Enthalpy of decomposition

The enthalpy of decomposition has been determined to be approximately 10 kJ/mol (2.4 kcal/mol) of KMnO (105). The decomposition has been shown to occur ia two stages. In the first stage essentially all of the KMnO decomposes iato K2(Mn0 2 5-Mn02 accompanied by the release of oxygen in... 

Comparative studies [1028,1052,1053] of the decompositions of Ni, Co and Cu alkanoates from formate to valerate showed that both the cation present and the length of the alkane chain influenced the temperature and enthalpy of decomposition. No such relationship was found [1048], however, between chain length and temperature of reaction of a series of nickel salts between the propionate and the stearate in a study which included some qualitative identifications of the products. Mass... 

The program sets four criteria, leading to a three-level qualitative classification low risk, medium, high for each of them. Each criterion quantifies an aspect of the decomposition risk. So these four classifications need to be taken into account to arrive at a final estimation. Someworkers have tried to use a sole criterion, which mathematically combines the four criteria, but failed. Three out of these four criteria involve calculating the enthalpies of decomposition and combustion of the particular compound. In order to do so it is necessary to know the enthalpies of formation of the compound and of the decomposition and combustion products. A lot of these values are inevitably absent in Part Three, so it was thought necessary to include estimation methods for enthaipies of formation as weil as for enthalpies of vapourisation/condensation, since in many cases there is only available the value for the physical state of the compound that is not always appropriate. 

Criterion estimates the enthalpy of decomposition of the particular molecule. 

The comparison of the experimental and estimated enthalpies of decomposition according to CHETAH shows that the latter are almost always overestimated. This is why AWd is calculated using AHf of the substance in the physical state at 25°C. 

In the previous examples, the calculated enthalpies of decomposition are taken. The enthalpies of formation of the decomposition substances come from the corresponding chapters in Part Two. The published values of enthalpies of formation are favoured and use of the values estimated is only made when there is no experimental data. A few inorganic compounds have been added which are noted for their instability eg ammonium dichromate and ammonium nitrate. 

Most substances which appear in the examples of this chapter are analysed In Part Two and their enthalpy of decomposition determined experimentally. This is because most of them are considered hardly stable. This is one of the reasons for assigning no Tow risk in the suggested classifications. But it is also indisputable that criterion Cf overestimates the instability risk. It is the case for all aromatic compounds that are generally very stable. In the examples above, N-methylaniline, dichlorobenzene... 

Criterion C2 takes into account criterion C as well as the enthalpy of combustion of the compound. This criterion is the result of the observation that compounds of simiiar enthalpies of decomposition and combustion are the only ones that present a risk bonded to instability. 

The enthalpy of decomposition is now replaced by the enthalpy of reaction to analyse the potential danger. Since the danger of a chemical reaction is usually related to a modification in its procedure, which makes it uncontrollable and causes destruction of the molecular groups, it seems to make more sense to write down the most energetic reaction possible. The risk will indicate the maximum potential danger considering the stoichiometry chosen. This approach may be... 

The enthalpy of decomposition, which is rather low compared with criterion C, in the CHETAH programme (see p.117), was determined by DSC and corresponds to 0.76 KJ/g at 230-260°C. In a confined atmosphere the decomposition conditions would be explosive. This reaction has been used many times because of its spectacular aspect. This is why it is called Vesuvius fire by artificers and green volcano by lecturers. These demonstrations should be stopped because of the detonation risks they represent as well as chromium (III) oxide toxicity. 

Other sources indicate that decomposition can take place at 350 C under 170 bar and in the absence of air. In this case, the temperature reaches 1350°C and the pressure is multiplied by six. An estimation of the enthalpy of decomposition gives AH = -4.33 kJ/g. The violent pressure increase also causes the decomposition of ethylene. 

Therefore allylic alcohol is regarded as unstable. At 360-500°C, its enthalpy of decomposition is 0.67 kJ/g. This is rather low according to the CHETAH method (see para 2.3) but high according to T. Grewer, who regards a compound as unstable when IaH I > 0.5 kJ/g. 

The relative chemical inertia of the C-0 bond has already been emphasised. Structural factors can increase this reactivity significantly. This can lead to ruptures that can be dangerous in certain situations. In this case, the higher propensity of this bond to rupture makes the particular ether rather unstable. The following table gives the enthalpies of decomposition for a few ethers. 

