On the Relative Acidity of Hydrohalic Acids

The acidity of the series of hydrohalic acids, HF, HCl, HBr and HI increases from HF through HI, a fact that appears counterintuitive offered that fluorine is the many electronegative -- and iodine the least -- of the four halides. Several factors come right into play consisting of warmth of development of the hydrohalides (ΔHfo), bond dissociation energy (BDE), electron affinity (EA) and heat of hydration. To evaluate the relative acidity of this series, the 2 too much cases will be thought about first: HF and also HI. The first step in the evaluation is to think about the stcapacity of the 2 hydrohalides. Clearly, the stcapability of HF is greater than HI as saw by their particular heats of formation, -65 and +6 kcal/mole. The bond dissociation energy of HF (+136 kcal/mol) is a lot better than that of HI (+71 kcal/mole). These two factors area the atoms hydrogen and also fluorine 71 kcal/mol over the Standard State while the hydrogen atom and iodine atom are 77 kcal/mol over the Standard State. Due to the fact that all 4 hydrohalides develop a hydrogen atom, we need not track its fate because it will be the same for all 4. We look for just the loved one acidity of the hydrohalides, not the absolute value.

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At this juncture we have actually a fluorine and also iodine atom in the gas phase. Since an aqueous acid has actually a negatively charged halide solvated by water, we need to obtain these gas phase atoms into an aqueous setting.. The electron affinity of fluorine atom to produce gas phase fluoride is -79 kcal/mol; for an iodine atom diminished to iodide, the value is -71 kcal/mol. Because fluoride is even more basic than iodide; its warmth of hydration is much more exothermic (-121 kcal/mol) than the hydration of iodide (-71 kcal/mol). It is not legitimate to compare the power of aqueous fluoride (-129 kcal/mol) through aqueous iodide (-65 kcal/mol) and assume, owing to the even more negative worth of aqueous fluoride, that hydrofluoric acid is even more acidic than hydroiodic acid. What is more vital is wbelow you begin and also wbelow you end. The effective comparichild is HF ---> fluoride vs. HI ---> iodide .

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HI is a more powerful acid than HF. The term "effective" is employed bereason the conversion of gaseous hydrogen atom to aqueous proton has actually been ignored in both analyses.