Problem 92 - Chemistry The Central Science In Si Units

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 \(\mathrm{H}-\) A molecules increases with increasing size of \(\mathrm{A} .(\mathbf{b})\) 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 (b) False, increases (c) False, HF is not the strongest.

Step 1: Evaluating Statement (a)

The statement (a) claims that acid strength in a series of H–A molecules increases with increasing size of A. This statement is generally true because as the size of A increases, the H-A bond becomes weaker and more polarizable, which makes it easier for the acid to donate a proton (H+). As a result, larger atoms like I- form stronger acids than smaller atoms like F- within the same group.

Step 2: Evaluating Statement (a) Correction

Statement (a) is true as is; there is no need for correction. The larger the atom A in H–A molecules, the weaker the bond and the stronger the acid generally becomes.

Step 3: Evaluating Statement (b)

Statement (b) posits that for acids of the same general structure but differing central atom electronegativities, acid strength decreases with increasing electronegativity. This statement is false. In fact, among acids with the same general structure, such as oxyacids, acid strength generally increases with increasing electronegativity because more electronegative atoms can stabilize the negative charge on the conjugate base better.

Step 4: Correcting Statement (b)

To correct statement (b), it should read: 'For acids of the same general structure but differing electronegativities of the central atoms, acid strength increases with increasing electronegativity of the central atom.'

Step 5: Evaluating Statement (c)

Statement (c) claims that the strongest acid known is HF because fluorine is the most electronegative element. This statement is false. HF is not the strongest acid; acids such as HCl, HBr, and HI are stronger. Although fluorine is very electronegative, HF forms a very strong hydrogen bond and does not dissociate completely in solution, making it a weak acid compared to other haloacids.

Step 6: Correcting Statement (c)

To correct statement (c), it should read: 'HF is not the strongest acid known despite fluorine's high electronegativity, because HF does not dissociate completely in solution due to its strong hydrogen bonds.'

Key Concepts

Electronegativity

Electronegativity is a measure of how strongly an atom attracts electrons in a bond. When considering acid strength, electronegativity plays a crucial role, especially in oxyacids. For acids of a similar structure, an increase in the electronegativity of the central atom often leads to stronger acids. This is because more electronegative atoms can better stabilize the negative charge on the conjugate base after the acid donates a proton (H⁺). For example, in oxyacids like HClO versus HIO, chlorine, being more electronegative than iodine, makes HClO a stronger acid. However, it is essential to recognize different effects depending on the specific group of acids. While higher electronegativity often strengthens oxyacids, for hydrohalic acids like HI, HBr, or HF, bond strength and dissociation factors dominate acid strength more than just electronegativity. In summary:

• Oxyacids: Higher electronegativity → Stronger acid
• Hydrohalic acids: Check bond strength and dissociation

Bond Dissociation

Bond dissociation energy is the energy needed to break a bond between two atoms. In acids, especially hydrohalic acids, the strength of the bond between hydrogen and the halogen atom significantly influences acid strength. When this bond is weaker, the acid is stronger because the H⁺ ion can more easily dissociate from the acid molecule. For instance, as we move down the halogen group in the periodic table from HF to HI: - The bond between hydrogen and the halogen atom gets weaker. - Consequently, HI is a much stronger acid than HF because the H-I bond dissociates more readily than the H-F bond. In short, bond dissociation is pivotal in determining how easily the acid can donate its proton. Thus, consider bond dissociation when comparing the strengths of acids within a series.

Molecular Polarity

Molecular polarity refers to the distribution of electrical charges across a molecule. In the context of acids, polarity influences how easily the molecule can release a proton. A highly polar bond between hydrogen and another atom, like in HCl, means the hydrogen atom is slightly positive while the other atom is slightly negative. This charge separation makes it easier for the hydrogen to break away as a proton, thus increasing the acidity. Acid strength often correlates with high molecular polarity because:

• Greater charge separation eases proton release.
• Polar molecules stabilize the conjugate base better after losing H⁺.

Thus, understanding molecular polarity will help students grasp why certain acids are stronger and why others are weaker even within similar chemical families.