Law of thermoneutrality

Germain Henri Hess is noted today for two fundamental principles of thermochemistry the law of constant summation of heat (known simply as Hess s law) and the law of thermoneutrality. These discoveries were remarkable in that they were postulated without any supporting theoretical framework and took place in a field of study almost totally neglected by his contemporaries. Hess s law is of immense practical importance and is used to this day to determine heats of reaction when their direct measurements are difficult or impossible. 

Hess was immediately aware of the significance of his discovery and continued his studies in the same realm. In 1842 he proposed the law of thermoneutrality, which stated that no heat is evolved by the exchange reactions of neutral salts in aqueous solutions. He was not able to fully explain these observations, and it was not until forty-five years later that the process of electrolytic dissociation was more completely elucidated by the Swedish physicist and chemist Svante Arrhenius. 

Reactions between ions and molecules occurring under the experimental conditions in a FT-ICR or FA instrument should be exothermic or thermoneutral. In both types of instrument the concentration of ions is approximately 104 times lower than the concentration of molecules. A simple pseudo-first order kinetic law is therefore observed for the ion/molecule reactions, and these proceed much faster in the gas phase than in solution. This is well demonstrated by the corresponding rate constants measured for the reaction between methyl bromide and the hydroxide ion as a function of the number of water molecules added to the OH ion and summarized in Table 2 (Bohme and Mackay, 1981). Note that the reaction in the gas phase is actually quenched by the addition of only three water molecules to OH ... 

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