Page images
PDF
EPUB

These results show how much more stable methane is under the influence of heat than any of the other gaseous hydrocarbons studied.

It probably decomposes at first into acetylene and hydrogen, according to the equation

2CH, = C2H2+3H, and then the acetylene either polymerises or decomposes to carbon and hydrogen, according to the temperature.

These results also explain why it is that the flame of methane when burning at an open tube is practically non-luminous, as, under these conditions, the maximum temperature of the flame is below 1100° C., and no formation of acetylene takes place; whilst with increase of temperature the flame becomes rapidly more and more luminous, so that when burnt in a regenerative burner at 1500° C. the light emitted is of considerable illuminating value.

As a further step in securing factors by which to trace the decomposition, it seemed advisable to attempt to trace the action of heat upon the benzene vapour formed by the polymerisation of the acetylene; and in order to do this, pure hydrogen was allowed to pass through benzene at a known rate and a constant temperature, the amount of benzene in the gas being determined.

Table V.—The Action of Heat upon Hydrogen-borne Benzene.

Percentage of benzene in original gas ..
Temperature of gas in the decomposing tube

5.28 900° C.

5.28 1100'C.

5.28 1300° C.

Analysis of the gas after heating. Unsaturated hydrocarbons

5.00 Containing acetylene

0.00 Saturated hydrocarbons..

0.00 Hydrogen....

95.00

[blocks in formation]

Carbon
Oil..

Carbon deposited and oil forīñed in grams per 100 c.c. of gas.

0:0

trace trace 0.012

0.01 0.00

which shows that the diluted benzene breaks down to acetylene, methane, and carbon, and, finally to carbon and hydrogen.

Taking the experimental data, it seemed to show that the primary reaction on heating ethylene is the splitting up of 3 mols. into acetylene and methane,

3C,H, = 2C,H,+2CH., and that the acetylene then polymerises into higher bodies as 3C,H, = C.H.,

and that these compounds, by further polymerisation and interactions amongst themselves, of the kind studied by Berthelot and Carnelley, * give rise to a large number of others. As the temperature rises, the methane formed in the primary action splits up into acetyl. ene and hydrogen,

2CH, = C,H, +3H2; and when the temperature has reached the decomposing point of the acetylene, which varies with the degree of dilution, polymerisation takes place no longer, but the acetylene splits up directly into carbon and hydrogen, and all the products formed at lower temperatures doing the same thing, the final reaction is

C,H, = C2+2H. An attempt was now made to see how far analytical results would quantitatively bear out the inferences deduced from the foregoing experiments.

To do this, the method adopted was as follows:— The whole apparatus was filled with ethylene. The platinum tube and condensing tube were weighed, and the amount of ethylene in the gas

holder measured. The platinum tube was heated to the required temperature, and the gas aspirated through it at a uniform rate of 4.2 c.c. per minute. When sufficient gas had passed through (about 250 c.c.) the stopcocks were turned off, and the amount of gas left in the holder measured. The amount of water displaced from the aspirator was also measured, and in this manner the change in volume was determined. The tube was again weighed, and the gain in weight noted. The platinum tube was also weighed, and then slowly heated to dull redness in a stream of hydrogen, in order to drive off any

oil that might have been condensed in it, and again weighed. The tube was now heated to bright redness in a stream of oxygen, in order to burn off carbon, and again weighed. The sample tube was closed, and the contents analysed; the acetylene in the absorption bottles was also determined in the usual way.

Ethylene was first heated to 700–800° C., but no acetylene was formed, no alteration in volume took place, and the gas seemed

unacted upon.

The temperature was then raised to 800—900° C., and the following results obtained. Oil......

0·0131 gram per 100 c.c. gas.
Heavy oil.... 0.0055
Carbon ....

Nil
Decrease in volume 100 to 89
Acetylene formed. 0.057

per

cent.
* Chem. Soc. Journ.,' vol. 37, p. 701.

[blocks in formation]

0.0186 gram of oil = 17.3 c.c. of acetylene

17° C. and 760 mm. 100 vols. of gas after heating condense to 89 vols. gas and 154 vols. acetylene.

Calcalating the analysis from a percentage to a basis of 89, we getAcetylene.....

0.05 Unsaturated hydrocarbons

72:41 Saturated hydrocarbons

14:00 Hydrogen

0.00 Nitrogen...

2.76

[ocr errors]

89.22

Taking 72:41 from 98-03 = 25.62 of ethylene decomposed, and, according to the equation 3C,H, = 2C,H, + 2CH., 25.62 should give

Calculated 17 vols. of acetylene and 17 vols. of methane
Found.... 15.4

14

the discrepancy in the figures being due to interactions between the products of decomposition, and the result would certainly seem to point to the above equation as representing the initial decomposition.

The next temperature tried was 900—1000° C., and the following figures were obtained :

Rate of flow.... 42 c.c. of gas per minute.
Oil...

0·0213 gram per 100 c.c. gas.
Heavy oil

0.0080 Carbon

0·0363 Decrease in volume.. 100 to 98:6

[blocks in formation]

Explosion of the whole-.
Carbon dioxide.....

73.73 per cent.
calculated from 173.28

2 73.96

0·0363 gram of carbon = 363 c.c. of acetylene at 19° C. and 762

mm. 0·0293

gram of oil = 27.0 c.c. of acetylene at 19° C. and 762 mm. Calculating these from 100 c.c. of gas to 98-6 c.c. of gas, we get

Oil = 26.8 c.c. of acetylene at 19° C.

Carbon = 35.8 c.c. And calculating the gas analysis in the same manner as before, we obtain :Acetylene

1.17 Unsaturated hydrocarbons

15.00 Saturated hydrocarbons

39.90 Hydrogen....

40.72 Nitrogen

1.81

98.60

Then the amount of ethylene decomposed is 98:03—15:00 = 83.03, and this should give

3C,H, = 2C,H,+2CH.
83:03 55:35 55:35 calculated.

62.6 39.9 found.

Evidently, therefore, some of the methane has decomposed, forming acetylene and hydrogen. Amount of methane decomposed 55-3539:9 = 15:45,

2CH, = C,H,+3Hg.
15:45 7.72 23:16

Adding this acetylene on to that already calculated for the decomposition of ethylene, we get 55-35 +7.72 = 63:07, a figure nearly equal to the acetylene found.

Taking into consideration the complexity of the changes involved and the difficulty in obtaining great accuracy in gas analysis, these results seem to me to prove that the primary action of heat upon ethylene may be represented by the equation

3C,H, = 2C,H,+2CHA,

whilst the final decomposition is as represented by previous observers,

C,H, = C2+2H,,

and that between these two extremes there occur a large number of interactions due to the polymerisation of the acetylene formed from the ethylene, and also at higher temperatures from the methane, according to the equation

2CH, = C,H, +3H,. .

In conclusion, I desire to acknowledge my indebtedness to Mr. F. B. Grundy for the assistance he has rendered me in this investigation,

« EelmineJätka »