Name: predicting acid-base equilibrium direction
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 5 s
Channel: JoVE

Consider a reaction between ammonia and an ethoxide ion. Ammonia loses its proton to the ethoxide ion, forming an amide ion and ethanol.
The reaction, at equilibrium, has two acids and two bases. The stronger acid and the stronger base, along with the position of equilibrium, are identified by comparing the pKa values of each acid.
Here, ethanol with a pKa of 15.9 is a stronger acid than ammonia, whose pKa is 38. Because a strong acid results in a weak conjugate base, and conversely, a weak acid provides a strong conjugate base, the ethoxide ion is a weaker base than the amide ion. 
Fundamentally, any equilibrium controlled acid&#8211;base reaction favors the formation of more stable species &#8212; the weaker acid and the weaker base &#8212; owing to their lower potential energies.
This explains why, in this reaction, the position of equilibrium lies to the side of the ethoxide ion and ammonia.
Furthermore, in any reaction, if the difference in the pKa values of the acids is large, the reversible reaction is neglected, and the equilibrium arrows are replaced by an irreversible arrow.
The position of equilibrium in an acid&#8212;base reaction can also be estimated from the pKa values of the acids by calcul

position of equilibrium in acid-base reactions

Position of Equilibrium in Acid-Base Reactions

In any solution, the value of pKa indicates whether an acid is completely dissociated or not. A negative pKa corresponds to a stronger acid, whereas a positive pKa corresponds to a weaker acid. Consider the reaction between ammonia and an ethoxide ion. In this reaction, ethanol with a pKa of 15.9 is a stronger acid than ammonia with a pKa of 38. Recall that the strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base. Hence, the ethoxide ion is a weak base.
<img alt="Acid-base reaction diagram; ammonia, hydrogen chloride forming ammonium, chloride; molecular equilibrium." src="/files/ftp_upload/11736/11736_PT_figure%201.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Eq1" />
Figure 1. Acid&#8211;base reaction between ammonia and an ethoxide ion
In an equilibrium-controlled acid&#8211;base reaction, the equilibrium position always favours the formation of the weaker acid and the weaker base. This is because the weaker acid and the weaker base are the most stable species due to their lower potential energies. Therefore, in this reaction, the equilibrium favours the formation of the ethoxide ion and ammonia.
Additionally, the position of the equilibrium can also be identified using the equilibrium constant that is calculated from pKa values. For an acid&#8211;base reaction, the equilibrium constant, Keq, can be calculated by subtracting the pKa value of the acid on the left side from the pKa value of the acid on the right side and then taking the antilog of the result. Depending on the value of the equilibrium constant, the position of the equilibrium is determined. When the value of Keq is greater than 1, the position of the equilibrium lies far to the product side. On the other hand, with the equilibrium constant smaller than 1, the equilibrium lies far to the reactant side.

In any solution, the value of pKa indicates whether an acid is completely dissociated or not. A negative pKa corresponds to a stronger acid, whereas a ...

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

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Organic Chemistry

Unit 1

Acids and Bases

KEY TERMS AND CONCEPTS

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 is a weak base.

For an equilibrium-controlled acid-base reaction, the equilibrium position favors the weaker acid and the weaker base. These species are more stable because they have lower potential energy. In this example, the equilibrium therefore favors ethoxide ion and ammonia.

The equilibrium position can also be predicted from pKa values by calculating Keq. To do this, subtract the pKa of the acid on the left from the pKa of the acid on the right, and then take the antilog of that result. If Keq is greater than 1, the equilibrium lies far toward the products. If Keq is smaller than 1, the equilibrium lies far toward the reactants.

Predicting Acid-Base Equilibrium Direction

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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...

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...

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How Solvation Changes Acidity

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key terms and concepts

Learn how pKa values and Keq predict acid-base equilibrium direction, including why weaker acids and weaker bases are favored in equilibrium.

Predicting Acid-Base Equilibrium Direction | JoVE K12

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Predicting Acid-Base Equilibrium Direction

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