On the Relative Acidity of Hydrohalic Acids

The acidity of the series of hydrohalic acids, HF, HCl, HBr and HI rises from HF through HI, a truth that seems counterintuitive given that fluorine is the a lot of electronegative -- and iodine the least -- of the 4 halides. Several components come right into play consisting of warmth of development of the hydrohalides (ΔHfo), bond dissociation energy (BDE), electron affinity (EA) and also heat of hydration. To evaluate the family member acidity of this series, the two extreme cases will be thought about first: HF and also HI. The initially step in the analysis is to consider the stcapability of the two hydrohalides. Clbeforehand, the stcapability of HF is greater than HI as observed by their corresponding heats of development, -65 and +6 kcal/mole. The bond dissociation power of HF (+136 kcal/mol) is much higher than that of HI (+71 kcal/mole). These two factors area the atoms hydrogen and fluorine 71 kcal/mol over the Standard State while the hydrogen atom and also iodine atom are 77 kcal/mol over the Standard State. Since all 4 hydrohalides develop a hydrogen atom, we need not track its fate bereason it will certainly be the same for all 4. We look for only the loved one acidity of the hydrohalides, not the absolute worth.

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At this time we have a fluorine and also iodine atom in the gas phase. Because an aqueous acid has a negatively charged halide solvated by water, we should gain these gas phase atoms right into an aqueous setting.. The electron affinity of fluorine atom to create gas phase fluoride is -79 kcal/mol; for an iodine atom decreased to iodide, the value is -71 kcal/mol. Since fluoride is more fundamental than iodide; its heat of hydration is much even more exothermic (-121 kcal/mol) than the hydration of iodide (-71 kcal/mol). It is not legitimate to compare the energy of aqueous fluoride (-129 kcal/mol) through aqueous iodide (-65 kcal/mol) and assume, owing to the more negative worth of aqueous fluoride, that hydrofluoric acid is even more acidic than hydroiodic acid. What is even more crucial is where you start and wright here you end. The reliable comparikid is HF ---> fluoride vs. HI ---> iodide .

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HI is a stronger 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.