Comment on the accuracy of thermodynamic data derived from studies of gas phase ion-molecule equilibria by the third law and van't Hoff methods
✍ Scribed by Rod S. Mason; Peter D.J. Anderson
- Book ID
- 104307538
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 335 KB
- Volume
- 161
- Category
- Article
- ISSN
- 0168-1176
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✦ Synopsis
Much of the thermodynamic data in the literature, such as proton affinities, derived from studies of gas phase ion-molecule equilibria, is based on the "third law" method. The van't Hoff approach is more rigorous, but it can carry greater uncertainty (S.W. Benson, Thermochemical Kinetics, Wiley, 1976). Comparison is made here between the two methods when there is a probability of interfering reactions, which becomes greater for more complex systems at both low and high temperatures, The extrapolation procedure used in the third law method has the effect of signifcantly demagnifying the error introduced by an inaccurate measurement of the equilibrium constant, caused by competitive loss of ions or neutrals in other reactions. The point is illustrated by the results from a study of the proton transfer reactions of isomers of protonated fluorobenzaldehyde with both benzaldehyde and mesitylene. Measured equilibrium constants can be in error by a factor of five at 300 K but still give answers accurate to within 4 kJ tool q (less than 1 kcal mol-l)! Good thermodynamic data can therefore be generated even in systems in which there is significant interference and by techniques in which even the temperature is not well defined.