The thermal decomposition of water

The reflected shock tube technique with multipass absorption spectrometric detection of OH‐radicals at 308 nm, corresponding to a total path length of 1.749 m, has been used to study the reaction H 2 O + M → H + OH + M between 2196 and 2792 K using 0.3, 0.5, and 1% H 2 O, diluted in Kr. As a result of the increased sensitivity for OH‐radical detection, the existing database for this reaction could be extended downward by ∼500 K. Combining the present work with that of Homer and Hurle, the composite rate expression for water dissociation in either Ar or Kr bath gas is k 1,Ar(or Kr) = (2.43 ± 0.57) × 10 −10 exp(−47117 ± 633 K / T ) cm 3 molecule −1 s −1 over the T ‐range of 2196–3290 K. Applying the Troe factorization method to data for both forward and reverse reactions, the rate behavior could be expressed to within <±18% over the T ‐range, 300–3400 K, by the three‐parameter expression k 1,Ar = 1.007 × 10 4 T −3.322 exp(−60782 K / T ) cm 3 molecule −1 s −1 A large enhancement due to H 2 O with H 2 O collisional activation has been noted previously, and both absolute and relative data have been considered allowing us to suggest k 1 , H 2 O = 1.671 × 10 2 T −2.440 exp(−60475 K / T ) cm 3 molecule −1 s −1 for the rate constants with H 2 O bath gas over the T ‐range, 300–3400 K.