1364 PRICE THE DEUOMPOSITION OF CXXV1.-The Decomposition of Carbamide in the Presence of Nitric Acid. By TUDOR WILLIAMS PRICE. WHILE investigating the use of dilute nitric acid as a nitrating agent a t elevated temperatures it was found that nitration was entirely inhibited by the addition of carbamide although all the acid was used up. This reaction was considered worthy of further investigation the results of which are given in the present com-munication. The action of concentrated nitric acid on carbamide nitrate in the solid state at the ordinary temperature has been studied by Franchimont (Rec. trav. chim. 1884 3 216) who found that a gas was given off slowly consisting of equal parts of carbon dioxide and nitrous oxide. The volume of gas was such that it contained all the carbon of the carbamide as carbon dioxide and half the nitrogen as nitrous oxide the other half forming ammonium nitrate.No account has been found in the literature of the action of dilute nitric acid on carbamide although the action of hydro-chloric acid and sulphuric acid has been studied by Fawsitt (Zeitsch. physikal. Chem. 1902 41 601) and of hydrochloric acid by Werner (T. 1918 113 84). Fawsitt (Zoc. cit.) found the reaction between carbamide and dilute hydrochloric and sulphuric acids to be unimolecular whic CARBAMIDE IN THE PBESENCE OW NITRIC ACID. 1356 he explained by the assumption that the former is first transformed to a small extent into ammonium cyanate at a meaeurable rate, and if the inverse change is neglected according to a unimolecular reaction The ammonium cyanate is thereupon decomposed into the ammonium salt of the acid used and carbon dioxide a t a much greater rate than that a t which ammonium cyanate is formed and thus the decomposition of carbamide in the presence of hot acids is almost identical with the rate a t which it is transformed into ammonium cyanate.Werner (Zoc. cit.) on the other hand states that the above assumption is quite unnecessary and that the velocity of the reaction is regulated by the rate of dissociation of carbamide (at looo) into ammonia and cyanic acid when both products of dis-sociation are removed practically as fast as they are generated. Whichever scheme of the mechanism of the reaction is correct, the final products are the same namely carbon dioxide and the ammonium salt of the acid used.If the action of hot dilute nitric acid on carbamide is the same as that of hydrochloric acid, the reaction should be unimolecular and the products should be carbon dioxide and ammonium nitrate. Veley (Proc. Roy. SOC. 1892 52 27) has shown that nitric acid decomposes slowly a t high temperatures forming nitrous acid, the weaker the acid the higher being the temperature required for decomposition; also it is well known that carbamide decom-poses nitrous acid very readily forming carbon dioxide and nitrogen. Hence it is possible that the true explanation of the disappearance of nitric acid when heated with carbamide in solu-tion is the alternate formation of nitrous acid from nitric acid, and decomposition of the nitrous acid by carbamide.In this case, nitrogen will be present in the evolved gas as well as carbon dioxide. There are thus three separate methods of decomposition of carbamide by nitric acid the gaseous products in each case being different. An analysis of the gas produced will then be of extreme importance. As will be seen in the experimental part the reaction between carbamide and nitric acid is unimolecular and the gas evolved consists entirely of carbon dioxide. E X P E R I M E N T A L . The carbamide used was ordinary commercially pure carbamide recrystallised four times from absolute alcohol. The nitric acid solutions were made from pure distilled nitric acid and containe 1356 PRICE THE DECOMPOSITION UF only a trace of nitrous acid.A normal solation of carbamide is taken to be one containing half its molecular weight in grams in a litre of water; all the other solutions of carbamide were made up on this barris. Ten C.C. of the mixed solutions of carbamide and nitric acid were placed in hard-glass test-tubes which were then sealed and placed in water a t the required temperature. After various periods of heating the tubes