NH3 M - NH

This section provides a summary of four reactions in the N/H system for which new (since 1973) rate constant data are available. The reactions considered are NH3 + M NH2 -h H + M, NH3 + M - NH + H2 + M, H -h NH3 - NH2 + H2, and H + NH2 NH + H2. Reactions involving these species are relevant to the kinetics of fuel-nitrogen NO and to the removal of NO from flue gases by selective reaction with NH3 or other NH compounds. 

The thermal decomposition of ammonia has been the subject of several experimental studies, from which it is clear that the overall kinetic mechanism under the conditions used is complex and that the observed rates of decomposition of NH3 are strongly influenced by secondary reactions. Most early studies (prior to 1973 see the survey by Baulch et a/., 1973) were conducted with relatively high levels of NH3, which led to overestimates of the rate constant and underestimates of the activation energy for the unimolecular process 

Of the early studies, the most direct appears to be that of Henrici (1966) who used uv absorption to monitor NH3 decay behind incident shock waves in mixtures of 0.03-0.6% NH3 in Ar. 

More recently, there have been four shock tube studies conducted with sufficiently high dilution and sufficiently low pressure to determine the low-pressure, second-order rate constant directly. Dove and Nip (1979) studied NH3 pyrolysis behind reflected shock waves (0.14-6.0% NH3 in Kr) using mass spectrometric analysis of sampled gases to measure NH3,NH2,NH, and N2. A detailed kinetic mechanism was developed and utilized to infer k. Their results are in reasonable agreement with other recent studies, which were all conducted in NH3/Ar mixtures. 

Holzrichter and Wagner (1981) studied NH3 decomposition in argon (0.01-0.3% NH3) behind incident and reflected shocks over a sufficient concentration range (9 x 10 to 2 x lO mol cm ) to infer both the low pressure (see Table 7 and Fig. 7) and high-pressure rate constants, with 

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Solutions of different acids having the same concentration might not have the same pH. For instance, the pH of 0.10 M CH3COOH(aq) is close to 3 but that of 0.10 M HCl(aq) is close to 1. We have to conclude that the concentration of H,() ions in 0.10 M CH3COOH(aq) is lower than that in 0.10 M HCl(aq). Similarly, we find that the concentration of OH ions is lower in 0.10 M NH,(aq) than it is in 0.10 M NaOH(aq). The explanation must be that in water CH.COOH is not fully deprotonated and NH3 is not fully protonated. That is, acetic acid and ammonia are, respectively, a weak acid and a weak base. The incomplete deprotonation of CH3COOH explains why solutions of HC1 and CH3COOH with the same molarity react with a metal at different rates (Fig. 10.14). 

Precipitated silver chloride dissolves in ammonia solutions as a result of the formation of Ag(NH3)2+. What is the solubility of silver chloride in 1.0 M NH.(aq) ... 

The Henderson-Hasselbalch equation depends on the assumption [NH3] 1.8xl0 5 M [NH/]... 

Theoretical smdies on unsaturated systems such as the d moieties IMf ri -CgMes) (CO)] and [tra -M(PH3)2X] (M = Rh or Ir) show that the thermodynamics of addition of NH3 to form MiHjNH, are more favorable for the third row element due to stronger M—H and M—NH, bonding. Experimental work available to date is in harmony with these calculations and has shown that it is possible to add NH3 to displace a weakly bound ligand and form an amido-hydride. The first such reactions under mild conditions involved an lr(l) complex, as shown in Equation (6.11)." ... 

Krizanovic, O., Sabat, M., Beyerle-Pfhur, R., Lippert, B. (1993). Metal-modified nucleobase pairs Mixed adenine, fhynime complexes of tranx-a Pt" (a=NH3, CH NH ) with Wat-son-Crick and Hoogsteen orientations of the bases, J. Am. Chem. Soc., 115 5538,... 

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