Problem 92 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indicate whether each of the following statements is true or false. For each statement that is false, correct the statement to make it true. (a) Acid strength in a series of \(\mathrm{H}-\mathrm{A}\) molecules increases with increasing size of A. (b) For acids of the same general structure but differing electronegativities of the central atoms, acid strength decreases with increasing electronegativity of the central atom. (c) The strongest acid known is HF because fluorine is the most electronegative element.

Step-by-Step Solution

Answer

(a) True - Acid strength in a series of H-A molecules increases with increasing size of A. (b) True - For acids of the same general structure but differing electronegativities of the central atoms, acid strength decreases with increasing electronegativity of the central atom. (c) False - Fluorine is the most electronegative element, but HF is not the strongest acid. At standard conditions, HCl, HBr, and HI are stronger acids than HF due in part to the small size of the fluorine atom.

Step 1: Statement (a)

Acid strength in a series of H-A molecules increases with increasing size of A. In a series of H-A molecules, as the size of A increases, the bond between H and A weakens. As a result, the acid strength increases. Therefore, this statement is true.

Step 2: Statement (b)

For acids of the same general structure but differing electronegativities of the central atoms, acid strength decreases with increasing electronegativity of the central atom. For acids with the same structure, as the electronegativity of the central atom increases, the strength of the bond between the central atom and hydrogen also increases. This leads to a decreased ability of the central atom to donate a proton, hence the acid strength decreases. So this statement is also true.

Step 3: Statement (c)

The strongest acid known is HF because fluorine is the most electronegative element. While it is true that fluorine is the most electronegative element, the strongest acid is not HF. At standard conditions, HCl, HBr, and HI are stronger acids than HF. This is primarily due to the small size of the fluorine atom, which results in a shorter and stronger bond between the hydrogen atom and the fluorine atom, making it more difficult for the HF molecule to dissociate (donate a proton) when compared to other hydrogen-halogen molecules. Thus, this statement is false. To correct the statement, we can say: "Fluorine is the most electronegative element, but HF is not the strongest acid. At standard conditions, HCl, HBr, and HI are stronger acids than HF due in part to the small size of the fluorine atom."

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that refers to an atom's ability to attract and hold onto electrons. This property is important when considering acid strength because it influences how tightly a central atom holds its hydrogen atoms.
In a molecule like \(\mathrm{H}-\mathrm{A}\), if the atom \(\mathrm{A}\) has high electronegativity, it will pull electrons towards itself more strongly. This often results in a stronger \(\mathrm{H}-\mathrm{A}\) bond, making it harder for the hydrogen atom to be released as a proton. Thus, acids with highly electronegative central atoms typically have reduced acid strength compared to those with less electronegative atoms.
It's important to note that while electronegativity diminishes the ability of an acid to donate a proton, it can also help to stabilize the negative charge of the conjugate base once the proton is donated. This duality plays a crucial role in understanding the complex nature of acids and their behaviors.

H-A Bond Weakening

The concept of \(\mathrm{H}-\mathrm{A}\) bond weakening is central to understanding why certain acids are stronger than others. When considering acid strength, it is vital to examine what happens to the bond between the hydrogen atom (H) and the other atom (A) in the molecule.
Generally, a weaker \(\mathrm{H}-\mathrm{A}\) bond means the acid is stronger because the bond is more easily broken, allowing the hydrogen ion (proton) to be released into the solution.
Factors that influence bond weakening include:

• Size of the Atom \(\mathrm{A}\): Larger atoms tend to form weaker bonds with hydrogen due to longer bond lengths, making the acid stronger.
• Electronegativity: As previously mentioned, higher electronegativity can impact bond strength, but with conflicting effects based on molecular structure.

The interplay between these factors ultimately dictates the acid's ability to donate protons and hence its strength relative to other acids.

Hydrogen Halides

Hydrogen halides are compounds consisting of hydrogen and a halogen, represented as \(\mathrm{HX}\) (where \(\mathrm{X}\) is a halogen such as fluorine, chlorine, bromine, or iodine). These compounds vary in terms of acid strength, primarily due to the size of the halogen atom and the resulting bond strength with hydrogen.
For instance:

• HF has a much stronger \(\mathrm{H}-\mathrm{F}\) bond due to the small size and high electronegativity of fluorine, making it a weaker acid because it doesn't release protons easily.
• HCl, HBr, and HI, on the other hand, have progressively weaker bonds with hydrogen due to larger atomic sizes, making them stronger acids under normal conditions.

These differences illustrate the principles of electronegativity and bond weakening, demonstrating why hydrogen halides exhibit varying degrees of acidity. Understanding these factors helps in predicting and explaining the behavior of these common and important acids in chemical reactions.