were withdrawn cooled opened and their contents titrated with standard sodium hydroxide solution, using methyl-orange as indicator. The majority of the experiments were made a t looo but the reaction between N / 2-csrbamide and N / 2-nitric acid was studied a t 70° 80° 90° and looo. The influence of certain salts on the velocity of the reaction was also examined. In order to obtain figures comparable with those of Fawsitt the velocity-constants for a unimolecular reaction were calculated according to the equation instead of the correct equation, a - Xl loge - k=- 1 d 8 - t a-x, where a is the initial concentration of carbamide x1 the amount of carbamide decomposed in time t, and x2 the amount decomposed in time t,.In every case t was fixed a t sixty minutes so as t o obviate the error due to the time taken for the tube to reach the desired temperature. u zl and x2 were obtained from the titre of sodium hydroxide immediately after mixing the two solutions and the titre after times t and t,. In one case the constants for a bimolecular and termolecular reaction were calculated. Results.-The results of two experiments are given in detail, and all are summarised in table I.It will be noted that the velocity-constant tends to increase towards the end of an experi-ment and this is much more marked with 1- and 2N-solution than with the more dilute ones CARBAMI.DE TI!l T3E PRlPsENUE OF NXTRIC ACID. 1357 Ezperimcat 3. N-Curbamide + N-Nitric Acid at 100°. a = 50-40. Time in minutes. 60 120 1 80 270 360 480 600 900 1600 0 - x. 46-60 40.61 36-06 29.53 24-2 1 17-84 13.36 5.18 1-21 kl x 106 k x 108 & x l(r (mimolecular). (bmolecular). (termoleoular). - - -84 46 84 84 60 101 89 66 127 91 64 163 - - - - ( 97) ( 98) (106) (109) Mertn value b x lo6 = 87. - - - -Egperiment 5. N/ 2-Carbamide + N/ 2-Nitric Acid at looo. a = 25.43.60 22-75 -120 19.81 101 180 17.25 100 270 13.96 101 360 11.39 100 480 8-29 106 600 6-36 102 900 3.00 104 1200 1-42 107 Mean value k x 10s = 102. Time in minutes. a - z. k x 106. 1500 0.89 (169) TABLE I. 2N-N-N/2-N/2-N / 4 -NIP-N/8-N/16-N/2-N/2-N/2-N/2-N/2-N/2-N/2-Showing Vetocity-constants obtained in the Decomposition of Carbamde. Maxi-m? Dma- vane-tion of No. of Mean tion of Concentration of reaction Tern- expt. in observa- vahe of k from lnirrfure. perature. minutes. tions. k x 109 mean. W-Carbamide + 2N-HNO, ... 100’ 420 7 73 2.0 , +N-HNOs ... 100 270 6 105 4.0 , + N-HN08 100 360 5 87 4-0 , + N-HNOs 100 1800 9 88 3.0 , + NIS-HNOS ... 100 1200 9 10.2 5-0 , + N/4-HNOa ... 100 420 7 111 3.0 , + N/8-HNOa ...100 420 6 116 6.0 , + NIlG-HNOS 100 3 60 7 133 8-0 122 5.0 91 + N/2-NH,N03} loo l5o0 10 900 9 118 6-0 420 5 118 3.0 600 8 116 3-0 , + NIS-HNOI ... 100 600 5 108 2.0 -l- N/S-rnO* ... $- N/2-HNO ... ’ 9 + N/2-KW08 .. . } loo + NP-HMO ..*} 100 9’ -I- NIB-KCI ...... , + N)2-HNOS ...- 89 1890 9 23.0 2-8 , + N/2-ECNO9 ... 80 2790 9 9.3 0.7 , + N/2-HNO ... 79 9360 10 2 4 0. 1358 PBIOE THE DECOMPOSI!MON OR Experiment 17 .-Twenty-five C.C. of N-carbamide and 25 C.C. of N-nitric acid were heated a t looo in a boiling tube fitted with a reflux condenser. A delivery tube connected the top of the con-denser to a nitrometer. The total air space from the level of the liquid in the boiling tube to the nitrometer was 34 C.C. One hundred QC. of gas were collected in the nitrometer and rejected.It was considered that by this time all the air had been swept out of the apparatus and collection of the gas for analysis was com-menced. Fifty C.C. of this gas were almost entirely absorbed by a piece of moist potassium hydroxide only a minute bubble being left. The gas was thus composed entirely of carbon dioxide and did not contain any nitrous oxide or nitrogen the small residue being either air which had not been completely swept out of the apparatus or nitrogen from the trace of nitrous acid present in the nitric acid. The heating of the carbamide and nitric acid solution was con-tinued for fifty hours a t the end of which the solution was evaporated to dryness. On analysis the residue was found t o consist of ammonium nitrate with a little udchanged carbamide.Discussion of Results. The results show that the reaction between carbamide and nitric acid in dilute