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.
HCl(g) has an irritating and unpleasant odor and is toxic. Inhalation of the gas will cause coughing.
All of these experiments are suited for use as classroom demonstrations. These experiments are not advised for use as laboratory experiments conducted by typical high school students. Advanced students or students with special laboratory skills could be allowed to generate HCl(g) by the large test tube method (Variant II). In any case, all gas generation work should be performed under close supervision by the instructor.
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.)
- several 60-mL plastic syringes with a LuerLOK fitting
- Latex LuerLOK syringe cap fittings
- Small plastic weighing boats
- balance capable of measuring to 0.01 g
- two pieces, latex tubing, 1/8-inch (3.175 mm) ID, 5 cm lengths
- two 18 x 150 mm test tubes
- two-hole #1 stopper fitted with two short lengths (2 cm) of glass tubing
- pinch clamp or hemostat
- ring stand and three suitable clamps to hold test tube and syringes
- small Bunsen burner
- matches or a lighter
- ‘permanent’ marker pen
This quantity of reagents will produce approximately 60 mL of HCl. The production of HCl is relatively fast and it typically takes 15 seconds to fill a syringe. Upon heating this mixture, HCl(g) is produced according to the reaction:
1.1 g anhydrous sodium hydrogen sulfate (sodium bisulfate) NaHSO4 (or 1.3 g sodium hydrogen sulfate (sodium bisulfate) monohydrate NaHSO4.H2O) Note: NaHSO4 gives better results but NaHSO4.H2O gives adequate results and may be used if the anhydrate is not available 0.7 g sodium chloride, NaCl
NaHSO4(s) + NaCl(s)
HCl(g) + Na2SO4(s)
NaHSO4.H2O(s) + NaCl(s) HCl(g) + Na2SO4(s) + H2O(l)
of Hydrogen Chloride in the Microwave Oven.
Samples of HCl(g) also can be prepared conveniently in a microwave oven. (See Generating Gases in a Microwave Oven.)
of Solid Reagent Mixture.
The solid reagent mixture is a 2:1 mass ratio of sodium hydrogen sulfate monohydrate, NaHSO4.H2O, and sodium chloride, NaCl. It is convenient to prepare a large quantity of the solid reagent mixture (20 g NaHSO4.H2O + 10 g NaCl) in order to perform several gas preparations. The mixture should be pulverized together with the aid of a mortar and pestle. (During the pulverization, you may notice the faint smell of HCl(g). This is normal.) The mixture should be stored in a tightly sealed glass or plastic bottle. We have also obtained very nice results with a 2:1 mixture of anhydrous potassium hydrogen sulfate and sodium chloride.
of Hydrogen Chloride.
|The gaseous hydrogen chloride (HCl) samples used
in these experiments are generated by the thermal method. The assembled
apparatus is shown in Figure 1. Every component of this apparatus
must be dry! Syringe plungers should move easily in the barrels.
This can be facilitated by applying a drop or two of silicone oil or spray
to the groove in the plunger's rubber seal. A small burner is also
needed. The left syringe, labeled 'HCl' is used to collect relatively
pure HCl(g) and the syringe labeled 'Waste' is used to collect impure samples
of gas and unwanted air. A pinch clamp or hemostat is used to pinch
closed one of the latex tubes.
Start by pinching closed the syringe labeled 'HCl'. Place a boiling chip in the test tube. Add the 2-g of reagent mixture to the test tube. Insert the stopper firmly in order to form an air-tight seal. Caution: Do not crimp the latex tubing!
Hydrogen chloride is generated by the following 3-step maneuver described in Chapter 1 and summarized in Figure 2.
Hydrogen chloride is extremely soluble in water and cannot be washed. However, it is unnecessary to wash gases collected by the thermal method because the gas sample is collected in a clean, dry syringe.
Unwanted samples of HCl(g) can be discarded by dissolving in water. The solid remaining in the test tube is Na2SO4 can be dissolved in water and discarded down the drain. The test tubes can be reused unless they have been damaged.
Indicator/pH 8 Solution.
Several of the experiments require a slightly basic universal indicator solution. Prepare a solution by mixing 200 mL distilled water plus 20 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.
Experiment 1. HCl(g) Fountain.
