Experiments With Hydrogen Sulfide

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					Experiments With Hydrogen Sulfide
Generating Hudrogen Sulfide in Large Syringes Introduction: Hydrogen Sulfide, along with many other gases, can be safely generated in a large syringe. Zinc sulfide and hydrochloric acid are mixed to produce 50-55mL of H2S. The production of H2S is rapid, taking about 15 seconds to fill the syringe. The reaction is: ZnS(s) + 2 HCl(aq) → H2S(g) + ZnCl2(aq) The hydrogen sulfide can then be used in several experiments or demonstrations. Purpose: The purpose of this experiment is to generate hydrogen in a syringe and to perform several experiments with it. Equipment/Materials: ZnS powder 6M HCl 6M NaOH DI water universal indicator conc. NH4OH 6% H2O2 30% H2O2 MnO2 powder 2% CuSO45H2O 2% Cd(NO3)24H2O 2% Pb(NO3)2 0.5% Bi(NO3)35H2O syringe Safety: ● Goggles must be worn at all times in the lab. ● Use a Fume Hood or Work Outdoors! The gas-generation and gas-washing steps should be carried out inside a working fume hood or outdoors. ● Toxicity Warning!! Hydrogen sulfide is extremely toxic, and this part of the series should only be conducted by those individuals who have become experts with syringe manipulation of gases. Although the syringe method minimizes the risk of accidental exposure to the gases generated, as a precaution, the gas-generation and gas-washing steps should be performed in a working fume hood or outdoors. H2S(g) has the familiar smell of rotten eggs. Its odor can be detected at 2 ppb! Low level exposure can cause headache, dizziness and nausea. Inhaling higher concentration of the gas can cause

latex syringe cap pipet 15cm length of latex tubing one-holed stopper birthday candle well plate >/= 4gal Ziplok bag gloves 400mL beaker 250mL beaker 18x150mm test tube (30mL) w/ stopper Pasteur pipet 500mL Roundbottom flask

collapse, coma, and death from respiratory failure. The odor of H2S does not increase in proportion with its concentration, so higher concentrations of the gas do not smell worse than low levels. Hydrogen sulfide is also a flammable gas. Procedure: 1. Neutralization Solution. Pour 100 mL of 6M NaOH into a 400-mL beaker, and place it inside the hood for later use. This solution will be used to neutralize excess reagents and can be used in an emergency to neutralize and absorb the reagents, should they spill or accidentally discharge. 2. Remove the plastic cap that covers new syringes. Break in the plunger by moving it up and down several times. 3. Place a small vial cap on the balance. Tare. 4. Mass 0.22 g solid ZnS (powdered) in to the cap. Do not use more than the given amount! 5. Lower the lid into the syringe by water flotation. First, remove the plunger and fill the syringe with water. Place your finger over the hole to form a seal. Place the cap on top of the liquid – the powder should not get into the water. Release the seal and let the water out to lower the cap without spilling the powder. The syringe should be kept in a vertical position. Carefully replace the plunger – the bottom of the plunger should fit snugly against the vial cap. (See Figure 1.) 6. While keeping the syringe vertical, pull up 3-5 mL of 6M HCl from a small weighing boat. The vial lid should float on top of the solution. 7. Place the latex syringe cap over the luerLOK fitting. The set-up should look like that in Figure 2. Shake the syringe to mix the reagents and generate the hydrogen sulfide. It may be necessary to help the plunger move up the barrel. After gas generation has stopped, pull the plunger further outward an additional 5 mL in order to create a slightly reduced pressure inside the syringe. While working inside the fume hood, remove the latex syringe cap while it is directed upwards. Note: Never remove the cap with the syringe aimed downward; acid will spray from Figure 2. Set-up for generation of hydrogen the syringe.

Figure 1. Water flotation

8. Carefully turn the syringe so that the top is down, and discharge the excess acid into the neutralizing solution. Immediately cap the syringe to prevent loss of the hydrogen sulfide. 9. Washing H2S. In order to remove traces of unwanted chemicals from the inside surfaces of the H2S-filled syringe, the syringe must be “washed” prior to use in experiments. To do this, suction 5 mL distilled water into the syringe without discharging any gas, cap the syringe, and tip the syringe from side to side in order to move the water around to dissolve the contaminants on the inside of the syringe. Remove the cap, and discharge the wash water into the beaker of 6 M NaOH. Do not discharge any of the gas. Repeat the washing procedure at least once. Note: H2S(g) is somewhat soluble in water so the volume of gas will decrease by a few mL with every wash. Disposal. Unwanted samples of hydrogen sulfide should be destroyed. This is accomplished most efficiently by suctioning some of the 6 M NaOH into the syringe as described in Experiment C. Glassware and syringes should be washed inside the hood before they are removed. Suitability. Because hydrogen sulfide is extremely toxic, all of these experiments should only be conducted by those individuals who are experts with syringe manipulation of gases.

