Problem 91 - Chemistry: The Central Science

Question

How does the acid strength of an oxyacid depend on (a) the electronegativity of the central atom; (b) the number of nonprotonated oxygen atoms in the molecule?

Step-by-Step Solution

Answer

The acid strength of an oxyacid depends on (a) the electronegativity of the central atom, where a higher electronegativity results in a stronger acid as it stabilizes the conjugate base more effectively, and (b) the number of nonprotonated oxygen atoms in the molecule, with more nonprotonated oxygen atoms leading to a stronger acid due to increased resonance effects and better charge distribution.

Step 1: Understand oxyacids and their structure

Oxyacids are compounds that consist of a central atom (usually a non-metal) bonded to one or more oxygen atoms and one or more hydrogen atoms. The general formula for an oxyacid is: \[HOX_n\] where \(X\) is the central atom and \(n\) represents the number of oxygen atoms bonded to the central atom. The strength of an oxyacid is determined by its ability to donate a proton (\(H^+\)) to a solvent like water.

Step 2: Electronegativity of the central atom

Electronegativity is the measure of the tendency of an atom to attract a bonding pair of electrons. Generally, electronegative atoms are better at stabilizing negative charges. In oxyacids, the central atom is bonded to an oxygen atom, and oxygen has a high electronegativity. When an oxyacid donates a proton to a solvent like water, it forms its conjugate base. The conjugate base of an oxyacid typically has a negative charge on the oxygen atom bonded to the hydrogen in the parent acid. If the central atom has a higher electronegativity, it will more efficiently distribute the negative charge throughout the oxyanion. This will result in a more stable conjugate base. A stable conjugate base will correspond to a stronger acid since a strong acid tends to give away protons easily. Therefore, it can be concluded that the higher the electronegativity of the central atom, the stronger the resulting oxyacid will be.

Step 3: Number of nonprotonated oxygen atoms in the molecule

The number of nonprotonated oxygen atoms in the molecule refers to those oxygen atoms not bonded to a hydrogen atom. These oxygen atoms play a crucial role in determining the strength of an oxyacid. Nonprotonated oxygen atoms contain unshared electron pairs that can resonate with the electron pairs in the oxygen-hydrogen bond in an oxyacid. This resonance effect helps distribute the negative charge across the molecule, reducing the overall electron density at a specific region. When an oxyacid dissociates, the resonance effect of nonprotonated oxygen atoms greatly increases the stability of the resulting anion. Consequently, a more stable anion will lead to a stronger oxyacid. Therefore, more nonprotonated oxygen atoms in the oxyacid will increase its acid strength. In summary, the acid strength of an oxyacid depends on (a) the electronegativity of the central atom, with a higher electronegativity resulting in a stronger acid, and (b) the number of nonprotonated oxygen atoms in the molecule - the more nonprotonated oxygen atoms present, the stronger the acid will be.