Question

Choose the stronger acid and give your reason: (a) HOCl or \(\mathrm{HClO}_{2}\), (b) \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) or \(\mathrm{H}_{2} \mathrm{SeO}_{3}\)

Short answer

\(\mathrm{HClO}_{2}\) is stronger than HOCl because it has more oxygen atoms. \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) is stronger than \(\mathrm{H}_{2} \mathrm{SeO}_{3}\) also due to having more oxygen atoms.

Step-by-step solution

Comparing Acid Strength Based on Structure for Pair (a)

To compare which acid is stronger between HOCl and \(\mathrm{HClO}_{2}\), one should look at the oxyacids' structures. A key rule is that for oxyacids with the same central atom but a different number of oxygen atoms, the more oxygens, the stronger the acid. This is because additional oxygen atoms draw more electron density away from the O-H bond, making the proton release (acid dissociation) easier due to increased electronegativity.

Determining the Stronger Acid for Pair (a)

Since \(\mathrm{HClO}_{2}\) has more oxygen atoms than HOCl, it is the stronger acid.

Comparing Acid Strength Based on Structure for Pair (b)

For the pair \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) and \(\mathrm{H}_{2} \mathrm{SeO}_{3}\), we also use the rule that more oxygen atoms result in a stronger acid. Therefore, we need to compare the number of oxygen atoms in each acid.

Determining the Stronger Acid for Pair (b)

\(\mathrm{H}_{2} \mathrm{SeO}_{4}\) has one more oxygen atom than \(\mathrm{H}_{2} \mathrm{SeO}_{3}\), which makes \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) the stronger acid.

Key concepts

Oxyacid Properties

Oxyacids are acids that contain hydrogen, oxygen, and another nonmetal element. They are characterized by having at least one hydrogen atom bonded to an oxygen atom, which in turn is bonded to another element, commonly referred to as the central atom. The properties of oxyacids are largely determined by the electronegativity of the central atom and the number of oxygen atoms present. More electronegative central atoms and a greater number of oxygen atoms can lead to stronger acids. This is because such structures favor the loss of the hydrogen ion, a process pivotal to an acid's strength.

Educationally, it's beneficial to understand that as the number of oxygen atoms increases, the ability of the central atom to hold on to the extra negative charges also increases. This, in turn, makes the hydrogen-oxygen bond weaker, allowing for easier release of the hydrogen proton (H+), which is a defining characteristic of an acid.

Acid Dissociation

Acid dissociation is the process by which an acid releases hydrogen ions (H+) into a solution. The strength of an acid is often measured by its dissociation constant (Ka), which indicates the extent to which an acid can donate its hydrogen ions in an aqueous solution. Higher Ka values correspond to stronger acids because they indicate a greater degree of dissociation. Simplifying, the more willingly an acid parts with its hydrogen ion, the more potent it is. When comparing similar types of acids, it's useful for students to recognize this mechanism as it directly affects an acid's reactivity and its interactions within a chemical environment.

For instance, when looking at oxyacids, the ease of acid dissociation increases as the number of oxygen atoms around the central nonmetal atom increases, resulting in a stronger acid. This is a crucial takeaway for those grappling with acid-base chemistry concepts.

Electron Negativity

Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself within a covalent bond. In the context of acid strength, the electronegativity of atoms involved in an acid’s structure can greatly influence its behavior. The more electronegative atoms present, the more they will attract electron density, weakening certain bonds and therefore facilitating the release of the H+ ion.

As students progress in chemistry, they learn that atoms with higher electronegativity within an oxyacid, such as oxygen, influence the acid’s ability to dissociate. For example, within oxyacids, the electron-withdrawing effect of additional oxygen atoms can lead to an increase in acid strength by destabilizing the bond between hydrogen and oxygen, making the acid more prone to losing a hydrogen ion.

Acid Strength Rules

Several rules help predict the relative strength of acids. One significant rule is that within a series of oxyacids, those with more oxygen atoms are stronger because each additional oxygen atom draws more electron density away from the acidic hydrogen-oxygen bond. This facilitates easier ionization of the hydrogen ion and thus a stronger acid.

Another rule to consider is the electronegativity of the central atom: the more electronegative it is, the stronger the acid, as the central atom's ability to attract electrons can also affect the strength of the O-H bond. Furthermore, the position of the atoms can influence strength; for example, the hydrogens attached to oxygen in carboxylic acids result in stronger acids due to resonance stabilization of the carboxylate group. Engaging with these fundamental rules enriches students' understanding of acid chemistry and equips them with the tools to anticipate the behavior of acids in various chemical contexts.