Remove the syringe cap from the
HCl(g)-filled syringe and place it in the cup of water. Keep the
syringe’s LuerLOK fitting under the surface of the water. Because
HCl(g) is so soluble in water, water is rapidly suctioned into the syringe
producing a small fountain. Be sure to hold the plunger in place
to prevent it from being pulled into the syringe. There is a considerable
force pulling the plunger inward!
2. HCl(g) is an Acid.
Prepare a very dilute NaOH(aq) solution by adding 1 drop of 1 M NaOH(aq) to 75 mL distilled H2O. Add several mL of universal indicator solution. If the pH is <7, add one drop of 1 M NaOH(aq) until the pH is > 7. Pour the solution into a 100-mL graduated cylinder. Next, generate HCl(g) as described above. Remove the syringe cap and attach a 15 cm length of latex tubing to the syringe. Dispense some of the HCl(g) near the surface of the water (Figure 11.3) and notice the production of an acidic solution at the surface. You should also notice two other phenomena. An aerosol cloud of HCl/H2O is formed above the surface. This is caused by HCl(g) condensing H2O(g) into an aerosol because of the great affinity of HCl for water. You should also notice that the acidic solution sinks through the column of water and soon the entire contents of the graduated cylinder are the same color (acidic). The 'sinking' occurs because the HCl(aq) solution that is produced near the surface is more dense than water and sinks.
Repeat Experiment 2A but instead of using 75 mL distilled water, use 75 mL of 50% corn syrup and 50% distilled water. You may find that it takes 2 or 3 drops of 1 M NaOH(aq) to make the solution pH > 7. This time the HCl(aq) layer is less dense than the corn syrup solution and layers of various colors are produced along the pH gradient.
Experiment 3. Acid Snow?
In terms of laboratory technique,
this experiment is similar to Experiment 2. You will need a flashlight.
Generate HCl(g) as described above. Remove the syringe cap and attach
a 15 cm length of latex tubing to the syringe. Dispense some of the
HCl(g) near the surface of the NaCl(sat'd) (Figure 11.3) and notice the
production of NaCl(s) at the surface. The HCl(g) dissolves in the
NaCl(sat'd) which causes the Cl-(aq) concentration to exceed that allowed
by the solubility of NaCl. LeChatelier's principle predicts that
the equilibrium will shift left and NaCl(s) will form:
NaCl(s) Na+(aq) + Cl-(aq) Ksp = 37
It does and slowly falls through the NaCl(sat'd)
solution. By darkening the room and holding a flashlight up to the
graduated cylinder, the beautiful glittering crystals of NaCl can be observed
as they fall through the solution giving the appearance of snow falling.
|Experiment 4. HCl(g) Reacts
Experiment 5. Well-plate
Reactions Between HCl(g) and Various Aqueous Solutions.
5 mL H2O
Ag+(aq) + Cl-(aq) ---> AgCl(s)
5 mL H2O
Pb+2(aq) + 2 Cl-(aq) ---> PbCl2(s)
Mg(s) + 2 H+(aq) ---> Mg+2(aq) + H2(g)
5 mL H2O
HCO3-(aq) + H+(aq) ---> CO2(g) + H2O
Next, prepare a syringeful of
HCl(g) as described above. One syringeful of HCl(g) is adequate for all
five of these experiments. Equip the syringe with a 3-cm length of
latex tubing. In the case of Wells 1 - 3, add five 1-mL 'puffs' of
HCl(g) just above the surface. For Well 4, slowly discharge 5 mL
HCl(g) just at the surface (latex tubing just touching the surface).
For Well 5, bubble 5 mL HCl(g) just below the surface.
Experiment 6. HCl(g) Reacts
with Office Paper.
CaCO3(s) + 2 HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
After 15 minutes, remove the office papers from
the solutions and allow them to dry on a clean surface or paper towel.
Compare the treated paper with the 'control' sample side-by-side on a dark
surface. You should notice that the treated paper has lost some of
its opacity. It has also lost some of its mass. Calcium carbonate
is added to paper make it whiter and more opaque.
| Experiment 7. HCl(g)
Reacts with Antacids.
CaCO3(s) + 2 HCl(g) CaCl2(s, aq) + CO2(g) + H2O(l)
Experiment 8. Grahams Law
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
|This article first appeared
in Chem13 News in April, 1998. The authors of the original
Chem13 article are:
From the Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA:
Rimantas Vaitkus, Department of Chemistry, Vilnius Pedagogical University, 2034 Vilnius, Lithuania
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(This page last updated 29 January 2002)