Hydrogen Physical Properties
Hydrogen Physical Properties
|At ordinary temperatures hydrogen is a gas, without colour, odour, or taste, and very slightly soluble in water. For temperatures between 0° C. and 25° C., Timofeeff found that the volume of hydrogen dissolved in one volume of water at 760 mm., or the absorption-coefficient (a) is given by the equation |
a=0.021528 – 0.00010216t+0.000001728t2,
where t stands for the temperature. For alcohol he gives the value of a at 0° C. as 0.0676, at 6° C. as 0.0693, at 13.4° C. as 0.0705, and at 18.8° C. as 0.0740, indicating that rise of temperature is attended by increase of solubility. The same phenomenon was noted by Just with a number of other organic solvents. Its solubility in liquid air is considerable, Dewar having observed that this solvent dissolves one-fifth of its volume of gaseous hydrogen at temperatures between -200° C. and -210° C.
Morley gives the weight of one litre of hydrogen at normal temperature and pressure as 0.089873±0.0000027 gram, the density compared with air being therefore 0.0694. Guye gives for this constant the value 0.08987, the error not exceeding one ten-thousandth of this number. Victor Meyer's investigations of the density of hydrogen at higher temperatures furnished no evidence of dissociation.
Regnault's value for the coefficient of expansion of hydrogen at 760 mm. between 0° C. and 100° C. is 0.0036613. His experiments demonstrated that on subjection to pressures between 1 and 27 atmospheres the compressibility of the gas is less than that indicated by Boyle's law. The remarkable nature of this phenomenon was accentuated by his further observation that nitrogen, air, and carbon dioxide are more compressible than they would be if they obeyed this law.
Two series of experiments carried out by Amagat confirmed Regnault's results for hydrogen, and proved methane to resemble nitrogen in being more compressible than theory demands. In one set of observations he employed pressures up to 400 atmospheres, measured directly by means of a manometer; in the other the gas was submitted to pressures up to 3000 atmospheres. Amagat's work was confirmed by the researches of Wroblewsky.
The behaviour of hydrogen at very low pressures has been investigated by Rayleigh. At about 1-5 mm. it obeys Boyle's law, and continues to do so within very narrow limits up to 150 mm.
Holborn has studied the isothermals of hydrogen at 0° C., 50° C., and 100° C., the pressure limits beingb 20 and 100 atmospheres. As unit of pressure he selected that of a column of mercury having a height of one metre at 0° C. under the normal gravity, g=980-665 cm.-sec.2; and the unit of volume was the volume of the gas under this pressure. Within the limits of experimental error, amounting to a few parts in ten thousand, the isotherms for 50° C. and 100° C. were linear; whilst the deviation of that for 0° C. did not exceed one part per thousand. The results obtained can be expressed in the following formulae:
0° C.: pv=0.99918 + 0.00082094p + 0.0000003745p2;
50° C.: pv=1.18212+ 0.00089000p;
100° C.: pv=1.36506+ 0.00091400p.
Expressed as functions of the formulae are:
0° C.: pv=0.99918 + (0.00081613)/v + (0.000001220)/v2;
50° C.: pv=l.18112 + (0.0010505)/v + (0.000001015)/v2;
100° C.: pv=1.36506 + (0.0012450)/v + (0.000001240)/v2.
The effect of pressure on hydrogen is represented graphically in fig, the product pv being plotted against the pressure p. Neon and helium resemble hydrogen in being less compressible than Boyle's law demands.
| Effect of pressure on hydrogen. |
Travers observed that at ordinary temperature the expansion of hydrogen without doing work is attended by rise of temperature, indicating that under these conditions it behaves as an " ultra-perfect " gas; at - 80° C. and 200 atmospheres its effusion without doing work is unaccompanied by calorific effect, a property characteristic of a perfect gas. Dewar found that at -200° C. hydrogen begins to assume the character of an imperfect gas, expansion without external work being attended by a fall in temperature. Landolt gives the diffusion-coefficient of hydrogen with respect to oxygen as 0.677 sq. cm.sec. at 0° C. and 760 mm.
