Problem 33 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

In conclusion, HCl, HBr, and HI are strong acids because they have relatively weaker H-X bonds (X = Cl, Br, I) which allow them to dissociate completely into their ions in a solution. On the contrary, HF is a weak acid as it has a strong H-F bond, preventing it from complete ionization into its ions in a solution.

Step 1: Understanding strong acids and weak acids

Strong acids dissociate completely into their ions in a solution, whereas weak acids do not dissociate completely and exist partially as undissociated molecules and partially as ions. In the case of hydrohalic acids, the extent of ionization in the solution is a function of their bond strength, that is, the strength of the bond between the hydrogen atom and the halogen atom.

Step 2: Comparing bond strengths of the given acids

We need to compare the bond strengths of HCl, HBr, HI, and HF. The bond strength decreases down the Group 17 elements (halogens) from fluorine to iodine, with fluorine having the highest bond strength. So, the order of bond strengths will be \( HF > HCl > HBr > HI \).

Step 3: Relating bond strength to ionization

Ionization in a solution depends on the bond strength of a given acid. The stronger the bond, the lower the degree of ionization. Therefore, acids with weaker bonds are more likely to ionize completely. Based on the order of bond strengths, we can understand why HCl, HBr, and HI are strong acids and HF is a weak acid.

Step 4: Conclusion

In conclusion, HCl, HBr, and HI are strong acids because they have relatively weaker bonds and thus can dissociate completely into their ions in a solution. On the contrary, HF is a weak acid due to its strong bond, which prevents it from complete ionization into its ions in a solution.

Key Concepts

Strong Acids

Strong acids are substances that completely dissociate into their ions when dissolved in water. This means that virtually all the acid molecules break apart to release hydrogen ions ( H^+ ) and their corresponding anions.
When an acid is strong, you can be confident that the concentration of hydrogen ions in solution is equal to the initial concentration of the acid.
Examples of strong acids include ( HCl, HBr, HI ) —these acids dissociate almost completely due to their weaker bonds with hydrogen, allowing full ionization without much resistance from molecular bonds.

Weak Acids

Unlike strong acids, weak acids only partially dissociate in solution.
This means a significant portion of the acid remains in its original molecular form, with only some molecules releasing hydrogen ions ( H^+ ) into the solution.
As a result, weak acids do not generate as many hydrogen ions, leading to lower acidity levels compared to strong acids at equivalent concentrations.

• Weak acids establish an equilibrium between their ionized and non-ionized forms.
• Their ionization tends to be much lower than that of strong acids, often less than 5%.

Hydrofluoric acid ( HF ) serves as an example of a weak acid. Despite being a hydrohalic acid, its strong bond strength with hydrogen hinders complete ionization.

Bond Strength

The strength of the bond between an acid's hydrogen atom and its accompanying element greatly influences its ability to ionize.
Stronger bonds require more energy to break, thus reducing the likelihood of complete dissociation.

• In hydrohalic acids, bond strength varies within the Group 17 halogens.
• Fluorine has the strongest bond with hydrogen, while iodine has the weakest.

The trend in bond strength can be summarized as: ( HF > HCl > HBr > HI ).
Acids with weaker bonds, such as ( HI ), tend to ionize more easily, making them strong acids compared to those with stronger bonds like ( HF ), which resist ionization.

Hydrohalic Acids

Hydrohalic acids are acids that consist of hydrogen bonded to halogen elements from Group 17 of the periodic table.
These include ( HF, HCl, HBr, HI ).
Their acid strength largely depends on the bond strength between their hydrogen and halogen atoms.

• Acids with a weak bond between hydrogen and halogen, like ( HI ), tend to be strong acids because they dissociate readily.
• Those with a strong bond, like ( HF ), do not ionize fully and thus are weak acids.

Understanding the nature of hydrohalic acids involves considering both their position in the halogen group and the bond strength, both of which contribute to their behavior as acids in solution.