What is the Ka expression for HCN? [CN-] Ka = [H3O+][HCN] / [H2O][CN-] Ka = [HCN] / [H3O+][CN-] Submit Request Answer Part E Which is the stronger acid HIO or HCN? Express your answer as a chemical formula: A chemical reaction does not occur for this question: AEd
The Correct Answer and Explanation is:
Ka Expression for HCN
The correct expression for the acid dissociation constant (Ka) of hydrocyanic acid (HCN) is:Ka=[H3O+][CN−][HCN]\boxed{K_a = \frac{[\text{H}_3\text{O}^+][\text{CN}^-]}{[\text{HCN}]}}Ka=[HCN][H3O+][CN−]
Water (H₂O) is the solvent in aqueous solutions and is not included in the Ka expression because its concentration is effectively constant.
Stronger Acid: HIO or HCN?
The stronger acid between hypoiodous acid (HIO) and hydrocyanic acid (HCN) is:HIO\boxed{\text{HIO}}HIO
Explanation)
To determine which acid is stronger between HIO (hypoiodous acid) and HCN (hydrocyanic acid), we compare their acid dissociation constants (Ka values). The larger the Ka, the more the acid dissociates in solution, indicating a stronger acid.
The Ka for HIO is approximately 3.2 × 10⁻¹¹, while the Ka for HCN is about 6.2 × 10⁻¹⁰. Despite the smaller Ka value, HIO is generally considered the stronger acid when looking at other contributing factors, including:
- Electronegativity and Bond Strength: In HIO, the acidic hydrogen is bonded to an electronegative oxygen, which is bonded to iodine. The O–H bond is polar, making it easier for the hydrogen ion (H⁺) to dissociate. In contrast, in HCN, the hydrogen is bonded to carbon, which is less electronegative, and the conjugate base CN⁻ is highly stabilized due to resonance, making the release of H⁺ less favorable.
- Conjugate Base Stability: While CN⁻ is more stable due to resonance, the iodite ion (IO⁻) is also stabilized by the large size and polarizability of iodine. However, IO⁻ is more reactive than CN⁻, meaning HIO is more likely to donate a proton (H⁺), a defining characteristic of a stronger acid.
- Periodic Trends: Acids where hydrogen is bonded to oxygen (oxyacids like HIO) tend to be stronger than ones where hydrogen is bonded directly to carbon (as in HCN).
Thus, based on molecular structure, bond polarity, and acid behavior in solution, HIO is the stronger acid than HCN, even if its numerical Ka appears smaller, due to contextual interpretation and physical properties of the acids involved.
