Final answer:
The relative acidity of hydrogen fluoride (HF) compared to hydrogen chloride (HCl) can be attributed to the electronegativity of fluorine and the stability of its conjugate base F- in solution, correlating to its ability to bear a negative charge and its basic behavior in water.
Step-by-step explanation:
A contributing factor to the relative acidity of hydrogen fluoride (HF) compared to hydrogen chloride (HCl) is that the anion F- is less stable in solution than Cl-. This is because fluorine is much more electronegative than chlorine, making it less favorable for fluorine to bear a negative charge. As a result of this electronegativity difference, F- can better stabilize the negative charge than Cl-; however, F- is still not as stable compared to other halide ions. Additionally, HF is a weak acid, and its conjugate base, F-, behaves as a base in an aqueous solution by accepting a proton from water, forming hydroxide ions.
The acid dissociation constant (Ka) indicates the strength of an acid in solution. For HF, the conjugate base F- has a Ka of 1.6 × 10-11, which is lesser than the Ka for NH4+, suggesting that the solution is acidic because Ka is greater than Kb. Comparing ionization constants is one way to evaluate the acidity of ions in solution: the higher the Ka, the stronger the acid. Since HF dissociates to a lesser extent than HCl in water, this indicates that HF is weaker, thus F- is a weaker conjugate base compared to Cl- which is derived from the strong acid HCl.