Question

Rank the following hydrides in order of increasing acid strength: \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{Te} .\)

Short answer

The order is: \(H_2O \)< \(H_2S\)< \(H_2Te\).

Step-by-step solution

- Understand Acid Strength and Bond Polarity

Acid strength in hydrides generally depends on the bond polarity and the bond strength in the molecule. As we move down a group in the periodic table, the bond strength decreases and bond polarity increases, usually leading to stronger acids.

- Analyze the Elements Involved

The hydrides given are \(H_2S\), \(H_2O\), and \(H_2Te\). In the periodic table, sulfur (S), oxygen (O), and tellurium (Te) are in Group 16. The order from top to bottom in the group is O, S, Te.

- Determine Bond Strength

As we go down the group, the bond strength between hydrogen and the element decreases. However, a weaker bond makes it easier for the molecule to dissociate and release \(H^+\) ions. Therefore, the bond strength decreases in the order: \(H_2O > H_2S > H_2Te\).

- Determine Bond Polarity

Polarity also affects acid strength. More polar bonds make stronger acids because they can more easily release \(H^+\) ions. Polarity increases as we go from \(H_2Te\) to \(H_2S\) to \(H_2O\).

- Rank the Hydrides

Combining both factors, we find that bond strength has a more pronounced effect. Therefore, the order of increasing acid strength is: \(H_2O \)< \(H_2S\)< \(H_2Te\).

Key concepts

bond polarity

Bond polarity refers to the distribution of electrical charge between two atoms in a bond. In a polar bond, one atom has a partial negative charge, and the other has a partial positive charge. This happens because one atom attracts the shared electrons more strongly due to its higher electronegativity.
For hydrides like \(H_2O\), \(H_2S\), and \(H_2Te\), bond polarity is essential in determining their acid strength.

• Oxygen (O) is more electronegative than Sulfur (S) and Tellurium (Te).
• This makes the O-H bond in water (\( H_2O \)) more polar than the S-H bond in hydrogen sulfide (\( H_2S \)), and much more so than the Te-H bond in hydrogen telluride (\( H_2Te \)).

Stronger polar bonds can release \(H^+\) ions more quickly, affecting acid strength. However, in hydrides, bond strength often plays a more significant role than bond polarity in determining overall acid strength.

periodic table trends

Periodic table trends are essential for understanding various properties of elements, including acid strength. The key trends affecting the hydrides of Group 16 elements (O, S, Te) are:

• **Electronegativity**: Decreases down the group (O > S > Te). This affects bond polarity.
• **Bond Strength**: This usually decreases as we move down a group. Elements lower down form weaker bonds with hydrogen because of the increasing size of the atoms.
• **Atomic Size**: Increases as we move down the group, making the bond length longer and weaker.

These trends explain why \(H_2Te\) has the highest acid strength among the hydrides mentioned, despite having the least polar bond. The weaker the bond, the easier it is to dissociate, making the compound more acidic.

bond strength

Bond strength is a measure of how strongly two atoms are held together in a bond. In the context of hydrides, this greatly influences their acid strength. Some crucial points are:

• **Weaker Bonds Lead to Stronger Acids**: A weaker bond means that it is easier to break, allowing the hydrogen ions (\(H^+\)) to dissociate more readily, which increases acidity.
• **Periodic Trends**: As you move down Group 16 in the periodic table, the bond strength between the hydrogens and the group elements decreases: \(H_2O > H_2S > H_2Te\).
• **Consequences**: \(H_2Te\) has the lowest bond strength, making the hydrogen telluride molecule the easiest to dissociate and therefore the strongest acid among the three.

Because bond strength decreases more significantly than changes in bond polarity as you move down the group, it becomes the dominant factor in determining the acid strength of these hydrides.