Question

\(\mathrm{HCl}, \mathrm{HBr}\), and \(\mathrm{HI}\) are strong acids, yet \(\mathrm{HF}\) is a weak acid. What does this mean in terms of the extent to which these substances are ionized in solution?

Short answer

The difference in ionization extent between HCl, HBr, HI, and HF lies in the strength of their H-A bond (A being the halogen atom). HCl, HBr, and HI have weaker H-A bonds and thus, dissociate completely in a solution resulting in a higher concentration of H+ ions. On the other hand, the H-F bond in HF is strong and difficult to break, leading to partial ionization in a solution and making it a weak acid. This demonstrates why HCl, HBr, and HI are considered strong acids while HF is a weak acid.

Step-by-step solution

Defining strong and weak acids

Strong acids are substances that completely ionize in water, meaning they dissociate into their respective ions, resulting in a higher concentration of H+ ions. Weak acids only partially ionize in water, so they don't release many H+ ions in a solution.

Acid ionization reaction and equilibrium constant

To compare the extent of ionization of these substances, let's write down the generic ionization reaction for an acid HA: \( HA_{(aq)} \rightleftharpoons H^+_{(aq)} + A^-_{(aq)} \) For this reaction, we can define the acid dissociation constant, Ka, as follows: \( K_a = \frac{[H^+][A^-]}{[HA]} \) Higher values of Ka mean that more of the acid dissociates, and the resulting solution has higher acidity.

Discussing factors affecting acidic strength

Two critical factors affecting the acidic strength are: 1. Electronegativity of the atom bonded to the hydrogen: A more electronegative atom attracts the shared electron pair more, weakening the H-A bond. A weaker H-A bond leads to easier ionization and stronger acidic behavior. 2. Bond length between hydrogen and the other atom: Generally, the weaker the bond (the larger the bond length), the greater the ionization of that particular acid.

Comparing acidic strengths of given substances

In this case, the acids are formed by hydrogen bonded to halogens. Between halogens, fluorine is the most electronegative element and has the shortest bond length with hydrogen. So, at first glance, we might expect HF to be a stronger acid than the rest. However, the electronegativity of fluorine makes the H-F bond very strong and difficult to break, unlike the case for HCl, HBr, and HI. They have longer and weaker bonds, which can break more easily. As a result, HCl, HBr, and HI are strong acids, and HF is a weak acid.

Conclusion

The difference in ionization extent between HCl, HBr, HI, and HF lies in the strength of their H-A bond (A being the halogen atom). HCl, HBr, and HI have weaker H-A bonds and thus, dissociate completely in a solution resulting in a higher concentration of H+ ions. On the other hand, the H-F bond in HF is strong and difficult to break, leading to partial ionization in a solution and making it a weak acid. This demonstrates why HCl, HBr, and HI are considered strong acids while HF is a weak acid.