Experiments with Hydroden Sulfide
Experiment A. H2S is Slowly Oxidized. Hydrogen sulfide is fairly soluble in water; 100 mL water at 0o C will dissolve up to 437 mL H2S(g), producing a solution that is about 0.2 M. However, the solution process is fairly slow so that we are able to wash the gas as described above. Fresh solutions of H2S(g) are clear and colorless but become cloudy white upon standing. The white suspension of elemental sulfur begins to appear within an hour and is produced from the reaction between H2S(g) and dissolved oxygen in water: 2 H2S(aq) + O2(aq) -> 2 S(s) + 2 H2O(l) 1. Place 10 mL distilled water in a 18 x 150 mm test tube (capacity 30 mL). Stopper the test tube with a rubber stopper. 2. Prepare a syringeful of H2S as described above. Wash the gas. Before removing the syringe cap, pull the plunger outward by 5 mL, thus creating slightly reduced pressure within the syringe. 3. Replace the syringe cap with a 15-cm length of latex tubing, and bubble 10 mL of the gas below the surface of the water in the test tube. Remove the syringe/tubing assembly, and pull about 5 mL air into the syringe to remove most of the H2S from the latex tubing. Replace the tubing with the syringe cap, and set the syringe aside for use in Experiments B and C.

4. Stopper the test tube and shake the it vigorously to dissolve some of the H2S(g). Set the stoppered test tube aside, and observe it over the next several hours. After 24 hours most of the H2S will have been destroyed by oxidation. Discard the resulting solution by adding it to the Neutralization Solution. Experiment B. H2S is a Weak Acid. Hydrogen sulfide is a weak acid with a dissociation constant that is considerably larger than that of water: H2S(aq) + H2O(l)  H3O+(aq) + HS-(aq) H2O(l) + H2O(l)  H3O+(aq) + OH-(aq) Ka= 1 x 10-7 Ka= 1 x 10-14

Thus, a 0.01 M H2S solution will have a pH = 4.5. 1. If necessary, prepare a syringeful of H2S as described above. Wash the gas. Before removing the syringe cap, pull the plunger outward by 5 mL, thus creating slightly reduced pressure within the syringe. 2. Remove the syringe cap, suction 20 mL of the universal indicator/pH 8 solution into the H2S-filled syringe, replace the latex cap, and shake to mix the reagents. The pH of the solution will drop from 8 to 4 as the H2S dissolves in the solution. 3. Discard the resulting solution by adding it to the Neutralization Solution. Keep the syringe capped when not in use. Excess H2S(g) can be used in Experiment C. Experiment C. Reaction Between H2S(g) and NaOH(aq). Hydrogen sulfide reacts readily with 6 M NaOH. The reaction is: H2S(g) + NaOH(aq)-> NaHS(aq) + H2O(l) 1. Pour 25 mL 6 M NaOH into a 150 mL beaker. Use the H2S(g) that remains from Experiments A and B or prepare a fresh syringeful of H2S as described above. It is unnecessary to wash the gas for this experiment. 2. Remove the latex syringe cap from the H2S-filled syringe, and suction a few milliliters of NaOH(aq) into the syringe. Hydrogen sulfide reacts instantaneously with the NaOH(aq). The plunger may move rapidly inward and/or the NaOH solution will be drawn rapidly into the syringe. The reaction is so rapid, it could be surprising. The beaker is used because the beaker walls will contain any splashed NaOH(aq). This solution can be slowly discarded down the drain with large amounts of water. If possible, use a sink in the hood and wear gloves.

Experiment D. H2S Burns in Oxygen with a Blue Flame. 1. Fit a 15 cm piece of latex tubing into the end of a glass Pasteur pipet (Flinn GP 7043) as shown in Fig. 1. It should make a snug fit.

Figure 1. Lates tubing in a Pasteur pipet.

2. Enrich a 500 mL flask with O2(g) by decomposing 25 mL 6% H2O2(aq) with 0.10 g MnO2(s) inside the flask.. (Do not drain the reagents from the flask.) Place an inverted beaker over the flask’s neck to minimize O2 loss. 3. Prepare a syringeful of H2S as described above. 4. Equip a one-holed rubber stopper with a birthday candle. Set the candle a safe distance away from the syringe and light the candle. 5. The general arrangement of the experimental apparatus is shown in Figure 2. Fit the syringe with the pipet/tubing assembly. Two people are needed to complete this procedure.

