Question

Based on their compositions and structures and on conjugate acid-base relationships, select the stronger base in each of the following pairs: (a) \(\mathrm{BrO}^{-}\) or \(\mathrm{ClO}^{-},(\mathbf{b}) \mathrm{BrO}^{-}\) or \(\mathrm{BrO}_{2}^{-}\) (c) \(\mathrm{HPO}_{4}^{2-}\) or \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}.\)

Short answer

Based on their compositions and structures and on conjugate acid-base relationships, the stronger bases in each pair are: (a) ClO⁻, (b) BrO⁻, and (c) HPO₄²⁻.

Step-by-step solution

Understand the concept of conjugate acid-base pairs

A conjugate acid-base pair consists of two species that differ by the presence of a proton (H+ ion). The species which can donate a proton is known as the conjugate acid and the species which can accept a proton is called the conjugate base. When comparing the basic strength of two given species, the stronger base will be the one that can donate its proton more readily.

Analyze the basic strength of (a) BrO⁻ or ClO⁻

First, let's compare the basic strength of BrO⁻ and ClO⁻. Both of these species are in the same period and have the same charge. The only difference is in the central atom, where the Br atom is larger and less electronegative than the Cl atom. As a result, the negative charge on the BrO⁻ ion is more dispersed over a larger atom, making it less reactive, thus the BrO⁻ ion is a weaker base. In comparison, ClO⁻ has a more localized negative charge on the smaller, more electronegative Cl atom, which makes it a stronger base. Therefore, the stronger base is ClO⁻.

Analyze the basic strength of (b) BrO⁻ or BrO₂⁻

Next, let's compare the basic strength of BrO⁻ and BrO₂⁻. Both of these species have the same central atom, bromine. However, they differ in the number of oxygen atoms present. The BrO₂⁻ ion has one more oxygen atom than BrO⁻, which spreads the negative charge over more atoms. This makes BrO₂⁻ less reactive and hence a weaker base compared to BrO⁻. So, the stronger base is BrO⁻.

Analyze the basic strength of (c) HPO₄²⁻ or H₂PO₄⁻

Finally, let's compare the basic strength of HPO₄²⁻ and H₂PO₄⁻. Both of these species have the same central atom, phosphorus, and the same composition in terms of P, H, and O atoms. However, they differ in the degree of protonation (number of hydrogen ions attached). H₂PO₄⁻ has one more H+ ion than HPO₄²⁻, which means HPO₄²⁻ has a greater negative charge associated with it. This greater charge on HPO₄²⁻ makes it more eager to accept a proton, thus making it a stronger base. Therefore, the stronger base is HPO₄²⁻.

Key concepts

Conjugate Acid-Base Pair

An essential concept in acid-base chemistry is the conjugate acid-base pair. This involves two species that differ only by a proton ( H ^ + ). When a base gains a proton, it becomes its corresponding conjugate acid. Conversely, when an acid donates a proton, it forms its conjugate base.
Understanding this relationship is key to determining which species will act as the base or acid in chemical reactions.
- **Conjugate acid**: The species formed after a base gains a proton. - **Conjugate base**: The species that remains after an acid loses a proton. When analyzing a chemical system, spotting these pairs helps predict the flow of protons and the tendency of a molecule to accept or donate a proton. The ability to understand and identify conjugate acid-base pairs is fundamental to mastering acid-base equilibrium and predicting reaction outcomes.

Base Strength Comparison

Choosing the stronger base between two options can be tricky, but knowing what to focus on makes it easier. Base strength refers to the ability of a species to accept a proton. Several factors can determine the strength of a base: - **Size of the atom holding the charge**: Smaller atoms with high electronegativity generally hold the charge more tightly, making them stronger bases. For example, ClO^ - is a stronger base than BrO^ - because chlorine is smaller and more electronegative than bromine. - **Resonance effects**: If the charge can be delocalized over several atoms, the base will generally be weaker because it is more stable.
For instance, comparing the bases BrO⁻ and BrO₂⁻, the presence of additional oxygen in BrO₂⁻ allows for more delocalization, making it weaker as a base compared to BrO⁻. - **Inductive Effect**: Nearby electronegative atoms can pull electron density away, weakening the base. Understanding these points can significantly aid in correctly determining which base is stronger in any given pair.

Electronegativity and Basicity

Electronegativity is a crucial factor influencing basicity among chemical species. A more electronegative atom will draw electron density towards itself more strongly, so it's going to hold an added proton less tightly. This means an atom with higher electronegativity will often make a stronger base when it's part of a negative ion.
- **Localized Charge**: In molecules such as ClO^ - , a more electronegative atom like chlorine holds the negative charge more localized compared to a less electronegative atom like bromine. Hence, ClO^ - is a stronger base than BrO^ - . This is because Cl's higher electronegativity means it is more competent at sustaining the negative charge without spreading it out, which bolsters its ability to attract protons. Understanding electronegativity helps explain why some bases are more reactive than others. It also helps to see why such bases would accept protons more readily, reflecting strong basicity.

Protonation and Charge

A central theme in acid-base chemistry is how protonation affects the charge of a molecule and its reactivity. Protonation involves the addition of a proton ( H ^ + ) to an atom, ion, or molecule, typically enhancing its positive charge or reducing its negative charge.
Consider the species HPO₄²⁻ and H₂PO₄⁻: - **HPO₄²⁻ vs. H₂PO₄⁻**: HPO₄²⁻ has a greater negative charge due to fewer protons compared to H₂PO₄⁻. This increased negative charge enables HPO₄²⁻ to attract protons more actively, making it a stronger base.
The extent of protonation directly impacts the charge distribution and influences a molecule's eagerness to participate in acid-base reactions. A deeper understanding of how charge affects behavior in reactions allows for better predictions about a substance's reactivity.