Name: comparing acid and base strength with pka
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 35 s
Channel: JoVE

An acid protonates a water molecule to form a hydronium ion and its conjugate base. In the reverse reaction, the conjugate base accepts a proton from the hydronium ion. The extent to which an acid dissociates in water defines its strength. 
In effect, strong acids completely dissociate in water, and the equilibrium lies on the side of the products, meaning that their solutions contain a high concentration of hydronium ions.
In comparison, weak acids only partially dissociate in water, so the equilibrium favors the reactants. Therefore, their solutions mostly contain undissociated acid molecules with only a few hydronium ions. 
The degree of dissociation of a weak acid can be measured using the equilibrium constant, Keq.
In a dilute acid solution, because the change in the concentration of water is negligible, its value essentially remains constant. 
Thus, a new equilibrium constant &#8212; called the acidity constant, Ka&#8212; is defined. Note that the expression has the concentration of hydronium ions in the numerator. 
A higher Ka corresponds to a larger hydronium ion concentration, thereby indicating a stronger acid.
The values of Ka range several orders of magnitude for various organic acids.

acid and bases: ka, pka, and relative strengths

Acid and Bases: Ka, pKa, and Relative Strengths

This lesson delves into a critical aspect of the relative strengths of acids and bases. The strength of an acid is evaluated by the acid dissociation into its conjugate base and a hydronium ion in water. The complete dissociation of a strong acid is confirmed with a very high concentration of hydronium ions. As a result, an incomplete dissociation process affirms a weak acid. Therefore, the equilibrium is in the forward direction for strong acids and backward for weak acids in these reactions.
Accordingly, the acid strength is defined by the concentration of undissociated acid molecules and hydronium ions. While the weak acid can be estimated via the equilibrium constant (Keq), it is constant for a dilute solution, and the change in water concentration is negligible. This observation leads to a modified equilibrium constant known as the acidity constant or dissociation constant, Ka. To define the acidity constant that is a scale of acidity, consider the generic acid-base reaction:
<img alt="Acid-base equilibrium equation: HA + H&#8322;O &#8652; A&#8315; + H&#8323;O&#8314;, chemical reaction process." src="/files/ftp_upload/11735/Reaction%20equation-%20Generic%20acid%20dissociation%20in%20water.svg" style="text-align: center; height: 25px;" data-altupdatedat="1754904545000" data-alt="Acid dissociation equilibrium equation, HA + H2O &#8652; A&#8315; + H3O&#8314;, chemical reaction process." />
Figure 1: Dissociation of a generic acid in water
Here, HA denotes the generic acid, and A&#8722; denotes its conjugate base. The following expression represents the acidity constant for this reaction.
<img alt="Acid dissociation constant equation \(K_a = \frac{[A^-][H_3O^+]}{[HA]}\), chemistry formula." src="/files/ftp_upload/11735/Math%20equation-Acidity%20constant%20for%20generic%20acid%20dissociation.png" style="text-align: center; height: 45px;" data-altupdatedat="1754904545000" data-alt="Acid dissociation constant formula \(K_a = \frac{[A^-][H_3O^+]}{[HA]}\), equilibrium expression." />
Figure 2: Acidity constant for a generic acid dissociation
This relationship focuses on the concentration of hydronium ions in the numerator. Accordingly, an increase in these ions leads to an increasing acidity constant and a stronger acid. In organic acids, typically, the magnitude of Ka is spread across several orders. Hence, the strength of different acids is expressed in terms of pKa values, calculated as the negative logarithm of Ka:
<img alt="&#60;p&#62;pKa equation formula, mathematical expression in acid-base chemistry analysis&#60;/p&#62;" src="/files/ftp_upload/11735/Math%20equation-pKa.png" style="text-align: center;" data-altupdatedat="1754904545000" data-alt="pKa equation, pKa=-log10(Ka), formula, chemistry, acid dissociation equilibrium, calculation" />
Figure 3: Expression of pKa
Here, the minus sign indicates the inverse relationship between the pKa value and acidity. As elucidated with benzoic acid versus hydrobromic acid, a higher pKa value equals a lower Ka value, which indicates a weaker acid.
By extension of the above principle, the pKa values can also establish the strength of a base. Since the dissociation of a base forms a conjugate acid, a stronger conjugate acid corresponds to a weaker base. For instance, consider methanol versus ethylamine. The conjugate acid of methanol with a pKa value of &#8722;3.8 is more acidic than the conjugate acid of ethylamine with a pKa value of 10.6. Hence, methanol is a weaker base than ethylamine.

