Fuel-rich flames of H2+O2+N2 at 1 bar have been burnt with a flat reaction zone and sampled into a mass spectrometer. To these flames varying, but small, amounts of ammonia were added and the corresponding ion concentrations were measured along each flame. It was found that the total concentration of positive ions was increased on addition of ammonia, suggesting additional ionisation schemes were operating alongside the mechanisms of natural flame chemi-ionisation. New ions probably result from:H + H + NH2 → NH4+ + e−which quite likely involves electronically excited species, such as H2*, inH2* + NH2 → NH4+ + e−This production of the chemi-ion NH4+ early in a flame is followed by the fast equilibria:NH3 + H3O+ ⇌ NH4+ + H2ONH4+ + H2O ⇌ NH4+·H2Oespecially downstream in the burned gas where NH3 becomes the major NHi species. Reaction (4) was analysed, assuming NH4+ to be equilibrated with H3O+. However, it is apparent that the ammonium ion is also formed by reaction (4) shifting its equilibrium position when a sample from
a flame is cooled on entering the first chamber of the mass spectrometer. Nevertheless, the equilibrium constant of reaction (4) was measured in each flame. This enabled the enthalpy change of reaction (4) to be determined as ΔH°4 = −173 ± 30 kJ mol−1, together with ΔS°4 = −5 ± 4 J mol−1 K−1, both at 298 K. Thus the proton affinity of NH3 is deduced to be 864 ± 30 kJ mol−1 at 298 K. Similarly, the enthalpy change for mono-hydration of NH4+ in reaction (12) was measured to be −88 ± 30 kJ mol−1 and ΔS°12
= −92 ± 19
J mol−1 K−1 at 298 K. The rate constant of reaction (7) is in accord with its reverse electron–ion recombination step having a rate constant of ∼3 × 10−7 ml ions−1 s−1 at flame temperatures.
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