Comparing strengths of acids

[Casey Collet 1I]

When comparing the strength of weak acids, it is supposed to be that the more electronegative the compound, the stronger the acid. However, acids also get stronger as the bonds get weaker, so that acids get stronger moving down a period. For example, on problem #53 in chapter 11, it says that HCl>HF, and that HClO2>HBrO2. How do you know when to use which rule? Is it the presence of oxygen in the molecule that makes the second statement above no longer valid?

[Niharika Reddy 1D]

Bond strength is the dominating factor when determining the strength of binary acids such as HF and HCl; we look at the size of F and Cl to determine HCl is the stronger acid because Cl is larger than F and the H-Cl bond is longer and weaker, making it easier for water to remove a proton (H⁺).

For oxoacids, we look at electronegativity since oxoacids more readily lose a proton if the resulting anion can be stabilized by electron withdrawing atoms. Atoms with higher electronegativity stabilize the negative charge on the conjugate base, making the O-H bond more polar and the acid stronger. Since Cl is more electronegative than Br, HClO₂ is stronger than HBrO₂.

[Chem_Mod]

Niharika, good answer. Introduce yourself to me at the beginning of class.