Thermal Decomposition of Ammonium Chlorate
Abstract Generated abstract
The study investigates the thermal decomposition of ammonium chlorate, addressing disagreements in earlier work about product composition and reaction mechanism. Ammonium chlorate was prepared by exchange synthesis, decomposed at 80 to 95°C, and examined by kinetic weight-loss measurements, chemical analysis of condensed residues, and mass spectrometric analysis of gaseous products. The decomposition was found to be incomplete, with S-shaped kinetic curves and an effective activation energy of about 24.2 kcal/mol, while residues consisted mainly of unchanged salt with nitrate and ammonium chloride impurities. The authors identify final products including water, nitrogen, nitric oxide, nitrous oxide, oxygen, hydrogen chloride, chlorine, and nitric acid, propose several intermediate chlorine and nitrogen species, and suggest a mechanism initiated by proton transfer to form ammonia and chloric acid.
Full Text
Thermal Decomposition of Ammonium Chlorate
R. N. Isaev, O. A. Zakharov, N. M. Krasnovich, V. N. Chulov
(Presented by the scientific seminar of the Department of Radiation Chemistry)
The thermal decomposition of ammonium chlorate (AC) has been studied by a number of authors [1–4]; however, there is still no unified opinion concerning the composition of the products and the mechanism of AC decomposition. Table I gives information on the composition of the products of the thermal decomposition of AC.
Table I
Composition of decomposition products
| Literature sources | Composition of products | Residue from decomposition |
|---|---|---|
| 1 | $HNO_3,\ N_2,\ Cl_2,\ H_2O,\ HCl,\ —,\ —,\ —,\ —$ | $NH_4NO_3$ |
| 2 | $—,\ N_2,\ Cl_2,\ H_2O,\ —,\ O_2,\ —,\ —,\ —$ | — |
| 3 | $—,\ —,\ Cl_2,\ H_2O,\ —,\ O_2,\ NH_3,\ —,\ —$ | $NH_4ClO_4$ |
| 4 | $—,\ N_2,\ Cl_2,\ —,\ —,\ O_2,\ —,\ NO_2,\ ClO_2$ | $NH_4NO_3 + NH_4ClO_3$ |
Knowledge of the precise composition of the decomposition products will make it possible to construct a scheme for the thermal decomposition of AC, which is of undoubted interest for the study of the decomposition of ammonium salts of oxychlorine acids.
The present work presents the results of an investigation of the thermal decomposition of AC, mass-spectrometric analysis of the gaseous decomposition products of AC, and chemical analysis of the products in the condensed phase.
EXPERIMENTAL PROCEDURE
Ammonium chlorate was prepared by the method described in [5], by an exchange reaction:
\[ 2KClO_3+(NH_4)_2SO_4 = 2NH_4ClO_3+K_2SO_4 \tag{1} \]
followed by extraction with ethyl alcohol. AC was stored in a desiccator over \(P_2O_5\) in the dark at \(-5^\circ C\). Analysis for the content of \(ClO_3^-\) in AC was carried out by the method of [6]; the ion was determined argentometrically, \(NO_3^-\) by reaction with diphenylamine, and hypochlorite and chloride ions were determined by the method described in [7]. Mass-spectrometric analysis of the gaseous products of thermal decomposition was performed on spectrometers of the MKh-1302 type [8] (at an ionizing voltage of 70 V and a filament current of 2 mA) and MI-1305 (in this case AC decomposed inside a “hot” ion source).
The kinetics of thermal decomposition were studied on an apparatus that made it possible to record changes in weight by means of quartz spring balances of the Mac-Bain type with an accuracy up to \(10^{-5}\) g. Decomposition of 10 mg AC samples was carried out in air in the temperature range \(80\text{–}95^\circ C\) (at \(100^\circ C\) AC explodes). The activation energy was found from the dependence of the transformation coefficient on the reciprocal temperature [9]
\[ \chi = \chi_0 e^{-E/RT} \tag{2} \]
RESULTS OF THE EXPERIMENTS
In Fig. 1, kinetic curves of the thermal decomposition of AC in the range \(80\text{–}95^\circ C\) are given in coordinates of degree of decomposition—time. The decomposition curves have an S-shaped form, characteristic of most topochemical decomposition reactions. An increase in the decomposition temperature leads to a shortening of the induction period, an increase in the maximum decomposition rate, and an increase in the percentage of decomposition. The fact of incomplete decomposition of AC has also been noted by other investigators [1, 3, 4]; however, the qualitative composition of the residue from decomposition was not found by them (Table 1). The investigations we carried out showed that the main part of the residue consists of the initial salt
Fig. 1. Dependence of the rate of decomposition of ammonium chlorate on temperature.
