By Lee Wood,2014-08-03 15:16
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Experiments With Ethyne

    Chapter 7

The chemicals required for CH production as listed in the Master Table (back cover) are: 22

    ; 0.20-g calcium carbide, CaC 2

    ; 5 mL distilled water

This quantity of calcium carbide will produce approximately 60 mL of CH. The 22

    production of CH is relatively fast, and it typically takes 15 seconds to fill a syringe. 22

    The reaction is:

     CaC(s) + 2 HO(l) ? CH(g) + Ca(OH)(s,aq) 22222

The CH gas samples used in these experiments are generated by the General Method 22

    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 othe syringe 180 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 CH(g) samples can be safely discharged into the room. 22

    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.


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.

     CaC(s) + 2 HO(l) ? CH(g) + Ca(OH)(s,aq) 22222

    Discharge the aqueous portion of the spent reaction mixture into a test tube and stopper tightly. Allow the suspension of Ca(OH)(s) to settle for about 10 minutes. Remove the 2

    stopper, and add a drop of phenolphthalein solution. The reaction mixture is alkaline because calcium hydroxide is slightly soluble:

     +2- Ca(OH)(aq) ? Ca(aq) + 2 OH(aq) 2

Next, add a few drops of aqueous sodium carbonate or bicarbonate solution. A white +2CaCO precipitate confirms the presence of Ca(aq): 3

     +22- Ca(aq) + CO(aq) ? CaCO(s) 33

    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 CH-22

    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 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 CH(g) + 8 MnO(aq)+ 8 H(aq) ? 3 HCOOH(aq) + 3 CO(g) + 8 MnO(s) + 4 HO(l) 224222

    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 MnO(aq) can be tested. 4-Unlike alkenes and alkynes, alkanes do not react with MnO(aq). 4

    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 Br(aq) can be tested. 2

    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, 2-tetrabromoethane as follows:

     CH(g) + Br(aq) ? CHBrCHBr 222

     CHBrCHBr + Br(aq) ? CHBrCHBr222

    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 CH(g) for every 5 parts O(g). 222

     2 CH(g) + 5 O(g) ? 4 CO(g) + 2 HO(g) 22222

    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 CH(g). Equip the 22

    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 CH(g). 22

    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 CH(g) 22


Space for figures

Experiment 48. Very Loud Bang! Prepare a syringe with O(g) as described in Chapter 2

    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 CH(g). Replace the latex cap with the 22

    latex tubing. Remove the plunger from the O-filled syringe and transfer about 20 mL of 2

    the CH into the O-filled syringe as shown in Fig. 31. Caution! Do NOT stopper the 222

    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 O(g) and CH(g) syringes to a 222

    ringstand as shown in Fig. 33. Bubble various quantities of O(g) 2

    and CH(g) through a 3% dish soap solution forming a mound of 22

    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 CH and O. Pure 222

    ethyne will also produce soot. Caution! The combustion of CH/O 222

    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 O-filled syringe, and slip the water-filled pipet rocket 2

    over the LuerLOK fitting as shown in Fig. 34. Bubble theO(g) 2

    into the pipet rocket until it is about half full. Next, complete the water-displacement with CH(g) until the rocket is nearly 22

    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 Cl(g) using the General Procedure in the original 2

    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 Cl(g) generation as described. Cl(g) is rapidly produced. Vent unwanted 22

    Cl(g) into a fume hood or into water. Connect the two syringes to their respective pieces 2

    of tubing as shown with the arrows in Fig. 35. Immerse the taped end of the tubing into a

beaker of water. Simultaneously bubble CH(g) and Cl(g) into the water using small 222

    bursts of 5 mL at a time. The gases react to produce HCl(aq) and sooty carbon:

     CH(g) + Cl(g) ? 2 C(s) + HCl(aq) 222

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