Question

Acid Strength and Molecular Structure Based on their molecular structure, pick the stronger acid from each pair of binary acids. Explain your choice. $$\begin{array}{l}{\text { a. HF and } \mathrm{HCl}} \\ {\text { b. } \mathrm{H}_{2} \mathrm{O} \text { or } \mathrm{HF}} \\ {\text { c. } \mathrm{H}_{2} \mathrm{Se} \text { or } \mathrm{H}_{2} \mathrm{S}}\end{array}$$

Short answer

a. HCl b. HF c. H2Se. Acid strength increases with weaker H-X bonds and higher electronegativity of the X atom.

Step-by-step solution

Understanding Acid Strength

The strength of a binary acid depends on the polarity of the H-X bond and the strength of the bond itself. Higher electronegativity and a weaker bond make a stronger acid because H+ is more easily released.

Comparing HF and HCl

Chlorine is less electronegative than fluorine, but the H-Cl bond is weaker than the H-F bond. Thus, HCl is a stronger acid than HF because it more readily gives up its proton.

Comparing H2O and HF

While both water and hydrofluoric acid have strong H-X bonds, the increased electronegativity of fluorine in HF makes it more polarized than water, resulting in stronger acidic character. Therefore, HF is a stronger acid than H2O.

Comparing H2Se and H2S

Selenium is one period below sulfur in the periodic table, indicating a larger atomic radius which leads to longer, weaker bonds in H2Se compared to H2S. The weaker bond in H2Se makes it a stronger acid than H2S.

Key concepts

Molecular Structure

The molecular structure of an acid significantly impacts its strength, which is a measure of its ability to donate a proton (H⁺). In binary acids, which consist of hydrogen and one other element, the arrangement of atoms and the type of bonds present influence the ease with which hydrogen ions are released.

For example, when comparing molecular structures, larger atoms tend to form longer bonds with hydrogen. These longer bonds are generally weaker, making the acid stronger because it can more readily donate a hydrogen ion. In essence, molecular structure determines the acid's propensity to release a proton, which is the hallmark of its acidic strength.

Binary Acids

Binary acids are simple compounds composed of hydrogen and one other nonmetallic element. The strength of a binary acid is not just a product of what elements it contains, but also of the properties of those elements—namely their electronegativity and size.

Typically, a binary acid with a more electronegative element will be stronger since it draws the shared electron pair in the bond closer, enhancing the bond's polarity. Additionally, if the binary acid has the hydrogen atom bonded to a larger atom, the bond length increases and the strength usually decreases, contributing to an easier release of the H⁺ ion and thereby a stronger acid.

Electronegativity

Electronegativity is a property that describes how strongly an atom attracts a pair of electrons toward itself in a bond. In the context of acids, the more electronegative the atom bonded to hydrogen is, the more it will attract the shared electrons in the bond.

This attraction increases the bond's polarity, which in turn affects the acid's strength. A higher electronegativity typically means a stronger acid because the polarized bond tends to release the hydrogen ion more easily. However, while high electronegativity favors acid strength, the presence of exceptionally strong bonds due to high electronegativity can sometimes paradoxically result in a weaker acid, as seen with hydrofluoric acid (HF), where the bond is too strong to easily release the hydrogen ion.

Bond Polarity

Bond polarity is the phenomenon where there is an unequal sharing of electrons between two atoms in a bond, resulting in a partial positive charge on one end and a partial negative charge on the other. This is particularly relevant in the case of acids, as the polarity of the bond between hydrogen and its companion atom can dictate the ease of proton donation.

The more polarized the bond, typically, the weaker it is, facilitating the release of the H⁺ ion to the solution. Therefore, in assessing the strength of acids, understanding bond polarity aids in predicting which acid will dissociate more readily in water, thus being the stronger acid. It's worth noting that although a bond's polarity contributes to acid strength, a balancing act between electronegativity and bond strength ultimately determines the acidity of a compound.