Question

Why does hydrosulfuric acid, \(\mathrm{H}_{2} \mathrm{~S}\), have two acid ionization constants? Write equations that correspond to each \(K_{\mathrm{a}}\) value.

Short answer

Hydrosulfuric acid, H2S, has two acid ionization constants because it is a diprotic acid, meaning it can donate two protons (H+). The first ionization reaction is \(H2S \rightarrow H^{+} + HS^{-}\) with a corresponding Ka1. The second ionization reaction is \(HS^{-} \rightarrow H^{+} + S^{2-}\), with a corresponding Ka2.

Step-by-step solution

Understand Diprotic Acids

Diprotic acids like H2S have two replaceable hydrogen ions. The substance undergoes ionization in two steps with two different acid dissociation constants, Ka1 and Ka2.

Write the First Ionization Reaction

The first ionization of hydrosulfuric acid involves removing the first hydrogen ion, \(H^{+}\), from \(H2S\). It forms hydrosulfide ion, \(HS^{-}\). The equation can be written as: \[H2S \rightarrow H^{+} + HS^{-}.\] The corresponding acid dissociation constant is denoted as \(K_{a1}\).

Write the Second Ionization Reaction

The second ionization involves removing the second hydrogen ion from the resultant hydrosulfide ion. The equation can be written as: \[HS^{-} \rightarrow H^{+} + S^{2-}.\] The corresponding acid dissociation constant for this reaction is denoted as \(K_{a2}\).

Key concepts

Diprotic Acids

Understanding diprotic acids is crucial for comprehending acid-base reactions in chemistry. Diprotic acids are a class of acids that can donate two protons or hydrogen ions when in an aqueous solution. These acids possess two ionizable hydrogen atoms, which ionize in a stepwise manner. The sequential release of hydrogen ions is what sets diprotic acids apart from monoprotic acids, which only have one ionizable hydrogen.

When a diprotic acid dissociates in water, the first hydrogen ion is released, resulting in the formation of a conjugate base that still has another hydrogen ion to donate. This two-step dissociation process is essential for predicting the acid's reactivity and understanding the acid-base equilibria in solutions containing diprotic acids, such as hydrosulfuric acid, sulfuric acid, and carbonic acid.

Significance of Stepwise Ionization

Each ionization step has its own equilibrium constant, reflecting the strength of the acid in each dissociation phase. The first dissociation constant is typically larger, indicating a stronger acid reaction, while the second dissociation involves a weaker acid and typically has a smaller constant.

Acid Dissociation Constant

The acid dissociation constant (\(K_a\)) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for the ionization reaction of the acid in water. For a general acid dissociation reaction, where a hydrogen ion is released, the equation can be represented as \[ HA \rightarrow H^{+} + A^{-}. \] The dissociation constant expression is given by \[ K_a = \frac{[H^{+}][A^{-}]}{[HA]}. \]Where the square brackets represent the molar concentration of each species at equilibrium.

The value of the dissociation constant provides insight into the degree of ionization of the acid: a high value indicates a strong acid that fully ionizes in solution while a low value signifies a weak acid. For diprotic acids, two distinct constants, denoted as \(K_{a1}\) and \(K_{a2}\), are defined for the first and second stages of the ionization process respectively, with \( K_{a1} > K_{a2} \), reflecting the higher tendency for the first proton to dissociate.

Importance in Chemical Reactions

Understanding the \(K_a\) values is vital during titration and in buffer solutions where maintaining a stable pH is crucial. It also helps predict the direction of the chemical equilibrium in acid-base reactions.

Hydrosulfuric Acid Ionization

Hydrosulfuric acid (\(H_{2}S\)) is a diprotic acid, which means it undergoes two distinct stages of ionization in an aqueous solution. The acid ionization constants for hydrosulfuric acid represent the ionization strength of each step as the acid donates its protons.

The first ionization stage is the dissociation of one hydrogen ion, forming the bisulfide ion (\(HS^{-}\)). This reaction can be represented as \[ H_{2}S \rightarrow H^{+} + HS^{-}. \] The equilibrium expression for the first dissociation constant, \(K_{a1}\), hence signifies the extent to which this reaction occurs.

In the second stage, the bisulfide ion, having lost one hydrogen ion, can further dissociate to release the second hydrogen ion and form the sulfide ion (\(S^{2-}\)) with the reaction \[ HS^{-} \rightarrow H^{+} + S^{2-}. \] The corresponding second acid dissociation constant, \(K_{a2}\), quantifies this reaction's equilibrium. The \(K_{a2}\) value is smaller than \(K_{a1}\), indicating that the second ionization is less favorable.

Application in Chemical Analysis

Precision in understanding these two ionization steps of hydrosulfuric acid is important in analytical chemistry, especially when dealing with complex acid-base equilibria or calculating the pH of polyprotic acid solutions.