Gases over Water

When the product of a chemical reaction is gaseous, it is often collected by the displacement of water. For example, suppose the following reaction is used as a source of hydrogen gas. 

Afpcndlx HE ndifcs a M re c Mpl t takle f the vaper piessare ef water ver teaperatare. 

TABLE 5.4 Vapor Pressure of Water versus Temperature 

Temperature (°C) Pressure (mmHg) Temperature ( C) Pressure (mmHg) 

The partial pressure of water in the mixture, called its vapor pressure, depends on temperature (Table 5.4). Vapor pressure increases with increasing temperature because higher tanpera-tures cause more water molecules to evaporate. We discuss vapor pressure more thoroughly in Chapter 11. 

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Headspace analysis has also been used to determine trichloroethylene in water samples. High accuracy and excellent precision were reported when GC/ECD was used to analyze headspace gases over water (Dietz and Singley 1979). Direct injection of water into a portable GC suitable for field use employed an ultraviolet detector (Motwani et al. 1986). While detection was comparable to the more common methods (low ppb), recovery was very low. Solid waste leachates from sanitary landfills have been analyzed for trichloroethylene and other volatile organic compounds (Schultz and Kjeldsen 1986). Detection limits for the procedure, which involves extraction with pentane followed by GC/MS analysis, are in the low-ppb and low-ppm ranges for concentrated and unconcentrated samples, respectively. Accuracy and precision data were not reported. 

In the diagram (Fig. 16), A is a tank of chlorine B, a safety bottle C, a bubbling bottle to observe rate of flow of chlorine gas and D, a 2-1. bottle fitted with a three-hole rubber stopper. If available, a neckless jar, such as the cylindrical jars used in collecting gases, over water in elementary classes, is even better, since the solid PCI5 is more easily scraped out. 

Directions (a) Use the same apparatus as for the preparation of hydrogen in Experiment 8. Fill the generator about one-third full of lumps of marble, slipping the pieces in while the tube is inclined, so that they will not crack it. To 20 c.c. of water in a graduate add 20 c.c. of concentrated hydrochloric acid and pour through the thistle tube enough of this mixture to cover completely the m.arble. Collect three bottles of carbon dioxide as you have collected gases over water before. (1) Which side up should the Ailed bottles be kept (2) Why ... 

Sulfur dioxide was first studied in detail by the English physicist and chemist Joseph Priestley (1733-1804), who invented a method for collecting gases over water. 

The En ish chemist Stephen Hales (1677-1761) made a step in the right direction, in the early eighteenth century, when he collected gases over water. The vapors formed as a result of a chemical reaction could be led, through a tube, into a jar of water that had been upended in a basin of water. The gas bubbled upward into the jar, displacing the... 

We can see an application of Dalton s law in the collection of insoluble gases over water. When prepared in the laboratory, oxygen is commonly collected by the downward displacement of water. Thus the oxygen is not pure but is mixed with water vapor (see Figure 12.12). When the water levels are adjusted to the same height inside and outside the bottle, the pressure of the o gen plus water vapor inside the bottle is equal to the atmospheric pressure ... 

A useful application of the law of partial pressures arises when you collect gases over water (a method used for gases that do not dissolve appreciably in water). Figure 5.20 shows how a gas, produced by chemical reaction in the flask, is collected by leading it to an inverted tube, where it displaces water. As gas bubbles through the water, the... 

Optional experiment. When all the air has been displaced, collect a test-tube of the gas over water (by appropriate inclination of the end of the delivery tube beneath the mouth of a test-tube filled with water and supported in a beaker of water). Observe the colour and odour of the gas. Ignite the test-tube of gas, and note the luminosity of the flame and the amount of carbon deposited. Pure acetylene is almost odourless the characteristic odour observed is due to traces of hydrides of phosphorus, arsenic and sulphur. 

An existing lO-in. I.D. packed tower using 1-inch Berl saddles is to absorb a vent gas in water at 85°F. Laboratory data show the Henry s Law expression for solubility to be y = 1.5x, where y is the equilibrium mol fraction of the gas over water at compositions of x mol fraction of gas dissolved in the liquid phase. Past experience indicates that the Hog for air-water system will be acceptable. The conditions are (refer to Figure 9-68). 

Commonly Daltons law is used in calculations involving the collection of a gas over water, as in the displacement of water by oxygen gas. In this situation there is a gas mixture 02 and water vapor, H20(g). The total pressure in this case is usually atmospheric pressure, and the partial pressure of the water vapor is determined by looking up the vapor pressure of water at the temperature of the water in a reference book. Simple subtraction generates the partial pressure of the oxygen. 

Because you will collect the aerosol gas over water, the beaker contains both the aerosol gas and water vapor. Form a hypothesis about how the presence of water vapor will affect the calculated value of the molar mass of the gas. Explain. 