The CgHs-N Cl- (benzenediazonium chloride) enthalpy of decomposition was calculated. Comparing its value (AH j = -1.5 kJ/g) with the CHETAH criterion C, makes it moderately stable. Accidents have been observed during reactions that are schematised below and these illustrate what has been stated. They are all due to the explosive character of the diazonium sulphides formed. Here is one example ... 

According to many authors, amines are also relatively unstable. The following table gives the enthalpies of decomposition for a few amines. 

The enthalpy of decomposition of this peracid is relatively high (AHcj = -1.83 kJ/g - average risk according to CHETAH criterion C,). Its aqueous solutions are unstable. Solutions that contain 80% of peracid detonate when they are stirred (even at -10°C). The usual way of preparing this peracid involves the effect of hydrogen peroxide when metaboric acid is present however, although this operating method was followed, serious accidents have occurred. 

The enthalpy of decomposition of fumaric acid was determined experimentally -0.925 kJ/g. This value shows its unstable nature that is due to the second carboxyl group. 

In Chapter 7, it was shown how the enthalpy of decomposition of an ammonium salt can be used to calculate the proton affinity of the anion. The proton affinity is a gas-phase property (as is electron affinity) that gives the intrinsic basidty of a species. The reaction of H+ with a base B can be shown as... 

The first aim of a thermal stability screening test (e.g., DSC/DTA) is to obtain data on the potential for exothermic decomposition and on the enthalpy of decomposition (AHd). These data, together with the initial theoretical hazard evaluation, are used in reviewing the energetic properties of the substance (Box 4) and the detonation and deflagration hazards of the substance (Boxes 7 and 8). The screening tests also provide data on the thermal stability of the substance or mixture, on the runaway potential, on the oxidation properties, and to a lesser extent, on the kinetics of the reaction (Box 10). 

In the worst case, an enthalpy of decomposition of 50 to 70 cal/g results in an adiabatic temperature rise of approximately 100 to 200°C which is, as a rule-of-thumb, not regarded as critical under the condition that die substance does not easily produce a significant quantity of gas and thus, in a very general way, will not lead to a hazardous situation [23]. However, this has to be evaluated for each individual process. 

If not, what are the important characteristics of the thermal decomposition if it should occur, such as time to onset, enthalpy of decomposition, self-heat rate, rate of pressure rise, rate of temperature rise, moles of gas/mole of substance and... 

Similar arguments can be applied for the formation of N2 and its oxides. Enthalpies of decomposition, which are estimated on the basis of the products described above, usually result in a conservative prediction. In practice, decomposition is nearly always incomplete due to the evaporation of volatile reaction products and polymerization or tar formation by heavier molecules. 

Substances with an OB of zero or more show a greater tendency to decompose into small molecules thus, the theoretical and experimental values of enthalpies of decomposition will be in greater agreement. However, if the... 

OB of a substance is less than zero (a negative value), as is the case with TBPB, the calculated enthalpy of decomposition will generally be larger than the experimental value. A literature correction for the calculated value is the following [10] ... 

None of the programs can predict kinetics, that is, the rate of reaction, the activation energy, or the order of the reaction. These parameters can only be determined experimentally. Except for CHETAH, the primary use of the programs is to compute the enthalpies of decomposition and combustion. In fact, acid-base neutralization, exothermic dilution, partial oxidation, nitration, halogenation, and other synthesis reactions are not included in the programs except for CHETAH, which can be used to calculate the thermodynamics of essentially any reaction. 

Probability Correlation. The second criterion is based on the difference between the enthalpy of combustion in excess oxygen and the maximum enthalpy of decomposition. This second criterion follows the... 

Rank Degree of Hazard Maximum Enthalpy of Decomposition/Reactiona (kcal/g) Possible Qualitative Interpretations of the Classifications... 

In Figure 2.10, substances with a high hazard potential according to the first criterion are classified as such only if a difference between the enthalpy of combustion in excess oxygen and the maximum enthalpy of decomposition is less than 3 kcal/g. The hazard potential is changed to medium if the difference is between 3 and 5 kcal/g, and drops to a low hazard potential if the difference exceeds 5 kcal/g. Substances with a medium hazard potential according to the first criterion are classified as such only if the difference is less than 5 kcal/g. The hazard potential is changed to low if the difference exceeds 5 kcal/g. 

OB and enthalpy of decomposition versus the composition of a mixture (only if two or three substances are involved). 

Assessment of results heat production/enthalpy of decomposition... 

The enthalpy of decomposition is determined by integrating the peak area (Ap) above the base line of the scan, using the following equation ... 

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