Hydrogen bond
Hydrogen bonds can vary in strength from weak (1–2 kJ mol) to strong (161.5 kJ mol in the ion ). Typical enthalpies in vapor include:
Acid strength
While the p"K" value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or DMSO), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak aniline base) is measured by its Hammett acidity function, the "H" value. Although these two concepts of acid strength often amount to the same general tendency of a substance to donate a proton, the p"K" and "H" values are measures of distinct properties and may occasionally diverge. For instance, hydrogen fluoride, whether dissolved in water (p"K" = 3.2) or DMSO (p"K" = 15), has p"K" values indicating that it undergoes incomplete dissociation in these solvents, making it a weak acid. However, as the rigorously dried, neat acidic medium, hydrogen fluoride has an "H" value of –15, making it a more strongly protonating medium than 100% sulfuric acid and thus, by definition, a superacid. (To prevent ambiguity, in the rest of this article, "strong acid" will, unless otherwise stated, refer to an acid that is strong as measured by its p"K" value (p"K" < –1.74). This usage is consistent with the common parlance of most practicing chemists.)