In 1775, Lavoisier published his paper on the composition of the air. In this historically interesting memoir, Lavoisier refers first to the action of heat upon a mixture of iron calx (oxide) and charcoal in giving fixed air, and to the similar action of mercury precipitate and charcoal. He then describes an experiment, in which he subjected the mercury precipitate to strong heat by itself and collected the expelled gas over water. This gas on examination gave properties familiar to us as those of oxygen. 

Carbides, (a) The aluminum nitride made in Preparation 13 contains a considerable amount of carbide AI4C3. Treat some of this product, or some commercial aluminum carbide, with 6 N NaOH in a test tube with a delivery tube. Collect some of the gas over water, which will dissolve all the ammonia. Test the combustibility of this gas and find that it bums with a nearly colorless flame. 

Hales had, as early as 1727, prepared oxygon by boating potassium nitrate, but although he collected the gas over water and measured its volume, lie did not nwognise it as a new gas or determine its properties. 

You will collect the hydrogen gas over water in a graduated cylinder. 

A student collects 55.0 mL of hydrogen gas over water at 23.0°C and 750 torr. What volume will the dry hydrogen occupy at 40°C and 775 torr ... 

The outer end of the delivery tube reaches just below the shelf of a pneumatic trough arranged to collect the gas over water. Fill the bottle full of water, invert it, and stand it on the shelf near the hole. (See App. A, 15.)... 

Experiment 77. — I. Construct an apparatus like Fig. 30 and arrange it to collect a gas over water as in the preparation of hydrogen (Exp. 19). Put from 10 to 15 gm. of copper borings in the flask, insert... 

Process Cover the bottom of the flask with sand, add a little water, and carefully slip into it a dozen or more small lumps of marble. Arrange the apparatus to collect the gas over water, as previously directed. Add through the safety tube just enough concentrated hydrochloric acid to cover the marble. Reject the first portion of the gas evolved. Collect five bottles, cover with glass plates or wet filter paper, and stand aside till needed. 

Appai atus The essential part of the apparatus is shown in Fig. 64. A carbon dioxide generator (see Fig. 46) is connected with the U-tube Ay which is to be filled with calcium chloride. The combustion tube BE is the same as that used in Exp. 87. The test tube C contains a solution of sodium hydroxide to absorb the excess of carbon dioxide. A delivery tube is connected with D and passes into a pneumatic trough arranged to collect a gas over water. The apparatus may be supported by clamps placed at B and B or any other convenient points. It is advisable, though not absolutely necessary, to add a little sodium hydroxide solution to the water in the trough. Four or five bottles and several lumps of charcoal are also needed. 

Experiment 94. — Construct an apparatus similar to that used for the preparation of chlorine (see Fig. 52). The flask should be smaller, though this modification is not necessary, and the tube B should pass to the bottom of a wash bottle fitted with a two-hole rubber stopper. The wash bottle is filled two-thirds full of sodium hydroxide solution. A delivery tube passes from the wash bottle into a pneumatic trough arranged to collect a gas over water. 

Experiment 96.—Apparatus Use the apparatus shown in Fig. 63. Arrange it to collect a gas over water. 

Add a small piece of aluminium to a test tube half full of a dilute solution of sodium hydroxide, and boil. Test any gas evolved. If only a little gas is liberated, attach a simple delivery tube and collect the gas over water. 

As described under Carbon Monoxide, the test may also be made by filling a test-tube with the gas over water, and applying a match to the mouth of the tube. 

Into a 6-inch test-tube place about 2 cc. of ethylene bromide, 2 cc. of alcohol, and about 0.5 gram of sheet zinc. Connect the tube with a delivery-tube, warm gently until reaction begins, and collect a test-tube full of the gas over water. (Eqi) Burn the gas. (Eq.)... 

Preparation and Properties of Acetylene (sections 41 to 43).—(a) Support a dry 200-cc. distilling flask by means of a clamp, and connect the side-arm of the flask with a delivery-tube arranged to collect a gas over water. Fit a dropping funnel into the neck of the flask by means of a cork. Place in the flask about 10 grams of calcium carbide, and let water fall very slowly,... 

The collection of H2 gas over water produced from the reaction of ethanol with sodium can easily be observed. 

Collecting a Gas over Water The law of partial pressures is frequently used to determine the yield of a water-insoluble gas formed in a reaction. The gaseous product bubbles through water and is collected into an inverted container, as shown in Figure 5.10. The water vapor that mixes with the gas contributes a portion of the total pressure, called the vapor pressure, which depends only on the water temperature. 

Aluminum reacts with excess hydrochloric acid to form aqueous aluminum chloride and 35.8 mL of hydrogen gas over water at 27°C and 751 mmHg. How many grams of aluminum reacted ... 

FIGURE 10.15 Collecting a water-insoluble gas over water. 

Gas jars can be filled with oxygen by using a cylinder or by generating the gas in a reaction and collecting the gas over water. For details of the generation see ... 

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