals:      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) + CO3-2(aq)  CaCO3(s)












    Experiment 2. Ethyne Reacts with Permanganate.
    Equipment:

    Chemicals:      Prepare a syringeful of ethyne as described above.  Wash the gas several times.  Suction 5-mL of the permanganate solution into the C2H2-filled 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 of 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 3. Ethyne Reacts with Aqueous Bromine.
    Equipment:

    Chemicals:     Prepare a syringeful of ethyne and wash the gas several times.  Pour 3 ? 5 mL of the bromine solution into the test tube and stopper.  Equip the C2H2-filled syringe with the latex tubing and transfer about 20-mL of the gas into the test tube, discharging the gas just above the surface of the bromine solution.  Stopper and shake to mix gas and liquid.  As a 'control,' the reaction between methane or propane and Br2(aq) can be tested. The red color of bromine will disappear in the ethyne-filled test tube but not in the alkane-filled test tube.

        Ethyne and aqueous bromine react to form first 2-bromoethanol as follows:

    C2H2(g) + Br2(aq)  CHBrCHOH

    The CHBrCHOH is an alkene alcohol which prefers the rearranged aldehyde version, however the two exist in an equilibrium:

    CHBrCHOH(aq)  CH2BrCHO

    The a-bromoaldehyde is unstable and likely reacts with something.  The ultimate products are uncertain.  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 4 - 6.  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 4. Sooty Combustible of Ethyne.
    Equipment:
    • test tubes, 18 x 150 mm 
    • beaker or plastic cup, 9-ounce or 250-mL
    • latex tubing, 15 cm length
    • birthday candle 
    • match or lighter
    • ring stand and clamp
    Chemicals:
    • C2H2(g), 60-mL

    Figure 1

     Figure 2
         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 Figure 1.  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 Figure 2.  Soot will be produced as the C2H2(g) burns.
     
     

    Experiment 5. Very Loud Bang!
    Equipment:

    Chemicals:     Clamp an O2-filled syringe with cap up and plunger down in a ring stand as shown in Figure 3.  (Do not remove the plunger yet.)  Replace the latex cap of the C2H2-filled syringe 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 Figure 3 (dispense the gas midway up the syringe).  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 Figure 4.  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.
     

    Figure 3

    Figure 4

     

    Experiment 6. Very Loud Banging Bubbles!
    Equipment:

    Chemicals:
        Clamp both the O2(g) and C2H2(g) syringes to a ring stand as shown in Figure 5.  Bubble various quantities of O2(g) and C2H2(g) through a 3% dish soap solution forming a mound of bubbles as shown in Figure 5.  Remove the tubes from the bubble solution and set it aside.  Ignite the bubbles with a piezoelectric lighter, candle or a match.  As with Experiment 5, the pop and flame produced will depend on the mixture of C2H2 and 02.  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.

    Figure 5

     
     

    Experiment 7. Ethyne/Oxygen Rockets.
    Equipment:

    Chemicals:
         An overview of filling and launching pipet mini-rockets is available (See: Filling and launching pipet rockets).  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 Figure 6.  Bubble the O2(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.


    Figure 6
    Photograph of ethyne/oxygen
    rocket launching in darkness.  A tremendous bang accompanies the launch and the rocket flies many meters. 

     
     
    Experiment 8. Spectacular Underwater Fireworks!
    Equipment:
    • Plastic cup or beaker, 250 mL
    • Latex tubing, 15-cm, 2 pc
    • Tape (electricians works well)
    Chemicals:      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 Figure 7.  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).  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)












     


    Figure 7


    Photograph of 'underwater fireworks.' The  formation of soot particles accompanies the reaction. 


    Clean-up and Storage.

        At the end of the experiments, wipe excess lubricant off of rubber diaphragm. Clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends the life of the plunger.  It may be necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.



     
    This article first appeared in Chem13 News in April, 1997.  The authors of the original Chem13 article are: 

    From Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA

    • Bruce Mattson, faculty member, principal investigator, 
    • Michael Anderson, co-PI
    • Joseph Nguyen, undergraduate student, chemistry major, graduated May 2000, now working in Residence Life at Idaho State


    Boyd Harrison, Muscatine High School Muscatine, Iowa

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    (last updated 29 January 2002)