Hydrogen is absorbed by wood-charcoal, and Kasper has shown that 1 c.c. of this substance at 0° C. and 430 mm. Absorbs 1.5 c.c.; at the same temperature and 1800 mm. it absorbs 11.7 c.c.
Lehfeldt gives for the electrochemical equivalent of hydrogen 96,590 coulombs, or 0.17394±0.00001 c.c. of gas per coulomb. The thermal conductivity of hydrogen is stated by Stefan1 to be seven times that of air.
At 16° C. the specific heat of hydrogen at constant volume is 4.875. The mean value of the specific heat between 0° C. and 2350° C. has been proved by Pier to be given by the expression 4.700 +4.5×10-4t. For the specific heat at constant pressure Lewis and Randall give the expression Cp=4.50+0.0009T.
Regnault's value for the molecular heat of hydrogen at constant pressure and the ordinary temperature is 6.81 cal., and at constant volume 4.81 cal. For the specific heat at constant pressure, Escher gives 3.4219, and for the molecular heat at constant volume 4.913. The mean value of the molecular heat at constant volume between 1300° C. and 1700° C. is given by Langen as 4.8+0.00061t, where t expresses degrees centigrade. According to Mallard and Le Chatelier, the mean molecular heat at constant pressure is given by 6.5+0.0006T, where T denotes absolute temperature. Rontgen found for the ratio of the two specific heats (y) at ordinary temperature Cp/Cv = l.41. Grlineisen and Merkel give the velocity of sound in hydrogen as 1260.6 m.-sec.; and the value of y at 0° C. and 760 mm. as Cp/Cv = 1.408.
Croullebois has determined the refractive indices at ordinary temperature and pressure for the C; E, and G lines of the solar spectrum. Merton and Barratt have studied the spectrum of hydrogen.
Liquefaction of hydrogen The fact that hydrogen cannot be liquefied solely by pressure engendered a belief in the impossibility of its liquefaction. In 1877 Cailletet allowed hydrogen at a pressure of 280 atmospheres to expand adiabatically to the pressure of the atmosphere, whereupon the temperature dropped below - 200° C., and a fine, transient mist of hydrogen appeared. Olszewski confirmed Cailletet's results, and by the aid of liquid air as a cooling-agent Dewar effected complete liquefaction. He cooled the gas to -205° C. at a pressure of 180 atmospheres, and then allowed it to expand to atmospheric pressure, collecting the condensed hydrogen in a double-walled vacuum-flask, silvered to retard absorption of heat. On the initiative of Travers and of Olszewski, the principle of Linde's air-liquefier has been utilized in the construction of a machine for the liquefaction of hydrogen on the large scale, the gas being first cooled to -200° C.
In the liquid state hydrogen is colourless and transparent, and a nonconductor of electricity. Although its surface-tension is low, being 1/35 that of water and 1/5 that of liquid air, it has a distinct meniscus and drops well. It obeys the law of Dulong and Petit, its specific heat being about 6. Its atomic volume at the boiling-point is 14.3, and its density is 0.07, or 1/14 of that of water. Its density at -252.83° C. and 745.52 mm. Is 0.07105. The latent heat of evaporation at the boiling-point is 123.1 cal. Dewar gives the boiling-point at atmospheric pressure as -252.5° C., or 20.5° abs. Olszewski gives -252.6° C., and for the critical temperature -240.8° C., and for the critical pressure 13.4-15 atmospheres. The value calculated by Goldhammer for the critical density is 0.02743. Travers and Jacquerod have tabulated the values obtained by them for the vapour-pressure -
|Pressure in mm of Mercury||Absolute Temperature|
|on the Hydrogen scale||On the Helium scale|
|50|| ... ||14.11|
Hydrogen Solidification By rapidly evaporating liquid hydrogen on the pump, Dewar cooled the residual liquid so much that it solidified to an ice-like mass, frothy on the surface, at -258° C., or 15° abs. This is the triple point of hydrogen at 55 mm., and further evacuation on the pump lowered the temperature to -260° C., or 13° abs. The heat of fusion of the solid is 16 cal. It crystallizes in the cubic system, thus resembling the metals. The most probable temperature of melting is regarded by Dewar as -257° C., and by Guertler and Pirani as -259° C.
