Problem 27 - Chemistry: An Atoms-Focused Approach

Question

Predict which acid in the following pairs of acids is the stronger acid: (a) \(\mathrm{H}_{2} \mathrm{SO}_{3}\) or \(\mathrm{H}_{2} \mathrm{SeO}_{3} ;\) (b) \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) or \(\mathrm{H}_{2} \mathrm{SeO}_{3}\)

Step-by-Step Solution

Answer

Answer: (a) \(\mathrm{H}_{2}\mathrm{SeO}_{3}\) is the stronger acid, and (b) \(\mathrm{H}_{2}\mathrm{SeO}_{4}\) is the stronger acid.

Step 1: Compare Bond Strengths

For this exercise, it is necessary to compare bond strengths of the elements involved, as weaker bonds tend to release protons more easily, making the acid stronger. We will use periodic trends, such as electronegativity and atomic size, to make our predictions. (a) Compare \(\mathrm{H}_{2}\mathrm{SO}_{3}\) and \(\mathrm{H}_{2}\mathrm{SeO}_{3}\)

Step 2: Electronegativity and Atomic Size

Considering that sulfur and selenium are in the same group on the periodic table, we can predict that as we move downwards in the group, the electronegativity will decrease and atomic size will increase. Moreover, weaker acid strength corresponds to weaker electronegativity and larger atomic size.

Step 3: Stronger Acid Prediction

Selenium has a larger atomic size and lower electronegativity than sulfur. This means that the \(\mathrm{H}_{2}\mathrm{SeO}_{3}\) will have weaker bonds and will be more likely to donate protons. Therefore, \(\mathrm{H}_{2}\mathrm{SeO}_{3}\) is likely the stronger acid. (b) Compare \(\mathrm{H}_{2}\mathrm{SeO}_{4}\) and \(\mathrm{H}_{2}\mathrm{SeO}_{3}\)

Step 4: Analyze the Number of Oxygen Atoms

Both of these acids contain the same central atom (Se), but they differ in the number of oxygen atoms bonded to it. More oxygen atoms will lead to more electron-withdrawing effects, resulting in a stronger acid.

Step 5: Stronger Acid Prediction

As \(\mathrm{H}_{2}\mathrm{SeO}_{4}\) has more oxygen atoms than \(\mathrm{H}_{2}\mathrm{SeO}_{3}\), it is expected to have a stronger electron-withdrawing effect and make it easier to donate protons. Hence, \(\mathrm{H}_{2}\mathrm{SeO}_{4}\) is likely the stronger acid. In conclusion, \(\mathrm{H}_{2}\mathrm{SeO}_{3}\) is the stronger acid out of the first pair, while \(\mathrm{H}_{2}\mathrm{SeO}_{4}\) is the stronger of the second pair.

Key Concepts

Periodic Trends

Understanding periodic trends is key to predicting many chemical properties, including acid strength. These trends refer to specific patterns observed in the periodic table that help explain variations in atomic characteristics and behavior. Breaking it down:

• Electronegativity: Generally decreases as you move down a group due to the increasing atomic size and the additional energy levels, which shield the outer electrons from the nucleus.
• Atomic Size: Increases as you go down a group because additional electron shells are added.

For acids like ( H_{2}SO_{3} ) and ( H_{2}SeO_{3} ), these trends help predict their ability to release protons, which is central to determining their strength.

Electronegativity

Electronegativity is an atom's ability to attract and hold onto electrons. In the context of acid strength, electronegativity is crucial because it impacts the atom's ability to polarize bonding electrons, influencing how easily an acid can donate a proton.
Sulfur ( S ) is more electronegative than selenium ( Se ), meaning it holds onto its electrons more tightly. As a result, acids containing sulfur generally have stronger bonds, making it harder for them to release protons. Comparatively, ( H_{2}SeO_{3} ) can more readily donate protons due to selenium's lower electronegativity, enhancing its acidity.

Atomic Size

Atomic size, often referred to as atomic radius, has significant implications for acid strength. The size of an atom affects how easy it is for an acid to release a proton.
Moving down a group in the periodic table increases atomic size. With a larger atomic size, like selenium ( Se ), the nucleus is farther from the bonding electrons, resulting in weaker bonds.
In the pair ( H_{2}SO_{3} ) vs. ( H_{2}SeO_{3} ), selenium's larger size compared to sulfur means ( H_{2}SeO_{3} ) will form weaker bonds, facilitating proton donation and thus, stronger acidity.

Bond Strength

In acids, bond strength is directly related to the acid's ability to donate protons. Weaker bonds break more easily, making an acid stronger as it can more effortlessly release protons.
The strength of a bond in an acid is influenced by factors like electronegativity and atomic size.
In the exercise at hand, selenium-based acids like ( H_{2}SeO_{3} ) demonstrate weaker bonds compared to sulfur-based acids, due to selenium's larger atomic size and lower electronegativity.

Number of Oxygen Atoms

The number of oxygen atoms bonded to the central atom of an acid greatly influences its strength. Oxygen is electronegative, pulling electrons towards itself and away from the hydrogen atoms.

• More oxygen atoms mean stronger electron-withdrawing effects.
• This enhances the acid's propensity to lose a proton, increasing its strength.

For instance, in comparing ( H_{2}SeO_{4} ) and ( H_{2}SeO_{3} ), ( H_{2}SeO_{4} ) has more oxygen atoms, leading to a stronger ability to release its protons and thus exhibits stronger acidity.