1—80°C; 2—85°C; 3—90°C; 4—95°C.
Fig. 2. Accumulation of chlorine-containing products of spontaneous decomposition of \(NH_4ClO_3\).
Table 2.
| $m/e$ | Molecularly charged ion | Relative intensity |
|---|---|---|
| 12 | $C^{+}$ | 1 |
| 14 | $N^{+}$ | 7,6 |
| 15 | ${}^{15}N^{+},\ NH^{+}$ | 4 |
| 16 | $O^{+},\ NH_{2}^{+}$ | 78,6 |
| 17 | $OH^{+},\ NH_{3}^{+}$ | 180 |
| 18 | $H_{2}O^{+}$ | 428 |
| 28 | $N_{2}^{+},\ CO^{+}$ | 62 |
| 30 | $NO^{+}$ | 104 |
| 32 | $O_{2}^{+}$ | 19,3 |
| 35 | $Cl^{+}$ | 60 |
| 36 | $HCl^{+}$ | 51,5 |
| 37 | ${}^{37}Cl^{+}$ | 20,9 |
| 38 | $H{}^{37}Cl^{+}$ | 16,5 |
| 44 | $N_{2}O^{+},\ CO_{2}^{+}$ | 18,6 |
| 46 | $NO_{2}^{+}$ | 45,7 |
| 51 | $ClO^{+}$ | 19,3 |
| 52 | $HClO^{+}$ | 5,6 |
| 53 | ${}^{37}ClO^{+}$ | 5,7 |
| 54 | $H{}^{37}ClO^{+}$ | 1,3 |
| 63 | $HNO_{3}$ | 1 |
| 67 | $ClO_{2}^{+}$ | 25,8 |
| 69 | ${}^{37}ClO_{2}^{+}$ | 9,6 |
| 70 | $Cl_{2}^{+}$ | 100 |
| 72 | ${}^{35}Cl{}^{37}Cl$ | 57,2 |
| 74 | ${}^{37}Cl_{2}^{+}$ | 8,6 |
| 84 | $HClO_{3}^{+}$ | 22,3 |
| 86 | $H{}^{37}ClO_{3}^{+},\ Cl_{2}O^{+}$ | 15 |
| 88 | ${}^{37}Cl{}^{35}ClO^{+}$ | 1,9 |
| 90 | ${}^{37}Cl_{2}O$ | 0,45 |
| 118 | $Cl_{2}O_{3}^{+}$ | 2,7 |
| 120 | ${}^{35}Cl{}^{37}ClO_{3}^{+}$ | 1,5 |
| 122 | ${}^{37}Cl_{2}O_{3}^{+}$ | 0,71 |
with an admixture of nitrate and ammonium chloride. In addition, the composition may contain hypochlorite and ammonium chlorite, which were found by us in solid \(NH_4ClO_3\) when it was stored in air at room temperature (Fig. 2).
The effective activation energy for the decomposition of XA, calculated from equation (2), is equal to \(24.2\ \text{kcal/mol}\) and agrees with the value \([24.7]\ \text{kcal/mol}\) for the initial period of acceleration of decomposition [4].
Table 2 gives the mass spectrum of the gaseous decomposition products of \(NH_4ClO_3\) when it is decomposed in the MI-1305 ion source. The mass spectrum includes both intermediate and final products of XA decomposition. From comparison of the data in Table 2 and the results of mass-spectrometric analysis on the MX-1302, where only the final products of XA transformation are recorded, we concluded that the thermolysis products of \(NH_4ClO_3\) are: \(H_2O\), \(N_2\), \(NO\), \(N_2O\), \(O_2\), \(HCl\), \(Cl\), and \(HNO_3\). The intermediate products are: \(NH_3\), \(HClO_3\), \(ClO\), \(ClO_2\), \(Cl_2O\), \(HClO\), and \(Cl_2O_3\).
Ammonia is included among the products of XA decomposition, since in the mass spectrum there is a fragment peak with \(m/e = 15\) \((NH^+)\). Nitrogen dioxide apparently is absent, since the intensity of the molecular peak with \(m/e = 46\) is within the limits of the background peak of the same mass [10]. To identify the other nitrogen oxides as products of XA decomposition, we used the ratios of the principal and fragment peaks obtained in [11].
