Problem 94 - Chemistry The Central Science

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 \(\mathrm{H}-\mathrm{X}\) molecules increases with increasing size of \(\mathrm{X}\). (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) False. Acid strength in a series of H-X molecules generally decreases with increasing size of X, as larger atoms are less effective in stabilizing the resulting negative charge on the conjugate base. (b) True. For acids of the same general structure, the acid strength increases with increasing electronegativity of the central atom because a more electronegative atom will be better at stabilizing the negative charge on its conjugate base after deprotonation. (c) False. The strongest acid known is fluoroantimonic acid (HSbF6), which is formed by mixing hydrogen fluoride (HF) with antimony pentafluoride (SbF5). The acid strength of fluoroantimonic acid is much higher than that of HF, mainly due to the high electronegativity and electron-withdrawing nature of the SbF5 molecule, which stabilizes the negative charge on the resulting conjugate base.

Step 1: Statement (a) Analysis

In this statement, we are analyzing the relationship between acid strength and the size of the atom X in H-X molecules. Our goal is to determine if acid strength increases with increasing size of X.

Step 2: Statement (a) True or False

The statement is False. As the size of the atom X increases, the bond between H and X becomes weaker and it becomes easier for the proton (H+) to be released. However, acid strength depends not only on the bond strength but also on the stability of the conjugate base formed.

Step 3: Statement (a) Correction

Acid strength in a series of H-X molecules generally decreases with increasing size of X, as larger atoms are less effective in stabilizing the resulting negative charge on the conjugate base.

Step 4: Statement (b) Analysis

In this statement, we are analyzing the relationship between the acid strength and the electronegativity of the central atom in H-X acids with the same general structure.

Step 5: Statement (b) True or False

The statement is True. For acids of the same general structure, the acid strength increases with increasing electronegativity of the central atom because a more electronegative atom will be better at stabilizing the negative charge on its conjugate base after deprotonation.

Step 6: Statement (c) Analysis

In this statement, we are determining if HF is the strongest acid due to fluorine being the most electronegative element.

Step 7: Statement (c) True or False

The statement is False. Although fluorine is the most electronegative element, which can contribute to increased acid strength, HF is not the strongest known acid. The bond between hydrogen and fluorine in HF is quite strong, and the fluoride ion (F-) is not a very stable conjugate base due to its small size and high charge density.

Step 8: Statement (c) Correction

The strongest acid known is fluoroantimonic acid (HSbF6), which is formed by mixing hydrogen fluoride (HF) with antimony pentafluoride (SbF5). The acid strength of fluoroantimonic acid is much higher than that of HF, mainly due to the high electronegativity and electron-withdrawing nature of the SbF5 molecule, which stabilizes the negative charge on the resulting conjugate base.

Key Concepts

H-X Molecules

When we talk about H-X molecules, we are referring to acids where a hydrogen atom (H) is bonded to another atom X. The strength of an acid in this series depends significantly on the ability of the hydrogen ion (H⁺) to detach from the X atom.

One might assume that larger atoms would lead to stronger acids, but the opposite is usually true. The larger the atom X, the weaker the bond it forms with hydrogen. This weaker bond can make it easier for the hydrogen ion to detach, but larger atoms are less capable of stabilizing the negative charge that forms on the conjugate base. This results in weaker acid strength overall.

For example:

• As the size of X increases, such as moving from fluorine (F) to iodine (I), the bond strength decreases because larger atoms form weaker bonds.
• However, these larger atoms do not stabilize the resulting conjugate base effectively, resulting in lower acid strengths.

Electronegativity

Electronegativity is a measure of how strongly an atom attracts electrons in a bond. In the context of acids, electronegativity plays a crucial role in determining acid strength.

For acids with similar structures, an increase in the electronegativity of the central atom often translates into increased acid strength. This is because a more electronegative central atom stabilizes the negative charge on the conjugate base more effectively after the hydrogen proton has been donated.

To illustrate:

• A central atom with high electronegativity, like chlorine (Cl), can attract electrons more strongly than less electronegative atoms, like phosphorus (P).
• This ability to stabilize negative charge makes the conjugate base more stable, thus enhancing the acid's strength.

Conjugate Base Stability

A conjugate base forms when an acid donates its hydrogen ion. The stability of this conjugate base is essential in determining the overall strength of the acid.

If the conjugate base is stable, the acid can easily donate its hydrogen ion, thus making it a stronger acid. Several factors contribute to the stability of a conjugate base, including the size of the atom holding the negative charge and the overall structure of the molecule.

Consider:

• An atom that effectively spreads the negative charge over a larger area will form a more stable conjugate base.
• Highly electronegative atoms in the conjugate base structure also enhance stability by holding onto the negative charge more effectively.

While one might think that HF is a strong acid due to fluorine's electronegativity, the conjugate base F⁻ is relatively unstable. Hence, HF is not as strong as some other acids with more stable conjugate bases.