Microscale Gas Chemistry:

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.)

• 60-mL plastic syringes with a LuerLOK fitting
• Latex LuerLOK syringe cap fittings
• Plastic vial caps that fit within the barrel of the syringe
• Small plastic weighing boat
• Clear plastic beverage cup, 9 oz (260 mL)

0.20-g calcium carbide, CaC2

5 mL distilled water

   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 180^o 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.
 

Experiment 1. Byproducts of Ethyne Generation.
Equipment:

• test tube, 18 x 150 mm, with suitable stopper

• aqueous portion of the reaction syringe contents
• phenolphthalein solution
• aqueous sodium carbonate or sodium bicarbonate solution, approximately 1 M

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:

Next, add a few drops of aqueous sodium carbonate or bicarbonate solution.  A white CaCO3 precipitate confirms the presence of Ca⁺²(aq):

Experiment 2. Ethyne Reacts with Permanganate.
Equipment:

• weighing boat, small

• C2H2(g), 60-mL
• potassium permanganate, dilute aqueous solution (one small crystal per 50 mL water)

     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:

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:

• test tubes, 18 x 150 mm with suitable stoppers
• latex tubing, 15 cm length

• C2H2(g), 60-mL
• aqueous bromine solution (See: How to prepare dilute bromine solutions)

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

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

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).

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

Experiment 5. Very Loud Bang!
Equipment:

• Ring stand and clamp
• 15-cm length of latex tubing
• candle
• match or lighter

• C2H2(g), 60-mL
• O2(g), 60-mL (See: Instructions for the preparation of oxygen)

Figure 3

Figure 4

Experiment 6. Very Loud Banging Bubbles!
Equipment:

• Ring stand and two clamps
• 15-cm length of latex tubing, two pc
• match or lighter
• small weighing boat

• C2H2(g), 60-mL
• O2(g), 60-mL (See: Instructions for the preparation of oxygen)
• 3% v/v dish soap solution (3 g dish soap + 97 g H2O)

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:

• large disposable pipets
• piezoelectric sparker (See: How to construct a piezoelectric sparker)
• plastic cup
• small test tubes

• C2H2(g), 60-mL
• O2(g), 60-mL, (See: Instructions for the preparation of oxygen)

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: C2H2(g), 60-mL Cl2(g), 60-mL, (See: Instructions for the preparation of chlorine)      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.

    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.