Ka and pKa review (video) | Khan Academy
For example, nitric acid and hydrochloric acid both give up their protons very easily. Nitric acid in water has a pKa of and hydrobromic acid has a pKa of - When hydrochloric acid or other acid is added to water, the pH level decreases. Another important point is the relationship between pH and the pKa of an acid. The Ka refers to the Keq for the reaction where an acid donates a proton to water; and there are several of these, e.g. phosphoric acid, boric acid, and citric acid. . The relationship between pH and weak acids is also expressed as: pH pK.
How to Convert pKa to Ka | Sciencing
Carboxylic acids containing -COOHsuch as acetic and lactic acids, normally have a Ka constant of about to 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.
In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3. This relationship is described by the following equation. This equation can be rearranged as follows. If the pH changes by 1 near the pKa value, the dissociation status of the acid changes by an extremely large amount.
pKa and Dissociation Equilibrium
When the pH is 3. Conversely, to change the pH level near the pKa value of an acid, the dissociation status of the acid must be changed significantly, which requires using an extremely large amount of acid or base. Looking at Table 7, you see that the pKa of carboxylic acids are in the range, the pKa of sulfuric acid is —10, and the pKa of water is Alkenes and alkanes, which are not acidic at all, have pKa values above The lower the pKa value, the stronger the acid.
It is important to realize that pKa is not at all the same thing as pH: Our table of pKa values will also allow us to compare the strengths of different bases by comparing the pKavalues of their conjugate acids. The key idea to remember is this: Sulfuric acid is the strongest acid on our list with a pKa value of —10, so HSO4- is the weakest conjugate base. While Table 7 provides the pKa values of only a limited number of compounds, it can be very useful as a starting point for estimating the acidity or basicity of just about any organic molecule.
pKa and Dissociation Equilibrium : SHIMADZU (Shimadzu Corporation)
Here is where your familiarity with organic functional groups will come in very handy. What, for example, is the pKaof cyclohexanol? Likewise, we can use Table 7 to predict that para-hydroxyphenyl acetaldehyde, an intermediate compound in the biosynthesis of morphine, has a pKa in the neighborhood of 10, close to that of our reference compound, phenol.
Notice in this example that we need to evaluate the potential acidity at four different locations on the molecule. Aldehyde and aromatic protons are not at all acidic pKavalues are above 40 — not on our table.
16.4: Acid Strength and the Acid Dissociation Constant (Ka)
The two protons on the carbon next to the carbonyl are slightly acidic, with pKa values around according to the table. The most acidic proton is on the phenol group, so if the compound were to be subjected to a single molar equivalent of strong base, this is the proton that would be donated.
As you continue your study of organic chemistry, it will be a very good idea to commit to memory the approximate pKa ranges of some important functional groups, including water, alcohols, phenols, ammonium, thiols, phosphates, carboxylic acids and carbons next to carbonyl groups so-called a-carbons. These are the groups that you are most likely to see acting as acids or bases in biological organic reactions.
A word of caution: If you are asked to say something about the basicity of ammonia NH3 compared to that of ethoxide ion CH3CH2O-for example, the relevant pKa values to consider are 9.