What is pka chemistry

Stabilization of a conjugate base: hybridization. Stabilization of a conjugate base: solvation. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript we've already talked about how to write an equilibrium expression so if we have some generic acid H a that donates a proton to h2o h2o becomes h3o plus and H turns into the conjugate base which is a minus and so here's our equilibrium expression and the ionization constant ka for a weak acid we already talked about the fact that it's going to be less than 1 so here we have three weak acids so hydrofluoric acid acetic acid and methanol and over here are the KA values so you can see that hydrofluoric acid has the largest KA value so even though they're all considered to be weak acids all right 3.

Ka and acid strength. When an acid dissociates, it releases a proton to make the solution acidic, but weak acids have both a dissociated state A - and undissociated state AH that coexist according to the following dissociation equilibrium equation.

The concentration ratio of both sides is constant given fixed analytical conditions and is referred to as the acid dissociation constant Ka. Ka is defined by the following equation. The square brackets indicate the concentration of respective components.

Based on this equation, Ka expresses how easily the acid releases a proton in other words, its strength as an acid. Carboxylic acids containing -COOH , such as acetic and lactic acids, normally have a Ka constant of about 10 -3 to 10 Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive.

Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows. A pKa may be a small, negative number, such as -3 or It may be a larger, positive number, such as 30 or The lower the pKa of a Bronsted acid, the more easily it gives up its proton.

The higher the pKa of a Bronsted acid, the more tightly the proton is held, and the less easily the proton is given up. The pKa scale as an index of proton availability. Some Bronsted acidic compounds; these compounds all supply protons relatively easily. For example, nitric acid and hydrochloric acid both give up their protons very easily. Nitric acid in water has a pKa of 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.

Their pKas are reported as 4. Water does not give up a proton very easily; it has a pKa of Methane is not really an acid at all, and it has an estimated pKa of about Some not-so-acidic compounds. Water is very, very weakly acidic; methane is not really acidic at all.

The pKa measures the "strength" of a Bronsted acid. A strong Bronsted acid is a compound that gives up its proton very easily. A weak Bronsted acid is one that gives up its proton with more difficulty.

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. When a compound gives up a proton, it retains the electron pair that it formerly shared with the proton.