Question

Arrange each group of compounds in order of increasing acid strength. Explain your reasoning- (a) \(\mathrm{HCl}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{PH}_{3}\) (b) \(\mathrm{NH}_{3}, \mathrm{PH}_{3}, \mathrm{AsH}_{3}\) (c) \(\mathrm{HBrO}, \mathrm{HBrO}_{3}, \mathrm{HBrO}_{4}\)

Short answer

(a) \( \text{PH}_3 < \text{H}_2 \text{S} < \text{HCl} \); (b) \( \text{NH}_3 < \text{PH}_3 < \text{AsH}_3 \); (c) \( \text{HBrO} < \text{HBrO}_3 < \text{HBrO}_4 \).

Step-by-step solution

Considering Molecular Structure and Trend (Group a)

- **Compounds:** 1. \( \text{HCl} \) 2. \( \text{H}_2 \text{S} \) 3. \( \text{PH}_3 \)- **Reasoning:** Acid strength in hydrides of elements increases across a period and down a group due to increasing electronegativity and atomic size respectively.- **Order:** \( \text{PH}_3 < \text{H}_2 \text{S} < \text{HCl} \), as chlorine is more electronegative than sulfur and phosphorus.

Analyzing Trends in Hydrides of Group 15 (Group b)

- **Compounds:** 1. \( \text{NH}_3 \) 2. \( \text{PH}_3 \) 3. \( \text{AsH}_3 \)- **Reasoning:** In Group 15 (pnictogens), hydrides become weaker bases and hence stronger acids as you move down the group due to decreasing bond strength and increasing size.- **Order:** \( \text{NH}_3 < \text{PH}_3 < \text{AsH}_3 \), reflecting increasing acid strength.

Evaluating Oxoacids Based on Oxidation States (Group c)

- **Compounds:** 1. \( \text{HBrO} \) 2. \( \text{HBrO}_3 \) 3. \( \text{HBrO}_4 \)- **Reasoning:** The acid strength of oxoacids increases with higher oxidation states of the central atom due to better stabilization of the negative charge.- **Order:** \( \text{HBrO} < \text{HBrO}_3 < \text{HBrO}_4 \), as the oxidation states are 1+, 5+, and 7+ respectively, with increasing ability to delocalize negative charge.

Key concepts

Molecular Structure

Understanding the molecular structure of a compound helps to explain its properties, including acid strength. The arrangement of atoms and the type of bonds within a molecule influence how easily a compound can donate protons (H+ ions), which is a key aspect of acid strength.
For instance, molecules with polar bonds, where there is a significant difference in electronegativity between the atoms, tend to be better acids. This is because the bond becomes easier to break, facilitating the release of a proton.

• In group (a), the molecule HCl contains a highly polar bond compared to H₂S and PH₃. Hence, the proton can be more readily liberated.

Thus, understanding molecular geometry and bond polarity is vital in determining the acid strength of compounds.

Electronegativity

Electronegativity plays a crucial role in determining acid strength. It is a measure of an atom's ability to attract and hold electrons. In a chemical bond, the more electronegative an atom, the more it pulls electrons toward itself, creating a polar bond.
This polarity can enhance acid strength by making it easier for the molecule to release a proton, as seen in the following points:

• In group (a) compounds, Cl is more electronegative than S and P, making HCl a stronger acid than H₂S and PH₃.
• Electronegativity trends are observed across periods and groups in the periodic table—acid strength often increases across a period and down a group in the periodic table.

Understanding electronegativity trends helps predict whether certain compounds are likely to exhibit strong acidic behavior.

Oxoacids

Oxoacids are acids that contain oxygen, a central atom, and hydrogen. Their acid strength is significantly influenced by the oxidation state of the central atom.
Higher oxidation states often mean stronger acids because the central atom can stabilize the negative charge left on the conjugate base after the acid donates a proton.
Consider these points for a clear understanding:

• In group (c), HBrO, HBrO₃, and HBrO₄ are oxoacids where the oxidation state of bromine increases from +1 to +7.
• As the oxidation state increases, the acid strength increases due to the central atom's ability to draw electrons towards itself more effectively, thus aiding proton release.

Thus, in oxoacids, both the central atom's element and its oxidation state are critical in determining acid strength.

Oxidation States

An oxidation state or number indicates the degree of oxidation of an atom in a chemical compound. For oxoacids, as the oxidation state of the central atom increases, the acid strength usually increases.
This is because higher oxidation states often lead to greater stability of the conjugate base, which can better accommodate negative charges.
Here are a few insights:

• In the compounds HBrO, HBrO₃, and HBrO₄, the oxidation states of bromine go from +1 to +7.
• The higher the oxidation state, the stronger the pull on the oxygen's electrons, making proton release easier.

Understanding oxidation states helps in predicting the strength of an oxoacid, indicating how readily an acid can donate its hydrogen ion.

Hydrides of Group 15

The hydrides of Group 15 elements show varying acid strengths as you move down the group. Hydrides typically become weaker bases and therefore exhibit stronger acidic behavior as you move from nitrogen to arsenic.
This is attributed to their bond strength and atomic size:

• Ammonia (NH₃) has strong N-H bonds due to nitrogen's small size and high electronegativity, making it a weak acid.
• In contrast, phosphine (PH₃) and arsine (AsH₃) have weaker P-H and As-H bonds, respectively. As a result, they are stronger acids than ammonia.

As you descend the group, the atomic size increases, the bond strength decreases, leading to an easier release of protons, thus increasing the acid strength. Understanding this trend is crucial for predicting the behavior of Group 15 hydrides.