Acid-Base Equilibrium and pKa Values

Vocabulary flashcards covering relative acid strengths, pKa values, and guidelines for determining acidity from Table 6-1.

$$pK_a$$

A logarithmic value defined as $pK_a = -\log(K_a)$, where a less positive or more negative value indicates a stronger acid relative to another acid.

Trifluoromethanesulfonic acid

A very strong acid with a $K_a$ of $1 \times 10^{13}$ and a $pK_a$ of $-13$.

Sulfuric acid

An inorganic acid ($HO-SO_2-OH$) with a $K_a$ of $1 \times 10^9$ and a $pK_a$ of $-9$.

Hydronium ion

The species $H_3O^+$ with a $K_a$ of $55$ and a $pK_a$ of $-1.7$.

Trichloroethanoic acid (Trichloroacetic acid)

A chlorinated carboxylic acid with a $K_a$ of $0.17$ and a $pK_a$ of $0.77$.

Ethanoic acid (Acetic acid)

A carboxylic acid with a $K_a$ of $1.8 \times 10^{-5}$ and a $pK_a$ of $4.75$.

Phenol

An aromatic alcohol structure with a $K_a$ of $1 \times 10^{-10}$ and a $pK_a$ of $10.0$.

Water ($H_2O$)

A molecule with a $K_a$ of $2 \times 10^{-16}$ and a $pK_a$ of $15.7$.

Propanone (Acetone)

A ketone compound with a $K_a$ of $1 \times 10^{-20}$ and a $pK_a$ of $20$.

Ethyne (Acetylene)

An alkyne compound with a $K_a$ of $1 \times 10^{-25}$ and a $pK_a$ of $25$.

Ethane

An alkane compound ($CH_3CH_3$) with a $K_a$ of $1 \times 10^{-50}$ and a $pK_a$ of $50$.

Benzene Ring Hydrogen $pK_a$

The average hydrogen on a benzene ring starts at a $pK_a$ of about $44$ (the $pK_a$ of an $sp^2$ H) and is modified slightly by ring substituents.

Lowest $pK_a$ Selection Rule

When a single chemical structure has multiple hydrogens with different $pK_a$ values, you generally look for the LOWEST $pK_a$ on that entire structure to determine its overall acidity.

Conjugate Acid Strength Rule

To find conjugate acid strength, look at the species in the conjugate base column and apply the $pK_a$ value in that same row (e.g., benzene with a negative charge on a carbon corresponds to a $pK_a$ of $44$).