addition of the ammonium chloride; then find the weight of silver which has been used to precipitate the chlorine of 100 parts by weight of ammonium chloride. From these results

calculate (1) the atomic weight of silver if Cl = 35 37; (2) the atomic weight of chlorine if Ag=107.66; assuming that the ratio of the atoms of silver and chlorine in silver chloride is 1 : 1.

CHAPTER II.

DISSOCIATION.

THE molecular weights of gasifiable elements and compounds are determined by finding the specific gravities of their vapours (s. Part II. Chap. III.

Exp. 1). But some of the results obtained appear at first sight to be abnormal. The greater number of these apparently abnormal results are

ex

plained by the occurrence of dissociation. The phenomena of dissociation were briefly studied in Part II., but some other and more accurate Exps. must now be performed. (s. Pattison Muir's Elements of Thermal Chemistry, Chap. IV. Sect. 2.)

Exp. 1. Dissociation of hydriodic acid. Read Lemoine, Equilibres chimiques entre l'hydrogène et l'iode gazeux, Annales de Chim. et Phys. (5). 12, 145.

I. Lead a rapid stream of hydriodic acid gas for half an hour or so into a dry bulb (Fig. 54) of about 250 c.c. capacity, blown from the same glass as that from which pressure tubes are made. During the operation of filling, the bulb is surrounded by water of a known temperature. The neck of the bulb has been previously narrowed and thickened at a. The entrance of

b

Fig. 54.

moisture during the filling of the bulb with gas is prevented by suspending in the neck a small tube, b, filled with calcium chloride. The gas is conveniently prepared from a mixture of 120 grams iodine, 6 grams amorphous phosphorus, and 40 grams of a concentrated solution of hydriodic acid; the gas must be thoroughly dried by passage over phosphorus pentoxide. When the bulb may be presumed to be full of the gas it is sealed at a; at the same time the barometric pressure is read. The bulb is now heated for a couple of hours or more in the vapour of boiling sulphur (440°); and then cooled as rapidly as possible by wiping it with a damp cloth.

The sealed end of the neck is now broken off under a saturated solution of common salt which has been well boiled and cooled out of contact with the air. The iodine resulting from the dissociation of the hydriodic acid, as well as any undecomposed hydriodic acid, dissolves in the saline solution; the residual gas in the bulb is transferred to a graduated tube and its volume is read off, and reduced to 0o and 760 mm. (= v). It is necessary to transfer some of this gas to a eudiometer and analyse it, as it is impossible to get the bulb perfectly air-free. The volume of the bulb v' must also be accurately determined by weighing it full of water. We have now all the data necessary for determining the percentage dissociation of hydriodic acid under the conditions of the experiment.

The following example of the mode of calculation is taken from Lemoine's paper.

v=35.48 c. c. This gas on analysis gave 96.7% H and 3.3% N. Hence v consisted of 34.31 c.c. H and 1∙18 c.c. N (the latter number corresponds to 1·5 c.c. of air).

The bulb, which had a capacity v'394.8 c.c., had been filled with hydriodic acid gas at a temp. of 10.70 and under a pressure of 748.2 mm., hence it follows that the amount of gas operated on would occupy at 0° and 760 mm.

394.8 × 748.2
1.039 × 760

= = 374.1 c.c.

Now these 374.1 c. c. must have contained 1.5 c.c. of air. Hence the true quantity of HI used was 374.1-1.5=372.6 c.c. The half of this =1863 c.c. represents the hydrogen of this hydriodic acid; but on warming, the 32 c.c. of oxygen in the air present would oxidise '64 c.c. of this hydrogen; hence there are only 185.7 c.c. of "disposable" hydrogen. Hence the ratio of free hydrogen to total disposable hydrogen (which is a measure of the dissociation)

374.1 623

or about

At the temperature of 350° at which this experiment was conducted it is easy to see that the pressure must have been 2.15 atmospheres.

394.8 273

Now perform another experiment at 440°; but this time let the hydriodic acid gas be introduced into the experimental bulb at such a diminished pressure as will become about an atmosphere at 440°; this is done by connecting the neck of the bulb with a three-way tube, the other limbs of which are connected, respectively, with a vacuum-pump, and a large globe containing dry hydriodic acid. In this case it will be necessary to leave the bulb in sulphur vapour for 25 hours or The warming may be effected in periods of 12 hours each, or less, provided after each warming the bulb and contents are cooled rapidly.

SO.

The results of the two experiments ought to shew that the limit of dissociation of hydrogen iodide at 440° varies very slightly indeed with the pressure of the reacting gases.

II. Perform two experiments similar to the above, but heat the bulbs to a temperature of about 360°, for say 8 hours. In neither case will equilibrium between the two inverse actions

be reached, as was the case at 440°; but your experiments ought to shew that in the bulb which has been submitted to the lower pressure there has been more dissociation than in that subjected to the higher pressure.

The constant temperature of 360° may be obtained by immersing the bulbs in the vapour of boiling mercury, contained either in a cylindrical vessel of sheet iron of such a length that the mercury vapour is condensed in its upper cooler portions, or preferably in an iron bottle of the kind designed by Deville for use in vapour density determinations. (Annales de Chim. et Phys. (3), 58, 257.)

III. Find as accurately as possible the volumes of two globes. Into one of these put as much iodine as is equivalent to the amount of hydrogen the globe will contain under atmospheric pressure, and into the other just half as much iodine as is equivalent to the amount of hydrogen the globe will contain under ordinary pressure [100 c.c. H at 0° and 760 mm. are

equivalent to 1·137 grams I]. Fill both globes with dry hydrogen, in the manner described under I. for hydriodic acid: seal, and heat to 440° for 4 hours. Then cool quickly, and determine the ratio of free hydrogen to total hydrogen as before.

Now calculate the ratio of hydriodic acid formed to hydriodic acid possible, in each case; from the equation

Your experiments ought to shew that as a decreases, à increases; or, in other words, that, other conditions being the same the dissociation-limit is lessened when either of the products of dissociation is present in excess.

Exp. 2. Dissociation of ammonium carbamate. Read Horstmann, Annalen der Chem. u. Pharm. 187, 48.

I. Dissociation of ammonium carbamate in vacuo. Procure a tube, closed at one end, about a metre in length and about 15 mm. in diameter. The tube is provided with a millimetre scale in terms of which it is accurately calibrated. Having thoroughly cleaned and dried this tube, lead into it perfectly dry ammonia and carbon dioxide gases. Make the delivery tubes so long that the gases first come into contact with each other near to the closed end of the tube. White crystalline plates of ammonium carbamate CO(NH)(ONH) form on the sides of the experimental tube. When this formation has gone on for some time, fill the tube very carefully with perfectly dry (but not warm) mercury, and invert it in a trough of mercury. It will be found that the height of the mercury in the experimental tube is now less than the height of the barometer. This is due to the fact that at ordinary temperatures, i.e. 17°-20°, the carbamate undergoes a notable amount of dissociation into carbon dioxide and ammonia.

The difference between the barometric height and the height of mercury in the experimental tube gives the dissociation-pressure for the temperature considered.

The following are some of the values obtained by Horst