6. Move the tip of the pipet into the vicinity of the candle flame. Slowly discharge the H2S into the candle flame at a rate suitable to ignite the gas and maintain a flame. 7. Carefully move the burning pipet into the O2-filled flask. The H2S will burn with a hotter flame, and the characteristic blue flame will be evident. An audible “roar” will be heard coming from the mouth of the flask. Important! Never withdraw the plunger while the pipet is lit! H2S forms explosive mixtures with air. The combustion of H2S(g) in O2(g) produces SO2(g) as follows: 2 H2S(g) + 3 O2(g) -> 2 SO2(g) + 2 H2O(g) ∆H = -1036 kj Sulfur dioxide is an acidic oxide. In order to test for the presence of SO2, withdraw 40-50 mL of the gas mixture (lf SO2 and O2) from the flask with a clean syringe equipped with a length of latex tubing. Suction 10 mL of a slightly basic aqueous solution of universal indicator into the SO2/O2-filled syringe and it will turn to its acidic color. Excess H2S(g) can be used in other experiments or should be destroyed as described above in the Disposal section. Experiment E. Precipitation Reactions. 1. Prepare (or obtain) stock solutions of Cd+2, Cu+2, Pb+2, and Bi+3 as follows.  Cu+2: 1.0 g CuSO4.5 H2O in 50 mL H2O  Cd+2: 1.0 g Cd(NO3)2 . 4 H2O in 50 mL H2O  Pb+2: 1.0 g Pb(NO3) 2 in 50 mL H2O  Bi+3: 0.5 g Bi(NO3) 3 .5 H2O in 100 mL H2O; stir for 30 minutes and filter; you will use the filtrate Also prepare (or obtain):  10 mL 6 M NaOH(aq): 2.4 g NaOH  10 mL 30% H2O2(aq) (only necessary if Experiment 56 is being performed) 2. Transfer 3 mL of the appropriate metal ion solution to each well of a 12=well plate shown in Figure 3. Transfer 5 mL of 6 M NaOH to each of two wells (labeled “OH-) and 5 mL of 30% H2O2(aq) to each of two wells. (If Experiment F is not being performed, only one well of each metal ion is required, and the wells of H2O2(aq) are not required.)

Prepare a syringeful of H2S(g) and wash the gas. Cap the syringe with a latex cap. Place the filled well plate along with the H2S-filled syringe and a plastic disposable pipet into a large >4 L or gallon size Ziplock bag. Zip the bag shut with a pocket of trapped

air to support the bag above the well plate. Dispense all of the H2S(g) above the surface of the metal ion solutions. (Avoid dispensing it over the NaOH and the H2O2 solutions.) An immediate reaction will be noted for each of the metal ions. Blue Cu+2(aq) will produce a brown web-like film of CuS(s) on the surface. Colorless Cd+2(aq) will produce a distinctive yellow precipitate of CdS(s). Colorless Pb+2(aq) will produce a spectacular silvery mirror of PbS(s) on the surface. The reactions between the various metal ions and H2S(g) are similar; the reaction for Cd+2(aq) is: Cd+2(aq) + H2S(aq) + 2 H2O(l) → CdS(s) + 2 H3O+(aq)

Allow the reactions to proceed for at least 5 minutes before going on to Experiment 56. Do NOT open the ZipLock bag. The two wells of NaOH(aq) will absorb the excess H2S(g) overnight. Clean-up: No clean-up is necessary at this point if Experiment 56 is being performed. If Experiment 56 is not being performed, follow the Clean-up procedure at the end of Experiment 56.

Experiment 56. Reoxidation of Metal Sulfides. Without opening the ZipLock bag, use the plastic disposable pipet to transfer at least 3 mL of H2O2(aq) to one of each pair of wells for each metal sulfide. Within a few minutes, bubbles will appear in the well containing CuS. Within 40 minutes the solutions containing CdS and PbS will have returned to clear. In both cases, the sulfide anion has been oxidized to the sulfate ion as per:

CdS(s) + 4 H2O2(aq) → Cd+2(aq) + SO42-(aq) + 4 H2O(l)

Within 2 – 3 hours the dark color of bismuth sulfide will be replaced with white, insoluble bismuth sulfate: Bi2S3(s) + 12 H2O2(aq) → Bi2(SO4)3(s) + 12 H2O(l) Readers may be interested to know that skunk scent contains trans-2-butene-1-thiol, CH3CHCHCH2SH, (or “TBT”) which is somewhat related to H2S in that both molecules possess the thiol group, SH. As with the oxidations studied in Experiment 56, TBT can be oxidized to the odorless sulfonic acid trans-CH3CHCHCH2SO3H. This chemistry is used in a “home skunk remedy” for treating pets who have been sprayed by skunks.1 Clen-up: Allow the bag to stand overnight. The NaOH(aq) will react with excess H2S(g). Without opening the ZipLock bag, suction a few mL of the NaOH(aq) into
1

“Skunk Non-scents,” Nancy Toughette, Chem Matters, page 7, October, 1996.

the syringe to remove traces of H2S(g). Wear gloves to avoid contact with unreacted H2O2(aq). It is now safe to open the bag indoors. Remove the contents carefully and discard the bag and pipet in the trash. Discard metal ions according to local regulations. Wash the syringe contents (NaSH(aq)) down the drain with plenty of water.

Universal Indicator/pH Solution. Experiments 52 and 54 require a slightly basic universal indicator solution. Prepare a solution by mixing 50 mL distilled water plus 1 mL universal indicator solution. Raise the pH to 8 by bubbling through the solution a pipetful of gaseous ammonia taken from the vapors above a solution of concentrated ammonium hydroxide solution.
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