This lesson delves into a critical aspect of the relative strengths of acids and bases. The strength of an acid is evaluated by the acid dissociation into ...

acid-and-bases-ka-pka-and-relative-strengths

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JoVE Core

Organic Chemistry

Unit 1

Acids and Bases

KEY TERMS AND CONCEPTS

comparing acid and base strength with pka

Acid and base strength can be compared using Ka and pKa. These values help show how much an acid dissociates in water and how strong a base is through its conjugate acid.

Acid strength depends on dissociation into a conjugate base and hydronium ion in water. A strong acid dissociates almost completely, so the hydronium ion concentration is very high. A weak acid dissociates only partly, so more acid remains undissociated and the equilibrium stays farther back.

For dilute weak acids, the equilibrium constant can be simplified because the water concentration changes very little. This modified equilibrium constant is the acidity constant, or dissociation constant, Ka. For a generic acid, HA dissociates to form A− and hydronium ions, and a larger hydronium ion concentration gives a larger Ka and a stronger acid.

Because Ka values can span many orders of magnitude in organic acids, chemists often use pKa instead. pKa is the negative logarithm of Ka. A higher pKa means a lower Ka and therefore a weaker acid, as shown by the comparison between benzoic acid and hydrobromic acid.

The same idea helps compare bases. When a base dissociates, it forms a conjugate acid, and a stronger conjugate acid means a weaker base. Methanol and ethylamine show this pattern: the conjugate acid of methanol has a pKa of -3.8 and is more acidic than the conjugate acid of ethylamine, which has a pKa of 10.6, so methanol is the weaker base.

Comparing Acid and Base Strength with pKa

comparing-acid-and-base-strength-with-pka

br&#248;nsted-lowry acids and bases

Proton Transfer in Brønsted-Lowry Reactions

Brønsted-Lowry acids and bases are defined by proton transfer in a chemical reaction. In this model, a Brønsted acid donates a proton and becomes its conjugate base. A Brønsted base accepts a proton and becomes its conjugate acid. This theory was proposed in 1923 by Johannes Brønsted and Thomas Lowry. It helps explain acid-base reactions by tracking how protons move between species. The electron shifts that drive these transfers are shown with curved arrows. A reaction between acetic acid and...

lewis acids and bases

Electron-Pair Reactions in Lewis Theory

Lewis theory explains acid-base reactions through electron pairs, especially for electron-deficient compounds that do not fit the Brønsted definition. It is often used to describe group 13 elements such as boron and aluminium. These elements have three valence electrons, so their trivalent compounds can have a sextet of electrons and an empty orbital at the central atom. Because of that electron shortage, these compounds can accept electrons from another species to complete their octet.

Acid and base strength can be compared using Ka and pKa. These values help show how much an acid dissociates in water and how strong a base is through its conjugate acid. Acid strength depends on dissociation into a conjugate base and hydronium ion in water. A strong acid dissociates almost completely, so the hydronium ion concentration is very high. A weak acid dissociates only partly, so more acid remains undissociated and the equilibrium stays farther back. For dilute weak acids, the...

position of equilibrium in acid-base reactions

position-of-equilibrium-in-acid-base-reactions

Predicting Acid-Base Equilibrium Direction

Acid-base equilibrium direction depends on acid strength and pKa values. A lower pKa means a stronger acid, while a higher pKa means a weaker acid. A negative pKa shows a stronger acid, and a positive pKa shows a weaker acid. In the reaction between ammonia and an ethoxide ion, ethanol has a pKa of 15.9 and ammonia has a pKa of 38. That means ethanol is the stronger acid in this pair. A strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base, so the ethoxide ion...

molecular structure and acidity

What Makes One Acid Stronger?

Acid strength depends on how stable the conjugate base is after the acid loses a proton. When deprotonation forms a more stable anion, the acid is stronger. If the anion is unstable, the acid is weaker. Chemists compare several structure-based factors to judge that stability. One factor is size. When acids come from elements in the same column of the periodic table, atomic size is compared. A larger atom can spread out a negative charge over a bigger space. That makes the conjugate base more...

leveling effect and non-aqueous acid-base solutions

leveling-effect-and-non-aqueous-acid-base-solutions

Solvent Effects on Strong Acids and Bases

Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other. For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the...

solvating effects

How Solvation Changes Acidity

Solvation changes acidity by affecting how stable a conjugate base is. The strength of an acid is commonly compared with its pKa value. When an ion dissolves, solvent molecules surround it and help stabilize it through solvation. Solvation of ions can happen in three main ways. In donor interaction, the solvent donates unshared electron pairs to the ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In charge-dipole interaction, polar solvent molecules align their partial...

key terms and concepts

Learn how Ka and pKa show acid strength, weak versus strong dissociation, and how conjugate acids help compare base strength in high school chemistry.

Comparing Acid and Base Strength with pKa | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Comparing Acid and Base Strength with pKa

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