solution is undoubtedly unimolecular. The products consist entirely of carbon dioxide and ammonium nitrate. Hence the reaction is analogous to the decomposition of carbamide by hydrochloric or sulphuric acid and is not due to the preliminary decomposition of nitric acid into nitrous acid with subsequent decomposition of the latter by carbamide. On comparing the results with those obtained by Fawsitt for hydrochloric acid in table 11 i t will be seen that a t all dilutions the velocity of reaction is greater with nitric acid thm with hydro-chloric acid. TABLE 11. Showing Comparison hetween Velocity of Reaction at looo of Carbamide with Nitric Acid and Hydrochloric Acid.k x 109 - Remtion mixture. HNOS. Ha." - 2N-Carbamide + 2N-acid ...... 73 N- , + N- , ...... 07 50 2712- , + N12- 9 ...... 107 77 N/4- , + N/4- , ...... 111 90 Xf0- , + N/8- , ...... 116 101 Nll6- , + N/16-, ...... 133 101 * Fawsitt's results CARBAMIDE IN THE PBESENCE OF NITBIO ACED. 1369 The velocity of reaction diminishes regularly with an increase in the concentration of the nitric acid and no maximum point was found such as Fawsitt found for hydroahloric acid (Eoc. cit. p. 612). Fawsitt and Werner both state that only free carbamide is active. According to Werner the equilibrium between carbamide and its nitrate can be represented as follows: and the reaction between carbamide and nitric acid on the analogy of the reaction between carbamide and hydrochloric acid can be represented thus : Phase 1.HN:C<lH3+ HN03=NH,N0,+ (HNCO LZ HO*CN). Phase 2. (HNCO HO*CN) + H,O + HN0,=NH,N03+ CO,. The diminution of the velocity with increase in concentration of the acid can thus be explained since it is only “free” carbamide, and not carbamide “fixed” as its nitrate which takes part in Phase 1 of the reaction. Fawsitt found that the decomposition of carbamide in the presence of hydrochloric acid was retarded slightly by the addition of ammonium chloride but was accelerated in the presence of sulphuric acid by the addition of ammonium sulphate. He also found that the decomposition of carbamide in the presence of water alone was accelerated by the addition of ammonium carbonate sodium chloride and potassium chloride whilst ammonium chloride and ammonium hydrochloride had a decided retarding effect.In the presence of nitric acid the present author has found that ammonium nitrate potassium nitrate ammonium chloride, and potassium chloride all have a distinct accelerating effect on the decomposition of carbamide a t N / 2-concentration. According to the dissociation theory of the decomposition of carbamide the first action of heat on it is the production of ammonia and cyanic acid; in the presence of acids this.is followed by combination of ammonia with acid forming an ammonium salt, and by hydrolysis of cyanic acid forming an ammonium salt and carbon dioxide. The addition of the ammonium salt or of any salt containing an ion common with the ammonium salt t o the reaction mixture should therefore have a retarding effect on the velocity of decomposition.As shown by the experiments this is not the case when carbamide is decomposed in the presence of nitric acid and hence it would seem that the dissociation theory o LEWIS STUDIES IN CATALYSIS. PART XEI. the decomposition of carbamide is not applicable in the preeence of nitric acid. The great influence of temperature on the velocity will be seen from table III. TABLE 111. Showing EfJect of Temperatwe on ‘Vetocity of Reaction between. N/2-Carbamide and N/2-Nitric Acid. looo 102.0 89 23.0 80 9.3 70 2.4 Temperature. k x los. Below 80° the velocity of decomposition of carbamide is small, and a t 30° to 40° would be negligible; hence the use of excess of carbamide for the removal of nitrous acid from a mixture of nitrous acid and nitric acid will not be accompanied by loss of nitric acid as such if the temperature is not allowed to rise above 40°. In conclusion the author’s best thanks are due to Meesrs. Nobel’s Explosives Company Limited and to Mr . William Rintoul Manager of the Research Section for the facilities afforded for carrying out this work and for permission to publish the results. THE RESEARCH LABORATORIES, ARDEER . [Received November 6th 1919.