Dissociation energy pressure

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

The new dinitrogen complex [Ni(CO)3N2] can be generated in a pressure cell by UV photolysis of tetracarbonylnickel in liquid krypton, doped with N2 at 114K. The decomposition of this complex was followed over the temperature range 122-127 K and a value of the Ni—N2 bond dissociation energy estimated at lOkcal moN1.2474... 

Increased pressures in a mass spectrometer lead to ion-molecule reactions, and measurements of such equilibria can be used to derive heats of formation or dissociation energies of complex fluoro ions. Ion cyclotron resonance spectrometers are mainly employed (222). 

To be consistent the value of E2 should be corrected to constant pressure so that it represents AH for the process involved (flow system studies and static system work with excess inhibitor are essentially constant pressure experiments). Then D < E < D+RT. In the present work a reasonable estimate gives D — E—0J = 57.0 kcal.mole-1. Similarly, D2+D2 should be corrected to 0 °K, giving an estimated value of 59.0 kcal.mole-1. This gives D2 = 2.0 kcal.mole-1. Such corrections are normally within the limits of experimental error, so that experimental values of E are associated directly with dissociation energies, and thermochemical data at 25 °C are used. 

The apparent first-order rate coefficient is 1.5x 1010 exp(—28,200/RT ) sec-1. This expression has undoubtedly been obtained for a pressure-dependent region. If, as an extreme case, it is assumed that the unimolecular process occurred in the second-order region and if approximately one half of the classical degree of vibrational freedom are active, an upper limit of kx — 1.5 x 1015 exp(—46,000/Rr) sec-1 is obtained. The mean Pb-CH3 bond dissociation energy in tetramethyl lead19,142 is 37.6 kcal.mole-1. Dx should therefore be about 40 kcal.mole-1. 

Nitrogen. Molecular nitrogen N2 has a dissociation energy of 950 kJ/mol, and the N-N triple bond is one of the strongest known chemical bond. Shock-wave experiments disclosed the possibility of N-N dissociation in condensed phases [224, 319-322]. From this an interest arose in the possible obtainment of arrays of N-N single bonds that could form in potentially energetic materials. Ab initio calculations of various kinds [323-327] showed that actually at high pressure... 

Carbon Monoxide. There are close similarities between carbon monoxide and nitrogen. The molecules are isoelectronic, and the bond lengths and dissociation energies are quite comparable. The phase diagrams of the two compounds show the same trends in the moderate pressure range with a variety of phase transitions between essentially alike crystal structures [333], when allowance is made for the lack of the inversion center and the presence of a weak electric dipole moment in carbon monoxide. However, the behavior and stability at higher... 

Unfortunately, there were two errors in Heicklen and Knight s analysis. Subsequent work66 has shown that their quantum yields were too low by a factor of two. Second, at the lowest pressures used (0.6 torr) some of the excited mercury will disappear by fluorescence, thus giving lower quantum yields than would otherwise have been obtained. Both of these errors would give too large a value for E0 it should be smaller than 2.2 kcal/mole. Consequently, the double bond dissociation energy in C3Fe should be between 112.7 and 114.9 kcal/mole. 

By comparison, the bond dissociation energies of the halogens range from 151 kj/mol for I2 to 243 kj/mol for Cl2. Because of the strong H-H bond, H2 is thermally stable. Even at 2000 K, only 1 of every 2500 H2 molecules is dissociated into H atoms at 1 atm pressure. 

By far, most of the evidence for excitation upon combination comes from an analysis of the kinetics of the reaction. The rate depends upon the pressure of the third body —that is, the newly formed molecule decomposes unless it is deactivated. In theory this deactivation need not remove a large amount of energy on each collision—merely enough to put the new molecule below its dissociation energy. From that point on, the higher efficiency for deactivation versus reacti-... 

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