Question

Predict the relative acid strengths of the following compounds: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{~S},\) and \(\mathrm{H}_{2} \mathrm{Se}\).

Short answer

\(\mathrm{H}_2\mathrm{Se} > \mathrm{H}_2\mathrm{S} > \mathrm{H}_2\mathrm{O}\) in acid strength.

Step-by-step solution

Understand Periodic Trends

The acid strength of hydrides (such as \(\mathrm{H}_2\mathrm{O}, \mathrm{H}_2\mathrm{~S}, \text{ and } \mathrm{H}_2\mathrm{Se}\)) generally increases down a group in the periodic table. This is because larger atoms form weaker bonds with hydrogen, making it easier to release an H+ ion, which corresponds to stronger acidity.

Identify the Element Trend

Determine the order of the elements based on their position in the periodic table:- Oxygen (\(\mathrm{O}\)): Period 2- Sulfur (\(\mathrm{S}\)): Period 3- Selenium (\(\mathrm{Se}\)): Period 4All of these elements are in Group 16; thus, they are in the same group and will exhibit increasing acid strength as we move down from \(\mathrm{O}\) to \(\mathrm{Se}\).

Apply the Trend to Determine Acidity

Using the periodic trend described in Step 1, we predict the relative acid strengths:- \(\mathrm{H}_2\mathrm{O}\)is the weakest acid because \(\mathrm{O}\) is the smallest atom in Group 16.- \(\mathrm{H}_2\mathrm{~S}\)is a stronger acid than \(\mathrm{H}_2\mathrm{O}\) because \(\mathrm{S}\) is larger than \(\mathrm{O}\).- \(\mathrm{H}_2\mathrm{Se}\)is the strongest acid because \(\mathrm{Se}\) is the largest atom in this group.

Key concepts

Periodic Trends

Periodic trends help us understand how certain chemical properties change across the periodic table. These trends are crucial when examining the behavior of elements and their compounds. One important trend involves the acid strength of hydrides, which refers to compounds formed between hydrogen and another element.
Acid strength tends to increase as you move down a group in the periodic table, such as Group 16. This is due to the increasing size of atoms. Larger atoms form weaker bonds with hydrogen, allowing the easier release of a proton (\(\mathrm{H}^+\)) which results in increased acidity.

• As you move downwards, atomic size increases.
• Larger atoms result in weaker bonds with hydrogen.
• Weaker bonds enable easier ionization, enhancing acid strength.

Understanding these periodic trends enables us to predict and rationalize the behavior of various compounds, like determining why \(\mathrm{H}_2\mathrm{Se}\) is a stronger acid than \(\mathrm{H}_2\mathrm{O}\).

Hydrides

Hydrides are compounds formed by the combination of hydrogen with other elements. They exhibit varying chemical properties, including acidity, depending on their molecular structure and the atoms involved.
In the context of Group 16 elements, hydrides such as \(\mathrm{H}_2\mathrm{O}\), \(\mathrm{H}_2\mathrm{~S}\), and \(\mathrm{H}_2\mathrm{Se}\) show a clear trend in acid strength.

• \(\mathrm{H}_2\mathrm{O}\) is a weak acid because its oxygen-hydrogen bond is relatively strong, preventing the easy release of \(\mathrm{H}^+\).
• \(\mathrm{H}_2\mathrm{~S}\) is stronger than water, as sulfur's larger atomic radius weakens its bond with hydrogen.
• The strongest among these is \(\mathrm{H}_2\mathrm{Se}\), where selenium's even larger size further weakens the hydrogen bond.

Hydrides of Group 16 provide a clear example of periodic trends affecting chemical behavior, specifically acidity, by revealing how atomic size influences these properties.

Group 16 Elements

Group 16 elements, also known as the oxygen family or chalcogens, comprise oxygen (\(\mathrm{O}\)), sulfur (\(\mathrm{S}\)), selenium (\(\mathrm{Se}\)), tellurium (\(\mathrm{Te}\)), and polonium (\(\mathrm{Po}\)). These elements exhibit notable trends across the group that significantly impact their chemical behaviors.
As you progress down the group, each element typically displays increased atomic and ionic size leading to various effects, including those on their hydrides' acid strengths.

• Atomic radius increases from oxygen to polonium.
• The increase in atomic size leads to decreased bond strength with hydrogen.
• This decrease results in easier \(\mathrm{H}^+\) dissociation and hence increased acidity.

By understanding the characteristics of Group 16 elements, we can better predict and explain the variations in molecular properties, like the acid strength of hydrides, as you move down the periodic table.