Chlorine oxides were assigned by us to the intermediate products, since the mass spectrum of \(HClO_3\) is unknown to us, and the intensity of its ion peaks is the sum of the intensities of the decomposition products of \(HClO_3\) and the products of its ionization.
Peaks with \(m/e = 118;\ 120\) and \(122\) were assigned by us to the compound \(Cl_2O_3\), the possibility of whose existence was first shown in work on the flash photolysis of \(ClO_2\) [12]. The authors believe that \(Cl_2O_3\) is formed as follows:
\[ ClO_2 + h\nu \longrightarrow ClO + O \tag{3} \]
\[ ClO_2 + ClO \longrightarrow Cl_2O_3 \tag{4} \]
The presence in the mass spectrum of peaks 120 and 122, due to the isotope composition of \(Cl_2O_3\), is confirmation of the possibility of formation of \(Cl_2O_3\).
DISCUSSION OF RESULTS
The data in Fig. 1 and the kinetic decomposition curves of XA show that the latter decomposes incompletely. This fact is also noted by other authors [1, 3, 4].
The incomplete decomposition of XA and the identity, in terms of composition, of the residue mass with the original salt make the thermal decomposition of XA similar to the decomposition of \(NH_4ClO_4\), in which, in the region of low temperatures, only 30% of the salt decomposes; this can apparently be explained by the following analogy.
First, what occurs primarily is decomposition of the intermolecular substance binding the elements of the mosaic structure of the crystal [13]; second, oxidation of ammonia may occur, which can suppress the reaction centers of decomposition [14]; and third, more thermostable products—nitrate and ammonium chloride—accumulate.
Seibrother [1] believes that the initial stage of decomposition of XA is the autooxidation of the \(NH_4ClO_4\) molecule with formation of nitric acid, nitrogen, chlorine, and water. The \(HNO_3\) formed reacts with chlorate and gives \(HCl\) and chloric acid, which oxidizes ammonia radicals with formation of \(HNO_3\) and \(Cl_2\).
Pavlyuchenko [2] and Ponyushkin [4] consider that the initial stage of decomposition of XA is electron transfer from the anion to the cation:
\[ NH_4^{+} + ClO_3^{-} \longrightarrow NH_4 + ClO_3, \tag{5} \]
Then, as Pavlyuchenko assumes, the radical–chain mechanism develops as follows:
\[ NH_4 \longrightarrow NH_3 + H \tag{6} \]
\[ H + ClO_3^{-} \longrightarrow OH + ClO_2^{-} \tag{7} \]
\[ ClO_3^{-} \longrightarrow ClO_3 + e \tag{8} \]
\[ NH_4^{+} + e \longrightarrow NH_4\ \text{and etc.} \tag{9} \]
The results of our studies make it possible to propose a different scheme for the decomposition of XA. The presence of \(NH_3\) and \(HClO_3\) in the products admits, as the first stage, proton transfer:
\[ NH_4^{+} + ClO_3^{-} \rightleftarrows NH_3 + HClO_3. \tag{10} \]
The molecules formed are partially desorbed from the surface and, in the cold parts of the reaction vessel, form a sublimate. \(HClO_3\) is rather unstable and therefore decomposes, possibly according to the scheme:
\[ HOClO_2 \longrightarrow ClO_2 + OH , \tag{11} \]
\[ ClO_2 \longrightarrow ClO + O \tag{12} \]
\[ ClO \longrightarrow Cl + O \tag{13} \]
\[ 2OHClO + M \longrightarrow M + H_2O(Cl_2O_2). \]
The decomposition products of \(HClO_3\) oxidize ammonia with the formation of final products. Since all the indicated processes [10–13] occur in the bulk of the crystal or on its surface, it may be assumed that the decomposition processes of \(HClO_3\) and the oxidation of \(NH_3\) proceed heterogeneously. It is very probable that the decomposition products of \(HClO_3\) may enter into interaction with an \(NH_4ClO_3\) molecule. The result of such interaction may possibly be compounds such as \(Cl_2O\), \(Cl_2O_3\), \(HClO\), \(HClO_2\).
The proposed scheme for the thermal decomposition of \(NH_4ClO_3\), allowing a deep analogy with the decomposition scheme of \(NH_4ClO_4\), may serve as a basis for the experimental study of the thermal decomposition of AC.
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