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Fast Flow Kinetic Studies of the Reaction CH 2 OH + HCl ⇋ CH 3 OH + Cl. The Heat of Formation of Hydroxymethyl

1993; Volume: 97; Issue: 7 Linguagem: Inglês

10.1002/bbpc.19930970708

0005-9021

Sándor Dóbé, M. Otting, F. Temps, H. Gg. Wagner, H. Ziemer,

Atmospheric Ozone and Climate

Abstract The kinetics of the chlorination equilibrium of methanol, CH 2 OH + HCl ⇋ CH 3 OH + Cl (1, – 1), have been studied using the fast flow technique with Laser Magnetic Resonance and Electron Paramagnetic Resonance detection. The rate constants of the forward and reverse reactions were found to be k 1 = (2.3±0.9) ·10 11 exp [‐(20.9±2.9) kJ·mol −1 / RT ] cm 3 /mol·s in the temperature range 500 K≤ T ≤812 K and k ‐1 = (3.7±0.4)±10 13 cm 3 /mol·s at room temperature. These kinetic results were utilized in third law and second law procedures to obtain the value of the heat of formation of the hydroxymethyl radical. The entropy of CH 2 OH, S 0 f,298 (CH 2 OH) = (254±4) J/mol·K, and the activation energy of reaction (‐1), E ‐1 = (0±4) kJ/mol, which are required for the calculations, were selected by critical assessment of the available literature data. The average of the third law and second law determinations provided S f H 0 298 (CH 2 OH) = ‐(9±6) kJ/mol. This value is higher by 17 kJ/mol than the heat of formation value in common use and implies a stronger C – H bond energy in methanol, D 298 (H–CH 2 OH) = 410 kJ/mol. These new thermochemical quantities are in excellent agreement with the very recent recommendations of Seetula and Gutman [J. Phys. Chem. 96 , 5401 (1992)] and have important implications for modeling studies of complex chemical systems such as the combustion of methanol.