Hydrogen Occlusion by Metals, and Diffusion through them Hydrogen has the property of diffusing through certain metals, such as platinum, palladium, iron, copper, and nickel, but at ordinary temperatures the velocity of occlusion and of diffusion are so slow as to preclude the possibility of measuring the volume of gas which has penetrated the metal within a reasonable time. At low red heat, however, the rate of diffusion is measurable, as is proved by the work of several investigators.7 When an evacuated vessel or tube fitted with a plate of platinum, palladium, or iron is heated to redness in an atmosphere of hydrogen, the vacuum is destroyed owing to diffusion of the hydrogen through the heated plate. The converse phenomenon, the production of a vacuum owing to diffusion of hydrogen out of a tube, can be demonstrated by filling with hydrogen a tube similar to that just described, and heating it in air. Since the hydrogen diffuses outwards, and the air cannot diffuse inwards to replace it, a vacuum is produced in the tube. A variation of the experiment is to fill the tube with nitrogen, which is superior to air because of its inertness towards hydrogen; on heating it in an atmosphere of hydrogen, diffusion inwards causes a rise in the pressure of the gas within the tube.
Sieverts observed that at high temperatures copper-wire, iron-wire, nickel, cobalt, and platinum occlude hydrogen, but that silver does not. He found that diffusion through copper begins at 640° C., through iron at 300° C., and slowly through nickel at 450° C., but that there is no diffusion through silver at 640° C. Sieverts's results also indicate the insolubility of the gas in cadmium, thallium, zinc, lead, bismuth, tin, antimony, aluminium, gold, tantalum, and tungsten, but Heald states that most freshly precipitated metals absorb hydrogen.
Metals permitting the passage of hydrogen at a red heat, but not of other gases, may be regarded as having the character of a semi-permeable membrane. Palladium is the most permeable of these metals, Graham having found that a sheet with a thickness of 1 mm. allows 327 c.c. to pass per sq. cm. in one minute at 265° C., and 3992 c.c. at 1062° C. The fact that the velocity of diffusion, although dependent on the pressure, does not decrease proportionally with it, is cited by Winkelmann as an argument in support of his theory of the atomic condition of hydrogen occluded by palladium.
Many metals occlude hydrogen, and there is a close connexion between their power of occlusion and their magnetic properties. At the ordinary temperature, elements with a specific magnetic susceptibility exceeding 0.9×106 occlude hydrogen readily; but, as a general rule, other elements lack this capacity.
The amount of hydrogen occluded by metals depends on the pressure, and diminishes with rise of temperature. It is also affected by the physical condition and previous history of the metal. Mond, Ramsay, and Shields found that at ordinary temperature and 1 to 4.6 atm. the absorption of hydrogen by palladium-black is 873 vols., and between these limits is independent of the pressure. For spongy palladium the absorption is 852 vols.; and for palladium-foil previously heated to redness, 846 vols. Most of the occluded gas is evolved in vacuum at the ordinary temperature, and the residual 2-8 per cent, at that of boiling sulphur. At different stages of the occlusion the heat evolved is the same, being 4.370 cal. for each gram of hydrogen. At ordinary temperatures spongy platinum absorbs 110 vols, of hydrogen, variations of pressure between 0.5 and 4.5 atm. producing little effect on the amount occluded. In vacuum a part of the gas is evolved, but the bulk comes off at 250° to 300° C., and the residue at red heat. Like that of palladium, the heat of occlusion is the same at different stages of the process, and amounts to 6.880 Cal. per gram absorbed.
