This idea is also true when considering the opposite: a base picking up a proton to form a conjugate acid. Is that a very, very, very, very weak acid? How many "verys" are there in a pKa unit? If something with a pKa of 4 is described as a weak acid, what is something with a pKa of 25? A very, very weak acid? It is certainly a better source of protons than something with a pKa of 35. pKa values that we have seen range from -5 to 50. However, the terms "strong" and "weak" are really relative. This term is often used to describe common acids such as acetic acid and hydrofluoric acid."Weak" Bronsted acids do not ionize as easily.This term is usually used to describe common acids such as sulfuric acid and hydrobromic acid."Strong" Bronsted acids ionize easily to provide H +.The pKa scale and its effect on conjugate bases.
The compound remains a Bronsted acid rather than ionizing and becoming the strong conjugate base.
Effectively, the strong base competes so well for the proton that the compound remains protonated. In a similar way, if a compound gives up a proton and becomes a strong base, the base will readily take the proton back again. It is not good at donating its electron pair to a proton. The Bronsted base does not easily form a bond to the proton.
Looked at another way, a strong Bronsted acid gives up a proton easily, becoming a weak Bronsted base. When a compound gives up a proton, it retains the electron pair that it formerly shared with the proton. Going to a farther extreme, a compound from which it is very, very difficult to remove a proton is not considered to be an acid at all. A strong Bronsted acid is a compound that gives up its proton very easily.Ī weak Bronsted acid is one that gives up its proton with more difficulty. A proton, H +, is a strong Lewis acid it attracts electron pairs very effectively, so much so that it is almost always attached to an electron donor. The pKa measures the "strength" of a Bronsted acid. Water is very, very weakly acidic methane is not really acidic at all. Methane is not really an acid at all, and it has an estimated pKa of about 50.įigure AB9.4. Water does not give up a proton very easily it has a pKa of 15.7. Their pKas are reported as 4.76 and 3.77, respectively. On the other hand, acetic acid (found in vinegar) and formic acid (the irritant in ant and bee stings) will also give up protons, but hold them a little more tightly. Nitric acid in water has a pKa of -1.3 and hydrobromic acid has a pKa of -9.0. Some Bronsted acidic compounds these compounds all supply protons relatively easily.įor example, nitric acid and hydrochloric acid both give up their protons very easily.