Question

Explain why \(\mathrm{HF}\) is a weak acid, whereas \(\mathrm{HCl}, \mathrm{HBr}\), and \(\mathrm{HI}\) are all strong acids.

Short answer

HF is a weak acid, while HCl, HBr, and HI are strong acids because of the differences in bond strengths and acid dissociation constants (Ka) between these compounds. HF has a stronger bond (569 kJ/mol) and a lower Ka value (\(6.8 \times 10^{-4}\)), indicating it only partially dissociates in solution. In contrast, HCl, HBr, and HI have weaker bonds and much larger Ka values, indicating they fully dissociate in solution, making them strong acids.

Step-by-step solution

Compare Bond Strengths

Bond strength is an important factor in determining the acidity of a compound. The weaker the bond strength between hydrogen and the other atom, the more likely it is for the compound to dissociate and release a hydrogen ion (H+). Comparing the bond strengths, we have: - HF - single bond with a bond strength of 569 kJ/mol - HCl - single bond with a bond strength of 431 kJ/mol - HBr - single bond with a bond strength of 366 kJ/mol - HI - single bond with a bond strength of 299 kJ/mol The bond strengths decrease as you move down the halogen group in the periodic table due to increasing atomic size. So, HI has the weakest bond making it a strong acid, and HF has the strongest bond, making it less prone to release H+ ions compared to the others.

Compare Acid Dissociation Constants (Ka)

Another factor that contributes to the acidity of these compounds is their Ka values. Ka, the acid dissociation constant, is a measure of the degree to which a weak acid will dissociate into its ions in a solution. Higher Ka values indicate stronger acids. Comparing the Ka values, we have: - HF - (\(Ka = 6.8 \times 10^{-4}\)) - HCl - (\(Ka >> 1\) - fully dissociates) - HBr - (\(Ka >> 1\) - fully dissociates) - HI - (\(Ka >> 1\) - fully dissociates) The Ka values of HCl, HBr, and HI are much larger than that of HF, which indicates they are strong acids that dissociate completely in solution. On the other hand, HF has a much smaller Ka value, indicating that it only partially dissociates in solution, making it a weak acid.

Conclusion

Based on the above comparisons of bond strengths and acid dissociation constants, we can conclude that HF is a weak acid due to its relatively strong bond and low Ka value. In contrast, HCl, HBr, and HI have weaker bonds and much larger Ka values, making them strong acids that dissociate fully in solution.