Question

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

Short answer

In summary, $\mathrm{HF}$ is a weak acid because fluorine is the most electronegative element, resulting in a strong bond with hydrogen, and fewer hydrogen ions released when dissolved in water. In contrast, $\mathrm{HCl}$, $\mathrm{HBr}$, and $\mathrm{HI}$ are strong acids because they have less electronegative halogen atoms, resulting in weaker bonds with hydrogen, and more hydrogen ions released when dissolved in water.

Step-by-step solution

Definition of Weak and Strong Acids

A weak acid is one that does not dissociate completely in water, meaning not all of its molecules release hydrogen ions (H+) when dissolved in it. Conversely, a strong acid completely dissociates in water, releasing a significant amount of H+ ions.

Acid strength and bond strength

The strength of an acid is mostly related to the strength of the bond between the hydrogen (H) atom and the other atom it's bound to, known as the conjugate base. In this case, we're comparing the bond between H and F (HF) to the bonds between H and Cl (HCl), H and Br (HBr), and H and I (HI). In general, the weaker the bond between the H atom and the conjugate base, the stronger the acid.

Electronegativity and bond strength

Electronegativity is the ability of an atom to attract electrons in a bond. A higher electronegativity means that the atom is more effective at attracting electrons. The bond strength between H and the conjugate base is affected by the electronegativity difference. When the electronegativity difference is large, the bond is more polar, and the bond is weaker. Conversely, when the electronegativity difference is smaller, the bond is less polar and more robust.

Electronegativity of halogens

The halogens (F, Cl, Br, and I) are all very electronegative, but there is a difference in their electronegativity values. Fluorine (F) is the most electronegative element with a value of 3.98, followed by chlorine (Cl: 3.16), bromine (Br: 2.96), and iodine (I: 2.66).

HF vs. HCl, HBr, and HI

Comparing acid strength, bond strength, and electronegativity of HF, HCl, HBr, and HI, we can see why HF is a weak acid, while the others are strong acids. Because F is the most electronegative element, it forms a very strong bond with hydrogen, meaning the bond is harder to break. Therefore, HF does not dissociate completely in water, releasing fewer H+ ions, making it a weak acid. On the other hand, HCl, HBr, and HI have less electronegative halogen atoms. This results in weaker bonds with hydrogen, which are more easily broken. Thus, these acids dissociate completely in water, releasing more H+ ions, making them strong acids.

Key concepts

Weak Acids

Weak acids are a fascinating topic in the study of chemistry. A weak acid is not like a strong acid in one crucial way: it doesn't let go of its hydrogen ions easily when dissolved in water. This means that only a small fraction of a weak acid's molecules will release hydrogen ions ( H^+ ), making them partially dissociated.
Understanding this can help you predict how a weak acid behaves in solution. Here are some key features of weak acids:

• They do not dissociate completely in water.
• They have higher pH values compared to strong acids.
• They are often involved in equilibrium reactions, showing reversible behavior.

When you put a weak acid like HF into water, most of its molecules remain intact, and only a few break into ions. This means it doesn't give off much H^+ , making it "weaker." This is largely because the bond between the hydrogen and the other atom in the molecule is relatively strong, making it less likely to break apart.

Strong Acids

Strong acids are strong because they easily let go of their hydrogen ions when in water. On adding a strong acid to water, it dissociates completely, releasing a lot of hydrogen ions ( H^+ ) into the solution. This full dissociation makes strong acids highly reactive.
Here's what distinguishes strong acids from weak ones:

• Complete dissociation in water.
• Lower pH values, indicating high acidity.
• More predictable behavior in reactions, often proceeding to completion.

For example, with acids such as HCl , HBr , and HI , the bonds between hydrogen and the halogen are weaker compared to HF , thus they split apart easily. These weak bonds are a direct result of the lesser electronegativity of Cl, Br, and I compared to F , resulting in a strong acid that readily dissociates in water.

Electronegativity

Electronegativity plays a critical role in defining acid strength. It represents how strongly an atom attracts electrons in a bond. In an acid, the electronegativity difference can indicate how tightly atoms hold together or how easily bonds break apart.
Here's why electronegativity is crucial for understanding acid behavior:

• High electronegativity means the atom pulls electrons strongly, creating strong, polar bonds.
• If the bond is too strong (as with HF ), it's difficult for the molecule to dissociate, leading to weak acidity.
• Lower electronegativity results in weaker bonds that are easier to break, typical in strong acids.

In the case of HF , fluorine's high electronegativity makes it hold tightly onto hydrogen, forming a very stable bond. Unlike HF , with elements like Cl , Br , and I , the lower electronegativity makes these acids' bonds less stable, leading to complete dissociation in water. Thus, these acids are considered strong, as they release H^+ ions readily.