Question

Choose the weaker acid in each of the following pairs: (a) HBr or \(\mathrm{H}_{2} \mathrm{Se}\) (b) \(\mathrm{HClO}_{4}\) or \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{H}_{2} \mathrm{SO}_{3}\) or \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Short answer

(a) \(\text{H}_{2} \text{Se}\), (b) \(\text{H}_{2} \text{SO}_{4}\), (c) \(\text{H}_{2} \text{SO}_{3}\)

Step-by-step solution

Determine the weaker acid in pair (a)

Compare HBr and \(\text{H}_{2} \text{Se}\). HBr is a stronger acid because Br is more electronegative than Se, leading to a stronger bond with hydrogen and easier dissociation of ions. Therefore, \(\text{H}_{2} \text{Se}\) is the weaker acid.

Determine the weaker acid in pair (b)

Compare \(\text{HClO}_{4}\) and \(\text{H}_{2} \text{SO}_{4}\). \(\text{HClO}_{4}\) is a stronger acid because it is a perchloric acid, which is one of the strongest known acids. Consequently, \(\text{H}_{2} \text{SO}_{4}\) is the weaker acid.

Determine the weaker acid in pair (c)

Compare \(\text{H}_{2} \text{SO}_{3}\) and \(\text{H}_{2} \text{SO}_{4}\). \(\text{H}_{2} \text{SO}_{4}\) is a stronger acid, as it completely dissociates in water and is a strong acid. Therefore, \(\text{H}_{2} \text{SO}_{3}\) is the weaker acid.

Key concepts

Acid Dissociation

Acid dissociation refers to the process where an acid breaks apart (or dissociates) in water to form ions. This is a crucial concept in understanding acid strength. When we dissolve an acid in water, it releases hydrogen ions (H⁺). For instance, when hydrochloric acid (HCl) dissociates in water, it produces H⁺ and Cl⁻ ions.

Dissociation can be represented by the equation:
\[\text{HA} \rightarrow \text{H}^+ + \text{A}^-\text{ (where \text{HA} is the acid)}\]

Strong acids dissociate completely, meaning they release a high concentration of H⁺ ions. Weak acids do not fully dissociate and thus, have a lower concentration of H⁺ in solution.

Understanding the degree of dissociation is vital. It allows us to identify and compare the strength of different acids, as seen in the exercise.

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself. This concept plays a significant role in determining acid strength. In a molecule, the more electronegative an element is, the more strongly it pulls electrons towards itself.

For example, in the case of HBr versus \[\text{H}_{2}\text{Se}\text{,}\] we find that the bromine (Br) atom is more electronegative than the selenium (Se) atom. This higher electronegativity means that Br attracts electrons more effectively, leading to a stronger and more stable negative ion when dissociation occurs.

Electronegativity also affects bond strength. A higher electronegativity difference between the hydrogen and another atom generally results in a weaker bond that dissociates more easily, increasing the acid strength. This understanding helps in predicting the strength of acids based on their elemental composition.

Strong Acids Versus Weak Acids

One of the core concepts in acid strength comparison is understanding the difference between strong acids and weak acids. Strong acids are those that completely dissociate in water. In other words, they release all their available hydrogen ions (H⁺). Common examples include hydrochloric acid (HCl) and sulfuric acid (\(\text{H}_{2}{\text{SO}_{4}}\) ).

Weak acids, on the other hand, only partially dissociate in water. This means only a fraction of the acid molecules release H⁺ ions. Examples of weak acids include acetic acid (\(\text{CH}_{3}{\text{COOH}}\) ) and sulfurous acid (\(\text{H}_{2}{\text{SO}_{3}}\) ).

The strength of an acid can be assessed by its dissociation constant (\text{K}_{a}). A higher \text{K}_{a} value indicates a stronger acid.

In the exercise, comparing acids like HBr and \[\text{H}_{2}{\text{Se}}\] or \[\text{HClO}_{4}\] and \[\text{H}_{2}{\text{SO}_{4}}\], the concepts of dissociation and electronegativity help determine why one acid is stronger or weaker than the other.