Question

Predict which is most likely to be the stronger acid: \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{~S}\). Explain your reasoning

Short answer

Based on the lower electronegativity and larger size of Sulfur, \(H2S\) (Hydrogen sulfide) is the stronger acid compared to \(H2O\) (Water).

Step-by-step solution

Understand Atomic Properties

First, consider the atomic properties of Oxygen and Sulfur. They are in the same group on the periodic table (Group 6 -Chalcogens), so they have similar properties. Oxygen is more electronegative (3.44) compared to Sulfur (2.58), so it will hold on to its electrons more strongly. However, Sulfur is larger and has more energy levels, which means its outer electrons are more shielded and thus can be given up more easily.

Consider Molecule Structure

Both \(H2O\) and \(H2S\) have a bent molecular geometry, with the H-O-H and H-S-H bond angles being less than 109.5 degrees. The molecular structure doesn't significantly influence the acidity here because Oxygen and Sulfur, both have two lone pairs of electrons and form two bonds with Hydrogen.

Determine Acidity

The key idea for acidity is the likelihood of a molecule to donate a proton (H+). This depends largely on the stability of the remaining ion after the proton is lost. In this case, \(H2O\) looses a proton to become \(OH^-\), and \(H2S\) looses a proton to become \(HS^-\). Because Sulfur is larger and less electronegative, the negative charge on the \(HS^-\) ion will be less concentrated and the ion is more stable. Therefore, \(H2S\) is a stronger acid than \(H2O\).

Key concepts

Periodic Table

To understand the strength of acids like water ( H_{2}O ) and hydrogen sulfide ( H_{2}S ), it's important to look at the periodic table. Both oxygen and sulfur belong to the same group in the periodic table, known as Group 6 or the Chalcogens. Elements in the same group often have similar chemical properties.

• Oxygen (O) is located at the second period, one row above sulfur.
• Sulfur (S) is located in the third period.

Being lower in the periodic table, sulfur has more energy levels than oxygen. This affects how easily each element can participate in chemical reactions. With more energy levels, the outer electrons in sulfur are further from the nucleus, making them less tightly bound and more available to participate in proton donation. This periodic trend is crucial in determining why H_{2}S is a stronger acid than H_{2}O .

Electronegativity

Electronegativity refers to an element's ability to attract electrons towards itself in a chemical bond. This property plays a significant role in determining acidity.
Oxygen is more electronegative than sulfur, with values of 3.44 for oxygen and 2.58 for sulfur on the Pauling scale. Higher electronegativity in an atom means it will hold shared electrons more tightly.

• Oxygen with higher electronegativity, holds onto its electrons more strongly, making the O-H bond less willing to break to release an H^+ ion (proton).
• Sulfur is less electronegative and does not hold its hydrogen atom as tightly, making it easier to donate a proton.

Hence, lower electronegativity in sulfur compared to oxygen supports the ease of proton donation in H_{2}S , making it a stronger acid than H_{2}O .

Molecular Geometry

The shape of a molecule, known as its molecular geometry, can influence its properties, including acidity. Both water ( H_{2}O ) and hydrogen sulfide ( H_{2}S ) have similar bent geometries due to their electron arrangement.
The central atom in both H_{2}O and H_{2}S carries two lone pairs of electrons, resulting in a bent shape with bond angles less than 109.5 degrees. This geometry inherently does not indicate a significant difference in acidity between the two molecules.
Even though H_{2}S and H_{2}O share this geometrical shape, the bent geometry's influence is secondary compared to the impact of electronegativity and atomic size differences on acidity. Therefore, the acidity is more significantly influenced by the factors of binding strength and atomic size rather than just molecular geometry alone.

Proton donation

Acidity is fundamentally tied to the ability of a molecule to donate protons ( H^+ ions). The process of a molecule donating a proton is what makes it behave as an acid.
In the case of water and hydrogen sulfide:

• Water ( H_{2}O ) donates a proton to form hydroxide ( OH^- ).
• Hydrogen sulfide ( H_{2}S ) donates a proton to form hydrosulfide ( HS^- ).

The ease of proton donation is affected by the atom's electronegativity and the stability of the resulting ion. Since sulfur is larger and less electronegative than oxygen, the negative charge in HS^- is better stabilized than in OH^- . Therefore, H_{2}S is more inclined towards donating a proton compared to H_{2}O , manifesting stronger acidic properties.

Ion Stability

After a molecule donates a proton, the stability of the resulting ion is a crucial factor in determining its acidity. A more stable ion formation means a stronger acid.
When hydrogen sulfide ( H_{2}S ) donates a proton, it forms the hydrosulfide ion ( HS^- ). Similarly, water ( H_{2}O ) forms the hydroxide ion ( OH^- ) when it donates a proton.

• The size of sulfur allows the HS^- ion to spread its charge over a larger area, reducing charge density and enhancing stability.
• On the other hand, oxygen in OH^- concentrates the charge due to its smaller size, making it less stable.

Thus, the greater ion stability of HS^- compared to OH^- is why H_{2}S has stronger acidic characteristics than H_{2}O . The ability for a larger ion to distribute and stabilize the negative charge is crucial in determining the relative strengths of acids.