Hoitsema has demonstrated the reversibility of the occlusion of hydrogen by palladium, and examined the influence exerted by pressure. He found that at low pressures the concentration of the hydrogen in the metal is proportional to the square root of the pressure. The results of his experiments, carried out at 100° C., can be tabulated thus
|Pressure in mm. (p).||Vol. in c.c. of 2 mg. of Occluded Hydrogen (1/c)||sqrt(p)/c|
From these values Hoitsema postulated the monatomicity of hydrogen occluded by palladium at low pressures. For higher pressures the concentration of the occluded gas is approximately proportional to the pressure, indicating that in these circumstances the molecular formula of the occluded gas is H2. The occlusion by palladium can be represented by the equilibrium
increase of pressure altering the equilibrium to the left, and decrease to the right.
Fall of temperature is accompanied by an increase in the volume of hydrogen occluded by palladium, a phenomenon observed by Schmidt for the range 140° to 300° C.; by Paal and Amberger down to -10° C.; and by Gutbier, Gebhardt, and Ottenstein down to -50° C.
The influence of temperature on the occlusion by palladium has also been investigated by Firth. He employs the term "adsorption " to denote the surface phenomenon of rapid occlusion of hydrogen, diffusion not being a determinable factor; limits "absorption " to slow occlusion in which the rate of diffusion or solution is a determinable factor; and uses the collective term "sorption" to include both adsorption and absorption. Firth observed adsorption only below 0° C.; between 0° and 150° C. he also observed absorption; but above 150° C. found absorption only.
Palladium-black contains both amorphous and crystalline palladium, and the proportion of each constituent and the sorptive capacity of the substance vary with the conditions of preparation. At low temperatures the sorptive capacity of palladium-black depends on the temperature at which sorption begins. When a sample saturated with hydrogen at 100° C. is cooled in the gas, it sorbs more hydrogen. From 100° to 20° C. the sorptive capacity decreases slightly, and increases continuously from 20° to -190° C. By heating palladium-black it is possible to increase the proportion of the crystalline variety. The relationship between the occlusive power of palladium for hydrogen and the activity of the metal for catalytic hydrogenation has been investigated by Maxted.
The occlusion of hydrogen by palladium decreases very rapidly with rise of temperature from 100° to 600° C., more slowly up to 800° C., and only very slightly between 800° and 1500° C.
The effect of "poisons " on the occlusion of hydrogen by palladium has been studied by Maxted. Hydrogen sulphide diminishes the occluding power of the metal, each atom of sulphur rendering almost exactly four atoms of palladium incapable of occluding the gas, while the remaining palladium occludes normally, de Hemptinne5 found that carbon monoxide deprives palladium of its sorptive power for hydrogen at low temperatures. Paal and Hartmann proved that carbon monoxide inhibits the activity of palladium for the catalytic reduction of sodium picrate, and observed mercury to exert a similar effect on palladium hydrosols.
A volumetric method for the estimation of hydrogen, either alone or in gaseous mixtures, is based by Paal and Hartmann on sorption by colloidal palladium, a simple gas-pipette being employed.
The occlusion of hydrogen by various metals has been investigated by Graham, and also by Neumann and Streintz. Their results are appended in tabular form, and give the volume of hydrogen under normal conditions sorbed by one volume of the metal. It is noteworthy that their observation regarding silver is not confirmed by the more recent work of Sieverts
|Silver (wire).||0.21||Iron (reduced)||0.4-19.2|
|Aluminium (foil).||1.1-2.7||Nickel (reduced)||17.18|
|Cobalt (reduced).||59-153||Gold (leaf).||0.48|
|Copper (wire)||0.3||Gold (precipitated)||37.46|
|Copper (reduced).||0.6-4.8||Lead (fused).||0.11-0.15|